This chapter describes the GNU facilities for interprocess
communication using sockets.
A socket is a generalized interprocess communication channel.
Like a pipe, a socket is represented as a file descriptor. Unlike pipes
sockets support communication between unrelated processes, and even
between processes running on different machines that communicate over a
network. Sockets are the primary means of communicating with other
machines; telnet, rlogin, ftp, talk and the
other familiar network programs use sockets.
Not all operating systems support sockets. In the GNU library, the
header file `sys/socket.h' exists regardless of the operating
system, and the socket functions always exist, but if the system does
not really support sockets these functions always fail.
Incomplete: We do not currently document the facilities for
broadcast messages or for configuring Internet interfaces. The
reentrant functions and some newer functions that are related to IPv6
aren't documented either so far.
When you create a socket, you must specify the style of communication
you want to use and the type of protocol that should implement it.
The communication style of a socket defines the user-level
semantics of sending and receiving data on the socket. Choosing a
communication style specifies the answers to questions such as these:
What are the units of data transmission? Some communication
styles regard the data as a sequence of bytes with no larger
structure; others group the bytes into records (which are known in
this context as packets).
Can data be lost during normal operation? Some communication
styles guarantee that all the data sent arrives in the order it was
sent (barring system or network crashes); other styles occasionally
lose data as a normal part of operation, and may sometimes deliver
packets more than once or in the wrong order.
Designing a program to use unreliable communication styles usually
involves taking precautions to detect lost or misordered packets and
to retransmit data as needed.
Is communication entirely with one partner? Some
communication styles are like a telephone call--you make a
connection with one remote socket and then exchange data
freely. Other styles are like mailing letters--you specify a
destination address for each message you send.
You must also choose a namespace for naming the socket. A socket
name ("address") is meaningful only in the context of a particular
namespace. In fact, even the data type to use for a socket name may
depend on the namespace. Namespaces are also called "domains", but we
avoid that word as it can be confused with other usage of the same
term. Each namespace has a symbolic name that starts with `PF_'.
A corresponding symbolic name starting with `AF_' designates the
address format for that namespace.
Finally you must choose the protocol to carry out the
communication. The protocol determines what low-level mechanism is used
to transmit and receive data. Each protocol is valid for a particular
namespace and communication style; a namespace is sometimes called a
protocol family because of this, which is why the namespace names
start with `PF_'.
The rules of a protocol apply to the data passing between two programs,
perhaps on different computers; most of these rules are handled by the
operating system and you need not know about them. What you do need to
know about protocols is this:
In order to have communication between two sockets, they must specify
the same protocol.
Each protocol is meaningful with particular style/namespace
combinations and cannot be used with inappropriate combinations. For
example, the TCP protocol fits only the byte stream style of
communication and the Internet namespace.
For each combination of style and namespace there is a default
protocol, which you can request by specifying 0 as the protocol
number. And that's what you should normally do--use the default.
Throughout the following description at various places
variables/parameters to denote sizes are required. And here the trouble
starts. In the first implementations the type of these variables was
simply int. On most machines at that time an int was 32
bits wide, which created a de facto standard requiring 32-bit
variables. This is important since references to variables of this type
are passed to the kernel.
Then the POSIX people came and unified the interface with the words "all
size values are of type size_t". On 64-bit machines
size_t is 64 bits wide, so pointers to variables were no longer
possible.
The Unix98 specification provides a solution by introducing a type
socklen_t. This type is used in all of the cases that POSIX
changed to use size_t. The only requirement of this type is that
it be an unsigned type of at least 32 bits. Therefore, implementations
which require that references to 32-bit variables be passed can be as
happy as implementations which use 64-bit values.
The GNU library includes support for several different kinds of sockets,
each with different characteristics. This section describes the
supported socket types. The symbolic constants listed here are
defined in `sys/socket.h'.
Macro: int SOCK_STREAM
The SOCK_STREAM style is like a pipe (see section 15. Pipes and FIFOs).
It operates over a connection with a particular remote socket and
transmits data reliably as a stream of bytes.
The SOCK_DGRAM style is used for sending
individually-addressed packets unreliably.
It is the diametrical opposite of SOCK_STREAM.
Each time you write data to a socket of this kind, that data becomes
one packet. Since SOCK_DGRAM sockets do not have connections,
you must specify the recipient address with each packet.
The only guarantee that the system makes about your requests to
transmit data is that it will try its best to deliver each packet you
send. It may succeed with the sixth packet after failing with the
fourth and fifth packets; the seventh packet may arrive before the
sixth, and may arrive a second time after the sixth.
The typical use for SOCK_DGRAM is in situations where it is
acceptable to simply re-send a packet if no response is seen in a
reasonable amount of time.
The name of a socket is normally called an address. The
functions and symbols for dealing with socket addresses were named
inconsistently, sometimes using the term "name" and sometimes using
"address". You can regard these terms as synonymous where sockets
are concerned.
A socket newly created with the socket function has no
address. Other processes can find it for communication only if you
give it an address. We call this binding the address to the
socket, and the way to do it is with the bind function.
You need be concerned with the address of a socket if other processes
are to find it and start communicating with it. You can specify an
address for other sockets, but this is usually pointless; the first time
you send data from a socket, or use it to initiate a connection, the
system assigns an address automatically if you have not specified one.
Occasionally a client needs to specify an address because the server
discriminates based on address; for example, the rsh and rlogin
protocols look at the client's socket address and only bypass password
checking if it is less than IPPORT_RESERVED (see section 16.6.3 Internet Ports).
Regardless of the namespace, you use the same functions bind and
getsockname to set and examine a socket's address. These
functions use a phony data type, struct sockaddr *, to accept the
address. In practice, the address lives in a structure of some other
data type appropriate to the address format you are using, but you cast
its address to struct sockaddr * when you pass it to
bind.
The functions bind and getsockname use the generic data
type struct sockaddr * to represent a pointer to a socket
address. You can't use this data type effectively to interpret an
address or construct one; for that, you must use the proper data type
for the socket's namespace.
Thus, the usual practice is to construct an address of the proper
namespace-specific type, then cast a pointer to struct sockaddr *
when you call bind or getsockname.
The one piece of information that you can get from the struct
sockaddr data type is the address format designator. This tells
you which data type to use to understand the address fully.
The symbols in this section are defined in the header file
`sys/socket.h'.
Data Type:struct sockaddr
The struct sockaddr type itself has the following members:
short int sa_family
This is the code for the address format of this address. It
identifies the format of the data which follows.
char sa_data[14]
This is the actual socket address data, which is format-dependent. Its
length also depends on the format, and may well be more than 14. The
length 14 of sa_data is essentially arbitrary.
Each address format has a symbolic name which starts with `AF_'.
Each of them corresponds to a `PF_' symbol which designates the
corresponding namespace. Here is a list of address format names:
AF_LOCAL
This designates the address format that goes with the local namespace.
(PF_LOCAL is the name of that namespace.) See section 16.5.2 Details of Local Namespace, for information about this address format.
AF_UNIX
This is a synonym for AF_LOCAL. Although AF_LOCAL is
mandated by POSIX.1g, AF_UNIX is portable to more systems.
AF_UNIX was the traditional name stemming from BSD, so even most
POSIX systems support it. It is also the name of choice in the Unix98
specification. (The same is true for PF_UNIX
vs. PF_LOCAL).
AF_FILE
This is another synonym for AF_LOCAL, for compatibility.
(PF_FILE is likewise a synonym for PF_LOCAL.)
AF_INET
This designates the address format that goes with the Internet
namespace. (PF_INET is the name of that namespace.)
See section 16.6.1 Internet Socket Address Formats.
AF_INET6
This is similar to AF_INET, but refers to the IPv6 protocol.
(PF_INET6 is the name of the corresponding namespace.)
AF_UNSPEC
This designates no particular address format. It is used only in rare
cases, such as to clear out the default destination address of a
"connected" datagram socket. See section 16.10.1 Sending Datagrams.
The corresponding namespace designator symbol PF_UNSPEC exists
for completeness, but there is no reason to use it in a program.
`sys/socket.h' defines symbols starting with `AF_' for many
different kinds of networks, most or all of which are not actually
implemented. We will document those that really work as we receive
information about how to use them.
Function: int bind(int socket, struct sockaddr *addr, socklen_t length)
The bind function assigns an address to the socket
socket. The addr and length arguments specify the
address; the detailed format of the address depends on the namespace.
The first part of the address is always the format designator, which
specifies a namespace, and says that the address is in the format of
that namespace.
The return value is 0 on success and -1 on failure. The
following errno error conditions are defined for this function:
EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
EADDRNOTAVAIL
The specified address is not available on this machine.
EADDRINUSE
Some other socket is already using the specified address.
EINVAL
The socket socket already has an address.
EACCES
You do not have permission to access the requested address. (In the
Internet domain, only the super-user is allowed to specify a port number
in the range 0 through IPPORT_RESERVED minus one; see
16.6.3 Internet Ports.)
Additional conditions may be possible depending on the particular namespace
of the socket.
Use the function getsockname to examine the address of an
Internet socket. The prototype for this function is in the header file
`sys/socket.h'.
Function: int getsockname(int socket, struct sockaddr *addr, socklen_t *length-ptr)
The getsockname function returns information about the
address of the socket socket in the locations specified by the
addr and length-ptr arguments. Note that the
length-ptr is a pointer; you should initialize it to be the
allocation size of addr, and on return it contains the actual
size of the address data.
The format of the address data depends on the socket namespace. The
length of the information is usually fixed for a given namespace, so
normally you can know exactly how much space is needed and can provide
that much. The usual practice is to allocate a place for the value
using the proper data type for the socket's namespace, then cast its
address to struct sockaddr * to pass it to getsockname.
The return value is 0 on success and -1 on error. The
following errno error conditions are defined for this function:
EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
ENOBUFS
There are not enough internal buffers available for the operation.
You can't read the address of a socket in the file namespace. This is
consistent with the rest of the system; in general, there's no way to
find a file's name from a descriptor for that file.
Each network interface has a name. This usually consists of a few
letters that relate to the type of interface, which may be followed by a
number if there is more than one interface of that type. Examples
might be lo (the loopback interface) and eth0 (the first
Ethernet interface).
Although such names are convenient for humans, it would be clumsy to
have to use them whenever a program needs to refer to an interface. In
such situations an interface is referred to by its index, which is
an arbitrarily-assigned small positive integer.
The following functions, constants and data types are declared in the
header file `net/if.h'.
Constant: size_t IFNAMSIZ
This constant defines the maximum buffer size needed to hold an
interface name, including its terminating zero byte.
Function: unsigned int if_nametoindex(const char *ifname)
This function yields the interface index corresponding to a particular
name. If no interface exists with the name given, it returns 0.
Function: char * if_indextoname(unsigned int ifindex, char *ifname)
This function maps an interface index to its corresponding name. The
returned name is placed in the buffer pointed to by ifname, which
must be at least IFNAMSIZ bytes in length. If the index was
invalid, the function's return value is a null pointer, otherwise it is
ifname.
Data Type:struct if_nameindex
This data type is used to hold the information about a single
interface. It has the following members:
This function returns an array of if_nameindex structures, one
for every interface that is present. The end of the list is indicated
by a structure with an interface of 0 and a null name pointer. If an
error occurs, this function returns a null pointer.
The returned structure must be freed with if_freenameindex after
use.
This section describes the details of the local namespace, whose
symbolic name (required when you create a socket) is PF_LOCAL.
The local namespace is also known as "Unix domain sockets". Another
name is file namespace since socket addresses are normally implemented
as file names.
In the local namespace socket addresses are file names. You can specify
any file name you want as the address of the socket, but you must have
write permission on the directory containing it.
It's common to put these files in the `/tmp' directory.
One peculiarity of the local namespace is that the name is only used
when opening the connection; once open the address is not meaningful and
may not exist.
Another peculiarity is that you cannot connect to such a socket from
another machine--not even if the other machine shares the file system
which contains the name of the socket. You can see the socket in a
directory listing, but connecting to it never succeeds. Some programs
take advantage of this, such as by asking the client to send its own
process ID, and using the process IDs to distinguish between clients.
However, we recommend you not use this method in protocols you design,
as we might someday permit connections from other machines that mount
the same file systems. Instead, send each new client an identifying
number if you want it to have one.
After you close a socket in the local namespace, you should delete the
file name from the file system. Use unlink or remove to
do this; see 14.6 Deleting Files.
The local namespace supports just one protocol for any communication
style; it is protocol number 0.
To create a socket in the local namespace, use the constant
PF_LOCAL as the namespace argument to socket or
socketpair. This constant is defined in `sys/socket.h'.
Macro: int PF_LOCAL
This designates the local namespace, in which socket addresses are local
names, and its associated family of protocols. PF_Local is the
macro used by Posix.1g.
Macro: int PF_UNIX
This is a synonym for PF_LOCAL, for compatibility's sake.
Macro: int PF_FILE
This is a synonym for PF_LOCAL, for compatibility's sake.
The structure for specifying socket names in the local namespace is
defined in the header file `sys/un.h':
Data Type:struct sockaddr_un
This structure is used to specify local namespace socket addresses. It has
the following members:
short int sun_family
This identifies the address family or format of the socket address.
You should store the value AF_LOCAL to designate the local
namespace. See section 16.3 Socket Addresses.
char sun_path[108]
This is the file name to use.
Incomplete: Why is 108 a magic number? RMS suggests making
this a zero-length array and tweaking the following example to use
alloca to allocate an appropriate amount of storage based on
the length of the filename.
You should compute the length parameter for a socket address in
the local namespace as the sum of the size of the sun_family
component and the string length (not the allocation size!) of
the file name string. This can be done using the macro SUN_LEN:
Macro: int SUN_LEN(struct sockaddr_un *ptr)
The macro computes the length of socket address in the local namespace.
Here is an example showing how to create and name a socket in the local
namespace.
#include <stddef.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>
int
make_named_socket (const char *filename)
{
struct sockaddr_un name;
int sock;
size_t size;
/* Create the socket. */
sock = socket (PF_LOCAL, SOCK_DGRAM, 0);
if (sock < 0)
{
perror ("socket");
exit (EXIT_FAILURE);
}
/* Bind a name to the socket. */
name.sun_family = AF_LOCAL;
strncpy (name.sun_path, filename, sizeof (name.sun_path));
/* The size of the address is
the offset of the start of the filename,
plus its length,
plus one for the terminating null byte.
Alternatively you can just do:
size = SUN_LEN (&name);
*/
size = (offsetof (struct sockaddr_un, sun_path)
+ strlen (name.sun_path) + 1);
if (bind (sock, (struct sockaddr *) &name, size) < 0)
{
perror ("bind");
exit (EXIT_FAILURE);
}
return sock;
}
This section describes the details of the protocols and socket naming
conventions used in the Internet namespace.
Originally the Internet namespace used only IP version 4 (IPv4). With
the growing number of hosts on the Internet, a new protocol with a
larger address space was necessary: IP version 6 (IPv6). IPv6
introduces 128-bit addresses (IPv4 has 32-bit addresses) and other
features, and will eventually replace IPv4.
To create a socket in the IPv4 Internet namespace, use the symbolic name
PF_INET of this namespace as the namespace argument to
socket or socketpair. For IPv6 addresses you need the
macro PF_INET6. These macros are defined in `sys/socket.h'.
Macro: int PF_INET
This designates the IPv4 Internet namespace and associated family of
protocols.
Macro: int PF_INET6
This designates the IPv6 Internet namespace and associated family of
protocols.
A socket address for the Internet namespace includes the following components:
You must ensure that the address and port number are represented in a
canonical format called network byte order. See section 16.6.5 Byte Order Conversion,
for information about this.
In the Internet namespace, for both IPv4 (AF_INET) and IPv6
(AF_INET6), a socket address consists of a host address
and a port on that host. In addition, the protocol you choose serves
effectively as a part of the address because local port numbers are
meaningful only within a particular protocol.
The data types for representing socket addresses in the Internet namespace
are defined in the header file `netinet/in.h'.
Data Type:struct sockaddr_in
This is the data type used to represent socket addresses in the
Internet namespace. It has the following members:
sa_family_t sin_family
This identifies the address family or format of the socket address.
You should store the value AF_INET in this member.
See section 16.3 Socket Addresses.
When you call bind or getsockname, you should specify
sizeof (struct sockaddr_in) as the length parameter if
you are using an IPv4 Internet namespace socket address.
Data Type:struct sockaddr_in6
This is the data type used to represent socket addresses in the IPv6
namespace. It has the following members:
sa_family_t sin6_family
This identifies the address family or format of the socket address.
You should store the value of AF_INET6 in this member.
See section 16.3 Socket Addresses.
Each computer on the Internet has one or more Internet addresses,
numbers which identify that computer among all those on the Internet.
Users typically write IPv4 numeric host addresses as sequences of four
numbers, separated by periods, as in `128.52.46.32', and IPv6
numeric host addresses as sequences of up to eight numbers separated by
colons, as in `5f03:1200:836f:c100::1'.
Each computer also has one or more host names, which are strings
of words separated by periods, as in `mescaline.gnu.org'.
Programs that let the user specify a host typically accept both numeric
addresses and host names. To open a connection a program needs a
numeric address, and so must convert a host name to the numeric address
it stands for.
Each computer on the Internet has one or more Internet addresses,
numbers which identify that computer among all those on the Internet.
An IPv4 Internet host address is a number containing four bytes of data.
Historically these are divided into two parts, a network number and a
local network address number within that network. In the
mid-1990s classless addresses were introduced which changed this
behavior. Since some functions implicitly expect the old definitions,
we first describe the class-based network and will then describe
classless addresses. IPv6 uses only classless addresses and therefore
the following paragraphs don't apply.
The class-based IPv4 network number consists of the first one, two or
three bytes; the rest of the bytes are the local address.
IPv4 network numbers are registered with the Network Information Center
(NIC), and are divided into three classes--A, B and C. The local
network address numbers of individual machines are registered with the
administrator of the particular network.
Class A networks have single-byte numbers in the range 0 to 127. There
are only a small number of Class A networks, but they can each support a
very large number of hosts. Medium-sized Class B networks have two-byte
network numbers, with the first byte in the range 128 to 191. Class C
networks are the smallest; they have three-byte network numbers, with
the first byte in the range 192-255. Thus, the first 1, 2, or 3 bytes
of an Internet address specify a network. The remaining bytes of the
Internet address specify the address within that network.
The Class A network 0 is reserved for broadcast to all networks. In
addition, the host number 0 within each network is reserved for broadcast
to all hosts in that network. These uses are obsolete now but for
compatibility reasons you shouldn't use network 0 and host number 0.
The Class A network 127 is reserved for loopback; you can always use
the Internet address `127.0.0.1' to refer to the host machine.
Since a single machine can be a member of multiple networks, it can
have multiple Internet host addresses. However, there is never
supposed to be more than one machine with the same host address.
There are four forms of the standard numbers-and-dots notation
for Internet addresses:
a.b.c.d
This specifies all four bytes of the address individually and is the
commonly used representation.
a.b.c
The last part of the address, c, is interpreted as a 2-byte quantity.
This is useful for specifying host addresses in a Class B network with
network address number a.b.
a.b
The last part of the address, b, is interpreted as a 3-byte quantity.
This is useful for specifying host addresses in a Class A network with
network address number a.
a
If only one part is given, this corresponds directly to the host address
number.
Within each part of the address, the usual C conventions for specifying
the radix apply. In other words, a leading `0x' or `0X' implies
hexadecimal radix; a leading `0' implies octal; and otherwise decimal
radix is assumed.
Classless Addresses
IPv4 addresses (and IPv6 addresses also) are now considered classless;
the distinction between classes A, B and C can be ignored. Instead an
IPv4 host address consists of a 32-bit address and a 32-bit mask. The
mask contains set bits for the network part and cleared bits for the
host part. The network part is contiguous from the left, with the
remaining bits representing the host. As a consequence, the netmask can
simply be specified as the number of set bits. Classes A, B and C are
just special cases of this general rule. For example, class A addresses
have a netmask of `255.0.0.0' or a prefix length of 8.
Classless IPv4 network addresses are written in numbers-and-dots
notation with the prefix length appended and a slash as separator. For
example the class A network 10 is written as `10.0.0.0/8'.
IPv6 Addresses
IPv6 addresses contain 128 bits (IPv4 has 32 bits) of data. A host
address is usually written as eight 16-bit hexadecimal numbers that are
separated by colons. Two colons are used to abbreviate strings of
consecutive zeros. For example, the IPv6 loopback address
`0:0:0:0:0:0:0:1' can just be written as `::1'.
IPv4 Internet host addresses are represented in some contexts as integers
(type uint32_t). In other contexts, the integer is
packaged inside a structure of type struct in_addr. It would
be better if the usage were made consistent, but it is not hard to extract
the integer from the structure or put the integer into a structure.
You will find older code that uses unsigned long int for
IPv4 Internet host addresses instead of uint32_t or struct
in_addr. Historically unsigned long int was a 32-bit number but
with 64-bit machines this has changed. Using unsigned long int
might break the code if it is used on machines where this type doesn't
have 32 bits. uint32_t is specified by Unix98 and guaranteed to have
32 bits.
IPv6 Internet host addresses have 128 bits and are packaged inside a
structure of type struct in6_addr.
The following basic definitions for Internet addresses are declared in
the header file `netinet/in.h':
Data Type:struct in_addr
This data type is used in certain contexts to contain an IPv4 Internet
host address. It has just one field, named s_addr, which records
the host address number as an uint32_t.
Macro: uint32_t INADDR_LOOPBACK
You can use this constant to stand for "the address of this machine,"
instead of finding its actual address. It is the IPv4 Internet address
`127.0.0.1', which is usually called `localhost'. This
special constant saves you the trouble of looking up the address of your
own machine. Also, the system usually implements INADDR_LOOPBACK
specially, avoiding any network traffic for the case of one machine
talking to itself.
Macro: uint32_t INADDR_ANY
You can use this constant to stand for "any incoming address" when
binding to an address. See section 16.3.2 Setting the Address of a Socket. This is the usual
address to give in the sin_addr member of struct
sockaddr_in when you want to accept Internet connections.
Macro: uint32_t INADDR_BROADCAST
This constant is the address you use to send a broadcast message.
Macro: uint32_t INADDR_NONE
This constant is returned by some functions to indicate an error.
Data Type:struct in6_addr
This data type is used to store an IPv6 address. It stores 128 bits of
data, which can be accessed (via a union) in a variety of ways.
Constant: struct in6_addr in6addr_loopback
This constant is the IPv6 address `::1', the loopback address. See
above for a description of what this means. The macro
IN6ADDR_LOOPBACK_INIT is provided to allow you to initialize your
own variables to this value.
Constant: struct in6_addr in6addr_any
This constant is the IPv6 address `::', the unspecified address. See
above for a description of what this means. The macro
IN6ADDR_ANY_INIT is provided to allow you to initialize your
own variables to this value.
These additional functions for manipulating Internet addresses are
declared in the header file `arpa/inet.h'. They represent Internet
addresses in network byte order, and network numbers and
local-address-within-network numbers in host byte order. See section 16.6.5 Byte Order Conversion, for an explanation of network and host byte order.
Function: int inet_aton(const char *name, struct in_addr *addr)
This function converts the IPv4 Internet host address name
from the standard numbers-and-dots notation into binary data and stores
it in the struct in_addr that addr points to.
inet_aton returns nonzero if the address is valid, zero if not.
Function: uint32_t inet_addr(const char *name)
This function converts the IPv4 Internet host address name from the
standard numbers-and-dots notation into binary data. If the input is
not valid, inet_addr returns INADDR_NONE. This is an
obsolete interface to inet_aton, described immediately above. It
is obsolete because INADDR_NONE is a valid address
(255.255.255.255), and inet_aton provides a cleaner way to
indicate error return.
Function: uint32_t inet_network(const char *name)
This function extracts the network number from the address name,
given in the standard numbers-and-dots notation. The returned address is
in host order. If the input is not valid, inet_network returns
-1.
The function works only with traditional IPv4 class A, B and C network
types. It doesn't work with classless addresses and shouldn't be used
anymore.
Function: char * inet_ntoa(struct in_addr addr)
This function converts the IPv4 Internet host address addr to a
string in the standard numbers-and-dots notation. The return value is
a pointer into a statically-allocated buffer. Subsequent calls will
overwrite the same buffer, so you should copy the string if you need
to save it.
In multi-threaded programs each thread has an own statically-allocated
buffer. But still subsequent calls of inet_ntoa in the same
thread will overwrite the result of the last call.
Instead of inet_ntoa the newer function inet_ntop which is
described below should be used since it handles both IPv4 and IPv6
addresses.
This function returns the network number part of the Internet host
address addr.
The function works only with traditional IPv4 class A, B and C network
types. It doesn't work with classless addresses and shouldn't be used
anymore.
Function: int inet_pton(int af, const char *cp, void *buf)
This function converts an Internet address (either IPv4 or IPv6) from
presentation (textual) to network (binary) format. af should be
either AF_INET or AF_INET6, as appropriate for the type of
address being converted. cp is a pointer to the input string, and
buf is a pointer to a buffer for the result. It is the caller's
responsibility to make sure the buffer is large enough.
This function converts an Internet address (either IPv4 or IPv6) from
network (binary) to presentation (textual) form. af should be
either AF_INET or AF_INET6, as appropriate. cp is a
pointer to the address to be converted. buf should be a pointer
to a buffer to hold the result, and len is the length of this
buffer. The return value from the function will be this buffer address.
Besides the standard numbers-and-dots notation for Internet addresses,
you can also refer to a host by a symbolic name. The advantage of a
symbolic name is that it is usually easier to remember. For example,
the machine with Internet address `158.121.106.19' is also known as
`alpha.gnu.org'; and other machines in the `gnu.org'
domain can refer to it simply as `alpha'.
Internally, the system uses a database to keep track of the mapping
between host names and host numbers. This database is usually either
the file `/etc/hosts' or an equivalent provided by a name server.
The functions and other symbols for accessing this database are declared
in `netdb.h'. They are BSD features, defined unconditionally if
you include `netdb.h'.
Data Type:struct hostent
This data type is used to represent an entry in the hosts database. It
has the following members:
char *h_name
This is the "official" name of the host.
char **h_aliases
These are alternative names for the host, represented as a null-terminated
vector of strings.
int h_addrtype
This is the host address type; in practice, its value is always either
AF_INET or AF_INET6, with the latter being used for IPv6
hosts. In principle other kinds of addresses could be represented in
the database as well as Internet addresses; if this were done, you
might find a value in this field other than AF_INET or
AF_INET6. See section 16.3 Socket Addresses.
int h_length
This is the length, in bytes, of each address.
char **h_addr_list
This is the vector of addresses for the host. (Recall that the host
might be connected to multiple networks and have different addresses on
each one.) The vector is terminated by a null pointer.
char *h_addr
This is a synonym for h_addr_list[0]; in other words, it is the
first host address.
As far as the host database is concerned, each address is just a block
of memory h_length bytes long. But in other contexts there is an
implicit assumption that you can convert IPv4 addresses to a
struct in_addr or an uint32_t. Host addresses in
a struct hostent structure are always given in network byte
order; see 16.6.5 Byte Order Conversion.
You can use gethostbyname, gethostbyname2 or
gethostbyaddr to search the hosts database for information about
a particular host. The information is returned in a
statically-allocated structure; you must copy the information if you
need to save it across calls. You can also use getaddrinfo and
getnameinfo to obtain this information.
The gethostbyname function returns information about the host
named name. If the lookup fails, it returns a null pointer.
Function: struct hostent * gethostbyname2(const char *name, int af)
The gethostbyname2 function is like gethostbyname, but
allows the caller to specify the desired address family (e.g.
AF_INET or AF_INET6) of the result.
The gethostbyaddr function returns information about the host
with Internet address addr. The parameter addr is not
really a pointer to char - it can be a pointer to an IPv4 or an IPv6
address. The length argument is the size (in bytes) of the address
at addr. format specifies the address format; for an IPv4
Internet address, specify a value of AF_INET; for an IPv6
Internet address, use AF_INET6.
If the lookup fails, gethostbyaddr returns a null pointer.
If the name lookup by gethostbyname or gethostbyaddr
fails, you can find out the reason by looking at the value of the
variable h_errno. (It would be cleaner design for these
functions to set errno, but use of h_errno is compatible
with other systems.)
Here are the error codes that you may find in h_errno:
HOST_NOT_FOUND
No such host is known in the database.
TRY_AGAIN
This condition happens when the name server could not be contacted. If
you try again later, you may succeed then.
NO_RECOVERY
A non-recoverable error occurred.
NO_ADDRESS
The host database contains an entry for the name, but it doesn't have an
associated Internet address.
The lookup functions above all have one in common: they are not
reentrant and therefore unusable in multi-threaded applications.
Therefore provides the GNU C library a new set of functions which can be
used in this context.
The gethostbyname_r function returns information about the host
named name. The caller must pass a pointer to an object of type
struct hostent in the result_buf parameter. In addition
the function may need extra buffer space and the caller must pass an
pointer and the size of the buffer in the buf and buflen
parameters.
A pointer to the buffer, in which the result is stored, is available in
*result after the function call successfully returned. If
an error occurs or if no entry is found, the pointer *result
is a null pointer. Success is signalled by a zero return value. If the
function failed the return value is an error number. In addition to the
errors defined for gethostbyname it can also be ERANGE.
In this case the call should be repeated with a larger buffer.
Additional error information is not stored in the global variable
h_errno but instead in the object pointed to by h_errnop.
Here's a small example:
struct hostent *
gethostname (char *host)
{
struct hostent hostbuf, *hp;
size_t hstbuflen;
char *tmphstbuf;
int res;
int herr;
hstbuflen = 1024;
/* Allocate buffer, remember to free it to avoid memory leakage. */
tmphstbuf = malloc (hstbuflen);
while ((res = gethostbyname_r (host, &hostbuf, tmphstbuf, hstbuflen,
&hp, &herr)) == ERANGE)
{
/* Enlarge the buffer. */
hstbuflen *= 2;
tmphstbuf = realloc (tmphstbuf, hstbuflen);
}
/* Check for errors. */
if (res || hp == NULL)
return NULL;
return hp;
}
Function: int gethostbyname2_r(const char *name, int af, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)
The gethostbyname2_r function is like gethostbyname_r, but
allows the caller to specify the desired address family (e.g.
AF_INET or AF_INET6) for the result.
Function: int gethostbyaddr_r(const char *addr, size_t length, int format, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)
The gethostbyaddr_r function returns information about the host
with Internet address addr. The parameter addr is not
really a pointer to char - it can be a pointer to an IPv4 or an IPv6
address. The length argument is the size (in bytes) of the address
at addr. format specifies the address format; for an IPv4
Internet address, specify a value of AF_INET; for an IPv6
Internet address, use AF_INET6.
Similar to the gethostbyname_r function, the caller must provide
buffers for the result and memory used internally. In case of success
the function returns zero. Otherwise the value is an error number where
ERANGE has the special meaning that the caller-provided buffer is
too small.
You can also scan the entire hosts database one entry at a time using
sethostent, gethostent and endhostent. Be careful
when using these functions because they are not reentrant.
Function: void sethostent(int stayopen)
This function opens the hosts database to begin scanning it. You can
then call gethostent to read the entries.
If the stayopen argument is nonzero, this sets a flag so that
subsequent calls to gethostbyname or gethostbyaddr will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
Function: struct hostent * gethostent(void)
This function returns the next entry in the hosts database. It
returns a null pointer if there are no more entries.
A socket address in the Internet namespace consists of a machine's
Internet address plus a port number which distinguishes the
sockets on a given machine (for a given protocol). Port numbers range
from 0 to 65,535.
Port numbers less than IPPORT_RESERVED are reserved for standard
servers, such as finger and telnet. There is a database
that keeps track of these, and you can use the getservbyname
function to map a service name onto a port number; see 16.6.4 The Services Database.
If you write a server that is not one of the standard ones defined in
the database, you must choose a port number for it. Use a number
greater than IPPORT_USERRESERVED; such numbers are reserved for
servers and won't ever be generated automatically by the system.
Avoiding conflicts with servers being run by other users is up to you.
When you use a socket without specifying its address, the system
generates a port number for it. This number is between
IPPORT_RESERVED and IPPORT_USERRESERVED.
On the Internet, it is actually legitimate to have two different
sockets with the same port number, as long as they never both try to
communicate with the same socket address (host address plus port
number). You shouldn't duplicate a port number except in special
circumstances where a higher-level protocol requires it. Normally,
the system won't let you do it; bind normally insists on
distinct port numbers. To reuse a port number, you must set the
socket option SO_REUSEADDR. See section 16.12.2 Socket-Level Options.
These macros are defined in the header file `netinet/in.h'.
Macro: int IPPORT_RESERVED
Port numbers less than IPPORT_RESERVED are reserved for
superuser use.
Macro: int IPPORT_USERRESERVED
Port numbers greater than or equal to IPPORT_USERRESERVED are
reserved for explicit use; they will never be allocated automatically.
The database that keeps track of "well-known" services is usually
either the file `/etc/services' or an equivalent from a name server.
You can use these utilities, declared in `netdb.h', to access
the services database.
Data Type:struct servent
This data type holds information about entries from the services database.
It has the following members:
char *s_name
This is the "official" name of the service.
char **s_aliases
These are alternate names for the service, represented as an array of
strings. A null pointer terminates the array.
int s_port
This is the port number for the service. Port numbers are given in
network byte order; see 16.6.5 Byte Order Conversion.
To get information about a particular service, use the
getservbyname or getservbyport functions. The information
is returned in a statically-allocated structure; you must copy the
information if you need to save it across calls.
The getservbyname function returns information about the
service named name using protocol proto. If it can't find
such a service, it returns a null pointer.
This function is useful for servers as well as for clients; servers
use it to determine which port they should listen on (see section 16.9.2 Listening for Connections).
The getservbyport function returns information about the
service at port port using protocol proto. If it can't
find such a service, it returns a null pointer.
You can also scan the services database using setservent,
getservent and endservent. Be careful when using these
functions because they are not reentrant.
Function: void setservent(int stayopen)
This function opens the services database to begin scanning it.
If the stayopen argument is nonzero, this sets a flag so that
subsequent calls to getservbyname or getservbyport will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
Function: struct servent * getservent(void)
This function returns the next entry in the services database. If
there are no more entries, it returns a null pointer.
Different kinds of computers use different conventions for the
ordering of bytes within a word. Some computers put the most
significant byte within a word first (this is called "big-endian"
order), and others put it last ("little-endian" order).
So that machines with different byte order conventions can
communicate, the Internet protocols specify a canonical byte order
convention for data transmitted over the network. This is known
as network byte order.
When establishing an Internet socket connection, you must make sure that
the data in the sin_port and sin_addr members of the
sockaddr_in structure are represented in network byte order.
If you are encoding integer data in the messages sent through the
socket, you should convert this to network byte order too. If you don't
do this, your program may fail when running on or talking to other kinds
of machines.
If you use getservbyname and gethostbyname or
inet_addr to get the port number and host address, the values are
already in network byte order, and you can copy them directly into
the sockaddr_in structure.
Otherwise, you have to convert the values explicitly. Use htons
and ntohs to convert values for the sin_port member. Use
htonl and ntohl to convert IPv4 addresses for the
sin_addr member. (Remember, struct in_addr is equivalent
to uint32_t.) These functions are declared in
`netinet/in.h'.
Function: uint16_t htons(uint16_t hostshort)
This function converts the uint16_t integer hostshort from
host byte order to network byte order.
Function: uint16_t ntohs(uint16_t netshort)
This function converts the uint16_t integer netshort from
network byte order to host byte order.
Function: uint32_t htonl(uint32_t hostlong)
This function converts the uint32_t integer hostlong from
host byte order to network byte order.
This is used for IPv4 Internet addresses.
Function: uint32_t ntohl(uint32_t netlong)
This function converts the uint32_t integer netlong from
network byte order to host byte order.
The communications protocol used with a socket controls low-level
details of how data are exchanged. For example, the protocol implements
things like checksums to detect errors in transmissions, and routing
instructions for messages. Normal user programs have little reason to
mess with these details directly.
The default communications protocol for the Internet namespace depends on
the communication style. For stream communication, the default is TCP
("transmission control protocol"). For datagram communication, the
default is UDP ("user datagram protocol"). For reliable datagram
communication, the default is RDP ("reliable datagram protocol").
You should nearly always use the default.
Internet protocols are generally specified by a name instead of a
number. The network protocols that a host knows about are stored in a
database. This is usually either derived from the file
`/etc/protocols', or it may be an equivalent provided by a name
server. You look up the protocol number associated with a named
protocol in the database using the getprotobyname function.
Here are detailed descriptions of the utilities for accessing the
protocols database. These are declared in `netdb.h'.
Data Type:struct protoent
This data type is used to represent entries in the network protocols
database. It has the following members:
char *p_name
This is the official name of the protocol.
char **p_aliases
These are alternate names for the protocol, specified as an array of
strings. The last element of the array is a null pointer.
int p_proto
This is the protocol number (in host byte order); use this member as the
protocol argument to socket.
You can use getprotobyname and getprotobynumber to search
the protocols database for a specific protocol. The information is
returned in a statically-allocated structure; you must copy the
information if you need to save it across calls.
The getprotobynumber function returns information about the
network protocol with number protocol. If there is no such
protocol, it returns a null pointer.
You can also scan the whole protocols database one protocol at a time by
using setprotoent, getprotoent and endprotoent.
Be careful when using these functions because they are not reentrant.
Function: void setprotoent(int stayopen)
This function opens the protocols database to begin scanning it.
If the stayopen argument is nonzero, this sets a flag so that
subsequent calls to getprotobyname or getprotobynumber will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
Function: struct protoent * getprotoent(void)
This function returns the next entry in the protocols database. It
returns a null pointer if there are no more entries.
Here is an example showing how to create and name a socket in the
Internet namespace. The newly created socket exists on the machine that
the program is running on. Rather than finding and using the machine's
Internet address, this example specifies INADDR_ANY as the host
address; the system replaces that with the machine's actual address.
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
int
make_socket (uint16_t port)
{
int sock;
struct sockaddr_in name;
/* Create the socket. */
sock = socket (PF_INET, SOCK_STREAM, 0);
if (sock < 0)
{
perror ("socket");
exit (EXIT_FAILURE);
}
/* Give the socket a name. */
name.sin_family = AF_INET;
name.sin_port = htons (port);
name.sin_addr.s_addr = htonl (INADDR_ANY);
if (bind (sock, (struct sockaddr *) &name, sizeof (name)) < 0)
{
perror ("bind");
exit (EXIT_FAILURE);
}
return sock;
}
Here is another example, showing how you can fill in a sockaddr_in
structure, given a host name string and a port number:
Certain other namespaces and associated protocol families are supported
but not documented yet because they are not often used. PF_NS
refers to the Xerox Network Software protocols. PF_ISO stands
for Open Systems Interconnect. PF_CCITT refers to protocols from
CCITT. `socket.h' defines these symbols and others naming protocols
not actually implemented.
PF_IMPLINK is used for communicating between hosts and Internet
Message Processors. For information on this and PF_ROUTE, an
occasionally-used local area routing protocol, see the GNU Hurd Manual
(to appear in the future).
The primitive for creating a socket is the socket function,
declared in `sys/socket.h'.
Function: int socket(int namespace, int style, int protocol)
This function creates a socket and specifies communication style
style, which should be one of the socket styles listed in
16.2 Communication Styles. The namespace argument specifies
the namespace; it must be PF_LOCAL (see section 16.5 The Local Namespace) or
PF_INET (see section 16.6 The Internet Namespace). protocol
designates the specific protocol (see section 16.1 Socket Concepts); zero is
usually right for protocol.
The return value from socket is the file descriptor for the new
socket, or -1 in case of error. The following errno error
conditions are defined for this function:
EPROTONOSUPPORT
The protocol or style is not supported by the
namespace specified.
EMFILE
The process already has too many file descriptors open.
ENFILE
The system already has too many file descriptors open.
EACCES
The process does not have the privilege to create a socket of the specified
style or protocol.
ENOBUFS
The system ran out of internal buffer space.
The file descriptor returned by the socket function supports both
read and write operations. However, like pipes, sockets do not support file
positioning operations.
When you have finished using a socket, you can simply close its
file descriptor with close; see 13.1 Opening and Closing Files.
If there is still data waiting to be transmitted over the connection,
normally close tries to complete this transmission. You
can control this behavior using the SO_LINGER socket option to
specify a timeout period; see 16.12 Socket Options.
You can also shut down only reception or transmission on a
connection by calling shutdown, which is declared in
`sys/socket.h'.
Function: int shutdown(int socket, int how)
The shutdown function shuts down the connection of socket
socket. The argument how specifies what action to
perform:
0
Stop receiving data for this socket. If further data arrives,
reject it.
1
Stop trying to transmit data from this socket. Discard any data
waiting to be sent. Stop looking for acknowledgement of data already
sent; don't retransmit it if it is lost.
2
Stop both reception and transmission.
The return value is 0 on success and -1 on failure. The
following errno error conditions are defined for this function:
A socket pair consists of a pair of connected (but unnamed)
sockets. It is very similar to a pipe and is used in much the same
way. Socket pairs are created with the socketpair function,
declared in `sys/socket.h'. A socket pair is much like a pipe; the
main difference is that the socket pair is bidirectional, whereas the
pipe has one input-only end and one output-only end (see section 15. Pipes and FIFOs).
Function: int socketpair(int namespace, int style, int protocol, int filedes[2])
This function creates a socket pair, returning the file descriptors in
filedes[0] and filedes[1]. The socket pair
is a full-duplex communications channel, so that both reading and writing
may be performed at either end.
The namespace, style and protocol arguments are
interpreted as for the socket function. style should be
one of the communication styles listed in 16.2 Communication Styles.
The namespace argument specifies the namespace, which must be
AF_LOCAL (see section 16.5 The Local Namespace); protocol specifies the
communications protocol, but zero is the only meaningful value.
If style specifies a connectionless communication style, then
the two sockets you get are not connected, strictly speaking,
but each of them knows the other as the default destination address,
so they can send packets to each other.
The socketpair function returns 0 on success and -1
on failure. The following errno error conditions are defined
for this function:
EMFILE
The process has too many file descriptors open.
EAFNOSUPPORT
The specified namespace is not supported.
EPROTONOSUPPORT
The specified protocol is not supported.
EOPNOTSUPP
The specified protocol does not support the creation of socket pairs.
The most common communication styles involve making a connection to a
particular other socket, and then exchanging data with that socket
over and over. Making a connection is asymmetric; one side (the
client) acts to request a connection, while the other side (the
server) makes a socket and waits for the connection request.
In making a connection, the client makes a connection while the server
waits for and accepts the connection. Here we discuss what the client
program must do with the connect function, which is declared in
`sys/socket.h'.
Function: int connect(int socket, struct sockaddr *addr, socklen_t length)
The connect function initiates a connection from the socket
with file descriptor socket to the socket whose address is
specified by the addr and length arguments. (This socket
is typically on another machine, and it must be already set up as a
server.) See section 16.3 Socket Addresses, for information about how these
arguments are interpreted.
Normally, connect waits until the server responds to the request
before it returns. You can set nonblocking mode on the socket
socket to make connect return immediately without waiting
for the response. See section 13.14 File Status Flags, for information about
nonblocking mode.
The normal return value from connect is 0. If an error
occurs, connect returns -1. The following errno
error conditions are defined for this function:
EBADF
The socket socket is not a valid file descriptor.
ENOTSOCK
File descriptor socket is not a socket.
EADDRNOTAVAIL
The specified address is not available on the remote machine.
EAFNOSUPPORT
The namespace of the addr is not supported by this socket.
EISCONN
The socket socket is already connected.
ETIMEDOUT
The attempt to establish the connection timed out.
ECONNREFUSED
The server has actively refused to establish the connection.
ENETUNREACH
The network of the given addr isn't reachable from this host.
EADDRINUSE
The socket address of the given addr is already in use.
EINPROGRESS
The socket socket is non-blocking and the connection could not be
established immediately. You can determine when the connection is
completely established with select; see section 13.8 Waiting for Input or Output.
Another connect call on the same socket, before the connection is
completely established, will fail with EALREADY.
EALREADY
The socket socket is non-blocking and already has a pending
connection in progress (see EINPROGRESS above).
This function is defined as a cancellation point in multi-threaded
programs, so one has to be prepared for this and make sure that
allocated resources (like memory, files descriptors, semaphores or
whatever) are freed even if the thread is canceled.
Now let us consider what the server process must do to accept
connections on a socket. First it must use the listen function
to enable connection requests on the socket, and then accept each
incoming connection with a call to accept (see section 16.9.3 Accepting Connections). Once connection requests are enabled on a server socket,
the select function reports when the socket has a connection
ready to be accepted (see section 13.8 Waiting for Input or Output).
The listen function is not allowed for sockets using
connectionless communication styles.
You can write a network server that does not even start running until a
connection to it is requested. See section 16.11.1 inetd Servers.
In the Internet namespace, there are no special protection mechanisms
for controlling access to a port; any process on any machine
can make a connection to your server. If you want to restrict access to
your server, make it examine the addresses associated with connection
requests or implement some other handshaking or identification
protocol.
In the local namespace, the ordinary file protection bits control who has
access to connect to the socket.
Function: int listen(int socket, unsigned int n)
The listen function enables the socket socket to accept
connections, thus making it a server socket.
The argument n specifies the length of the queue for pending
connections. When the queue fills, new clients attempting to connect
fail with ECONNREFUSED until the server calls accept to
accept a connection from the queue.
The listen function returns 0 on success and -1
on failure. The following errno error conditions are defined
for this function:
EBADF
The argument socket is not a valid file descriptor.
ENOTSOCK
The argument socket is not a socket.
EOPNOTSUPP
The socket socket does not support this operation.
When a server receives a connection request, it can complete the
connection by accepting the request. Use the function accept
to do this.
A socket that has been established as a server can accept connection
requests from multiple clients. The server's original socket
does not become part of the connection; instead, accept
makes a new socket which participates in the connection.
accept returns the descriptor for this socket. The server's
original socket remains available for listening for further connection
requests.
The number of pending connection requests on a server socket is finite.
If connection requests arrive from clients faster than the server can
act upon them, the queue can fill up and additional requests are refused
with an ECONNREFUSED error. You can specify the maximum length of
this queue as an argument to the listen function, although the
system may also impose its own internal limit on the length of this
queue.
Function: int accept(int socket, struct sockaddr *addr, socklen_t *length_ptr)
This function is used to accept a connection request on the server
socket socket.
The accept function waits if there are no connections pending,
unless the socket socket has nonblocking mode set. (You can use
select to wait for a pending connection, with a nonblocking
socket.) See section 13.14 File Status Flags, for information about nonblocking
mode.
The addr and length-ptr arguments are used to return
information about the name of the client socket that initiated the
connection. See section 16.3 Socket Addresses, for information about the format
of the information.
Accepting a connection does not make socket part of the
connection. Instead, it creates a new socket which becomes
connected. The normal return value of accept is the file
descriptor for the new socket.
After accept, the original socket socket remains open and
unconnected, and continues listening until you close it. You can
accept further connections with socket by calling accept
again.
If an error occurs, accept returns -1. The following
errno error conditions are defined for this function:
EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket argument is not a socket.
EOPNOTSUPP
The descriptor socket does not support this operation.
EWOULDBLOCK
socket has nonblocking mode set, and there are no pending
connections immediately available.
This function is defined as a cancellation point in multi-threaded
programs, so one has to be prepared for this and make sure that
allocated resources (like memory, files descriptors, semaphores or
whatever) are freed even if the thread is canceled.
The accept function is not allowed for sockets using
connectionless communication styles.
Function: int getpeername(int socket, struct sockaddr *addr, socklen_t *length-ptr)
The getpeername function returns the address of the socket that
socket is connected to; it stores the address in the memory space
specified by addr and length-ptr. It stores the length of
the address in *length-ptr.
See section 16.3 Socket Addresses, for information about the format of the
address. In some operating systems, getpeername works only for
sockets in the Internet domain.
The return value is 0 on success and -1 on error. The
following errno error conditions are defined for this function:
EBADF
The argument socket is not a valid file descriptor.
Once a socket has been connected to a peer, you can use the ordinary
read and write operations (see section 13.2 Input and Output Primitives) to
transfer data. A socket is a two-way communications channel, so read
and write operations can be performed at either end.
There are also some I/O modes that are specific to socket operations.
In order to specify these modes, you must use the recv and
send functions instead of the more generic read and
write functions. The recv and send functions take
an additional argument which you can use to specify various flags to
control special I/O modes. For example, you can specify the
MSG_OOB flag to read or write out-of-band data, the
MSG_PEEK flag to peek at input, or the MSG_DONTROUTE flag
to control inclusion of routing information on output.
The send function is declared in the header file
`sys/socket.h'. If your flags argument is zero, you can just
as well use write instead of send; see 13.2 Input and Output Primitives. If the socket was connected but the connection has broken,
you get a SIGPIPE signal for any use of send or
write (see section 24.2.7 Miscellaneous Signals).
Function: int send(int socket, void *buffer, size_t size, int flags)
The send function is like write, but with the additional
flags flags. The possible values of flags are described
in 16.9.5.3 Socket Data Options.
This function returns the number of bytes transmitted, or -1 on
failure. If the socket is nonblocking, then send (like
write) can return after sending just part of the data.
See section 13.14 File Status Flags, for information about nonblocking mode.
Note, however, that a successful return value merely indicates that
the message has been sent without error, not necessarily that it has
been received without error.
The following errno error conditions are defined for this function:
EBADF
The socket argument is not a valid file descriptor.
The socket type requires that the message be sent atomically, but the
message is too large for this to be possible.
EWOULDBLOCK
Nonblocking mode has been set on the socket, and the write operation
would block. (Normally send blocks until the operation can be
completed.)
ENOBUFS
There is not enough internal buffer space available.
ENOTCONN
You never connected this socket.
EPIPE
This socket was connected but the connection is now broken. In this
case, send generates a SIGPIPE signal first; if that
signal is ignored or blocked, or if its handler returns, then
send fails with EPIPE.
This function is defined as a cancellation point in multi-threaded
programs, so one has to be prepared for this and make sure that
allocated resources (like memory, files descriptors, semaphores or
whatever) are freed even if the thread is canceled.
The recv function is declared in the header file
`sys/socket.h'. If your flags argument is zero, you can
just as well use read instead of recv; see 13.2 Input and Output Primitives.
Function: int recv(int socket, void *buffer, size_t size, int flags)
The recv function is like read, but with the additional
flags flags. The possible values of flags are described
in 16.9.5.3 Socket Data Options.
If nonblocking mode is set for socket, and no data are available to
be read, recv fails immediately rather than waiting. See section 13.14 File Status Flags, for information about nonblocking mode.
This function returns the number of bytes received, or -1 on failure.
The following errno error conditions are defined for this function:
EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
EWOULDBLOCK
Nonblocking mode has been set on the socket, and the read operation
would block. (Normally, recv blocks until there is input
available to be read.)
This function is defined as a cancellation point in multi-threaded
programs, so one has to be prepared for this and make sure that
allocated resources (like memory, files descriptors, semaphores or
whatever) are freed even if the thread is canceled.
The flags argument to send and recv is a bit
mask. You can bitwise-OR the values of the following macros together
to obtain a value for this argument. All are defined in the header
file `sys/socket.h'.
Look at the data but don't remove it from the input queue. This is
only meaningful with input functions such as recv, not with
send.
Macro: int MSG_DONTROUTE
Don't include routing information in the message. This is only
meaningful with output operations, and is usually only of interest for
diagnostic or routing programs. We don't try to explain it here.
Here is an example client program that makes a connection for a byte
stream socket in the Internet namespace. It doesn't do anything
particularly interesting once it has connected to the server; it just
sends a text string to the server and exits.
The server end is much more complicated. Since we want to allow
multiple clients to be connected to the server at the same time, it
would be incorrect to wait for input from a single client by simply
calling read or recv. Instead, the right thing to do is
to use select (see section 13.8 Waiting for Input or Output) to wait for input on
all of the open sockets. This also allows the server to deal with
additional connection requests.
This particular server doesn't do anything interesting once it has
gotten a message from a client. It does close the socket for that
client when it detects an end-of-file condition (resulting from the
client shutting down its end of the connection).
Streams with connections permit out-of-band data that is
delivered with higher priority than ordinary data. Typically the
reason for sending out-of-band data is to send notice of an
exceptional condition. To send out-of-band data use
send, specifying the flag MSG_OOB (see section 16.9.5.1 Sending Data).
Out-of-band data are received with higher priority because the
receiving process need not read it in sequence; to read the next
available out-of-band data, use recv with the MSG_OOB
flag (see section 16.9.5.2 Receiving Data). Ordinary read operations do not read
out-of-band data; they read only ordinary data.
When a socket finds that out-of-band data are on their way, it sends a
SIGURG signal to the owner process or process group of the
socket. You can specify the owner using the F_SETOWN command
to the fcntl function; see 13.16 Interrupt-Driven Input. You must
also establish a handler for this signal, as described in 24. Signal Handling, in order to take appropriate action such as reading the
out-of-band data.
Alternatively, you can test for pending out-of-band data, or wait
until there is out-of-band data, using the select function; it
can wait for an exceptional condition on the socket. See section 13.8 Waiting for Input or Output, for more information about select.
Notification of out-of-band data (whether with SIGURG or with
select) indicates that out-of-band data are on the way; the data
may not actually arrive until later. If you try to read the
out-of-band data before it arrives, recv fails with an
EWOULDBLOCK error.
Sending out-of-band data automatically places a "mark" in the stream
of ordinary data, showing where in the sequence the out-of-band data
"would have been". This is useful when the meaning of out-of-band
data is "cancel everything sent so far". Here is how you can test,
in the receiving process, whether any ordinary data was sent before
the mark:
success = ioctl (socket, SIOCATMARK, &atmark);
The integer variable atmark is set to a nonzero value if
the socket's read pointer has reached the "mark".
Here's a function to discard any ordinary data preceding the
out-of-band mark:
int
discard_until_mark (int socket)
{
while (1)
{
/* This is not an arbitrary limit; any size will do. */
char buffer[1024];
int atmark, success;
/* If we have reached the mark, return. */
success = ioctl (socket, SIOCATMARK, &atmark);
if (success < 0)
perror ("ioctl");
if (result)
return;
/* Otherwise, read a bunch of ordinary data and discard it.
This is guaranteed not to read past the mark
if it starts before the mark. */
success = read (socket, buffer, sizeof buffer);
if (success < 0)
perror ("read");
}
}
If you don't want to discard the ordinary data preceding the mark, you
may need to read some of it anyway, to make room in internal system
buffers for the out-of-band data. If you try to read out-of-band data
and get an EWOULDBLOCK error, try reading some ordinary data
(saving it so that you can use it when you want it) and see if that
makes room. Here is an example:
struct buffer
{
char *buf;
int size;
struct buffer *next;
};
/* Read the out-of-band data from SOCKET and return it
as a `struct buffer', which records the address of the data
and its size.
It may be necessary to read some ordinary data
in order to make room for the out-of-band data.
If so, the ordinary data are saved as a chain of buffers
found in the `next' field of the value. */
struct buffer *
read_oob (int socket)
{
struct buffer *tail = 0;
struct buffer *list = 0;
while (1)
{
/* This is an arbitrary limit.
Does anyone know how to do this without a limit? */
#define BUF_SZ 1024
char *buf = (char *) xmalloc (BUF_SZ);
int success;
int atmark;
/* Try again to read the out-of-band data. */
success = recv (socket, buf, BUF_SZ, MSG_OOB);
if (success >= 0)
{
/* We got it, so return it. */
struct buffer *link
= (struct buffer *) xmalloc (sizeof (struct buffer));
link->buf = buf;
link->size = success;
link->next = list;
return link;
}
/* If we fail, see if we are at the mark. */
success = ioctl (socket, SIOCATMARK, &atmark);
if (success < 0)
perror ("ioctl");
if (atmark)
{
/* At the mark; skipping past more ordinary data cannot help.
So just wait a while. */
sleep (1);
continue;
}
/* Otherwise, read a bunch of ordinary data and save it.
This is guaranteed not to read past the mark
if it starts before the mark. */
success = read (socket, buf, BUF_SZ);
if (success < 0)
perror ("read");
/* Save this data in the buffer list. */
{
struct buffer *link
= (struct buffer *) xmalloc (sizeof (struct buffer));
link->buf = buf;
link->size = success;
/* Add the new link to the end of the list. */
if (tail)
tail->next = link;
else
list = link;
tail = link;
}
}
}
This section describes how to use communication styles that don't use
connections (styles SOCK_DGRAM and SOCK_RDM). Using
these styles, you group data into packets and each packet is an
independent communication. You specify the destination for each
packet individually.
Datagram packets are like letters: you send each one independently
with its own destination address, and they may arrive in the wrong
order or not at all.
The listen and accept functions are not allowed for
sockets using connectionless communication styles.
The normal way of sending data on a datagram socket is by using the
sendto function, declared in `sys/socket.h'.
You can call connect on a datagram socket, but this only
specifies a default destination for further data transmission on the
socket. When a socket has a default destination you can use
send (see section 16.9.5.1 Sending Data) or even write (see section 13.2 Input and Output Primitives) to send a packet there. You can cancel the default
destination by calling connect using an address format of
AF_UNSPEC in the addr argument. See section 16.9.1 Making a Connection, for
more information about the connect function.
Function: int sendto(int socket, void *buffer. size_t size, int flags, struct sockaddr *addr, socklen_t length)
The sendto function transmits the data in the buffer
through the socket socket to the destination address specified
by the addr and length arguments. The size argument
specifies the number of bytes to be transmitted.
The return value and error conditions are also the same as for
send, but you cannot rely on the system to detect errors and
report them; the most common error is that the packet is lost or there
is no-one at the specified address to receive it, and the operating
system on your machine usually does not know this.
It is also possible for one call to sendto to report an error
owing to a problem related to a previous call.
This function is defined as a cancellation point in multi-threaded
programs, so one has to be prepared for this and make sure that
allocated resources (like memory, files descriptors, semaphores or
whatever) are freed even if the thread is canceled.
The recvfrom function reads a packet from a datagram socket and
also tells you where it was sent from. This function is declared in
`sys/socket.h'.
Function: int recvfrom(int socket, void *buffer, size_t size, int flags, struct sockaddr *addr, socklen_t *length-ptr)
The recvfrom function reads one packet from the socket
socket into the buffer buffer. The size argument
specifies the maximum number of bytes to be read.
If the packet is longer than size bytes, then you get the first
size bytes of the packet and the rest of the packet is lost.
There's no way to read the rest of the packet. Thus, when you use a
packet protocol, you must always know how long a packet to expect.
The addr and length-ptr arguments are used to return the
address where the packet came from. See section 16.3 Socket Addresses. For a
socket in the local domain the address information won't be meaningful,
since you can't read the address of such a socket (see section 16.5 The Local Namespace). You can specify a null pointer as the addr argument
if you are not interested in this information.
The flags are interpreted the same way as for recv
(see section 16.9.5.3 Socket Data Options). The return value and error conditions
are also the same as for recv.
This function is defined as a cancellation point in multi-threaded
programs, so one has to be prepared for this and make sure that
allocated resources (like memory, files descriptors, semaphores or
whatever) are freed even if the thread is canceled.
You can use plain recv (see section 16.9.5.2 Receiving Data) instead of
recvfrom if you don't need to find out who sent the packet
(either because you know where it should come from or because you
treat all possible senders alike). Even read can be used if
you don't want to specify flags (see section 13.2 Input and Output Primitives).
Here is a set of example programs that send messages over a datagram
stream in the local namespace. Both the client and server programs use
the make_named_socket function that was presented in 16.5.3 Example of Local-Namespace Sockets, to create and name their sockets.
First, here is the server program. It sits in a loop waiting for
messages to arrive, bouncing each message back to the sender.
Obviously this isn't a particularly useful program, but it does show
the general ideas involved.
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>
#define SERVER "/tmp/serversocket"
#define MAXMSG 512
int
main (void)
{
int sock;
char message[MAXMSG];
struct sockaddr_un name;
size_t size;
int nbytes;
/* Remove the filename first, it's ok if the call fails */
unlink (SERVER);
/* Make the socket, then loop endlessly. */
sock = make_named_socket (SERVER);
while (1)
{
/* Wait for a datagram. */
size = sizeof (name);
nbytes = recvfrom (sock, message, MAXMSG, 0,
(struct sockaddr *) & name, &size);
if (nbytes < 0)
{
perror ("recfrom (server)");
exit (EXIT_FAILURE);
}
/* Give a diagnostic message. */
fprintf (stderr, "Server: got message: %s\n", message);
/* Bounce the message back to the sender. */
nbytes = sendto (sock, message, nbytes, 0,
(struct sockaddr *) & name, size);
if (nbytes < 0)
{
perror ("sendto (server)");
exit (EXIT_FAILURE);
}
}
}
Here is the client program corresponding to the server above.
It sends a datagram to the server and then waits for a reply. Notice
that the socket for the client (as well as for the server) in this
example has to be given a name. This is so that the server can direct
a message back to the client. Since the socket has no associated
connection state, the only way the server can do this is by
referencing the name of the client.
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>
#define SERVER "/tmp/serversocket"
#define CLIENT "/tmp/mysocket"
#define MAXMSG 512
#define MESSAGE "Yow!!! Are we having fun yet?!?"
int
main (void)
{
extern int make_named_socket (const char *name);
int sock;
char message[MAXMSG];
struct sockaddr_un name;
size_t size;
int nbytes;
/* Make the socket. */
sock = make_named_socket (CLIENT);
/* Initialize the server socket address. */
name.sun_family = AF_LOCAL;
strcpy (name.sun_path, SERVER);
size = strlen (name.sun_path) + sizeof (name.sun_family);
/* Send the datagram. */
nbytes = sendto (sock, MESSAGE, strlen (MESSAGE) + 1, 0,
(struct sockaddr *) & name, size);
if (nbytes < 0)
{
perror ("sendto (client)");
exit (EXIT_FAILURE);
}
/* Wait for a reply. */
nbytes = recvfrom (sock, message, MAXMSG, 0, NULL, 0);
if (nbytes < 0)
{
perror ("recfrom (client)");
exit (EXIT_FAILURE);
}
/* Print a diagnostic message. */
fprintf (stderr, "Client: got message: %s\n", message);
/* Clean up. */
remove (CLIENT);
close (sock);
}
Keep in mind that datagram socket communications are unreliable. In
this example, the client program waits indefinitely if the message
never reaches the server or if the server's response never comes
back. It's up to the user running the program to kill and restart
it if desired. A more automatic solution could be to use
select (see section 13.8 Waiting for Input or Output) to establish a timeout period
for the reply, and in case of timeout either re-send the message or
shut down the socket and exit.
We've explained above how to write a server program that does its own
listening. Such a server must already be running in order for anyone
to connect to it.
Another way to provide a service on an Internet port is to let the daemon
program inetd do the listening. inetd is a program that
runs all the time and waits (using select) for messages on a
specified set of ports. When it receives a message, it accepts the
connection (if the socket style calls for connections) and then forks a
child process to run the corresponding server program. You specify the
ports and their programs in the file `/etc/inetd.conf'.
Writing a server program to be run by inetd is very simple. Each time
someone requests a connection to the appropriate port, a new server
process starts. The connection already exists at this time; the
socket is available as the standard input descriptor and as the
standard output descriptor (descriptors 0 and 1) in the server
process. Thus the server program can begin reading and writing data
right away. Often the program needs only the ordinary I/O facilities;
in fact, a general-purpose filter program that knows nothing about
sockets can work as a byte stream server run by inetd.
You can also use inetd for servers that use connectionless
communication styles. For these servers, inetd does not try to accept
a connection since no connection is possible. It just starts the
server program, which can read the incoming datagram packet from
descriptor 0. The server program can handle one request and then
exit, or you can choose to write it to keep reading more requests
until no more arrive, and then exit. You must specify which of these
two techniques the server uses when you configure inetd.
The file `/etc/inetd.conf' tells inetd which ports to listen to
and what server programs to run for them. Normally each entry in the
file is one line, but you can split it onto multiple lines provided
all but the first line of the entry start with whitespace. Lines that
start with `#' are comments.
Here are two standard entries in `/etc/inetd.conf':
The service field says which service this program provides. It
should be the name of a service defined in `/etc/services'.
inetd uses service to decide which port to listen on for
this entry.
The fields style and protocol specify the communication
style and the protocol to use for the listening socket. The style
should be the name of a communication style, converted to lower case
and with `SOCK_' deleted--for example, `stream' or
`dgram'. protocol should be one of the protocols listed in
`/etc/protocols'. The typical protocol names are `tcp' for
byte stream connections and `udp' for unreliable datagrams.
The wait field should be either `wait' or `nowait'.
Use `wait' if style is a connectionless style and the
server, once started, handles multiple requests as they come in.
Use `nowait' if inetd should start a new process for each message
or request that comes in. If style uses connections, then
waitmust be `nowait'.
user is the user name that the server should run as. inetd runs
as root, so it can set the user ID of its children arbitrarily. It's
best to avoid using `root' for user if you can; but some
servers, such as Telnet and FTP, read a username and password
themselves. These servers need to be root initially so they can log
in as commanded by the data coming over the network.
program together with arguments specifies the command to
run to start the server. program should be an absolute file
name specifying the executable file to run. arguments consists
of any number of whitespace-separated words, which become the
command-line arguments of program. The first word in
arguments is argument zero, which should by convention be the
program name itself (sans directories).
If you edit `/etc/inetd.conf', you can tell inetd to reread the
file and obey its new contents by sending the inetd process the
SIGHUP signal. You'll have to use ps to determine the
process ID of the inetd process as it is not fixed.
This section describes how to read or set various options that modify
the behavior of sockets and their underlying communications protocols.
When you are manipulating a socket option, you must specify which
level the option pertains to. This describes whether the option
applies to the socket interface, or to a lower-level communications
protocol interface.
Here are the functions for examining and modifying socket options.
They are declared in `sys/socket.h'.
Function: int getsockopt(int socket, int level, int optname, void *optval, socklen_t *optlen-ptr)
The getsockopt function gets information about the value of
option optname at level level for socket socket.
The option value is stored in a buffer that optval points to.
Before the call, you should supply in *optlen-ptr the
size of this buffer; on return, it contains the number of bytes of
information actually stored in the buffer.
Most options interpret the optval buffer as a single int
value.
The actual return value of getsockopt is 0 on success
and -1 on failure. The following errno error conditions
are defined:
EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
ENOPROTOOPT
The optname doesn't make sense for the given level.
Function: int setsockopt(int socket, int level, int optname, void *optval, socklen_t optlen)
This function is used to set the socket option optname at level
level for socket socket. The value of the option is passed
in the buffer optval of size optlen.
The return value and error codes for setsockopt are the same as
for getsockopt.
Use this constant as the level argument to getsockopt or
setsockopt to manipulate the socket-level options described in
this section.
Here is a table of socket-level option names; all are defined in the
header file `sys/socket.h'.
SO_DEBUG
This option toggles recording of debugging information in the underlying
protocol modules. The value has type int; a nonzero value means
"yes".
SO_REUSEADDR
This option controls whether bind (see section 16.3.2 Setting the Address of a Socket)
should permit reuse of local addresses for this socket. If you enable
this option, you can actually have two sockets with the same Internet
port number; but the system won't allow you to use the two
identically-named sockets in a way that would confuse the Internet. The
reason for this option is that some higher-level Internet protocols,
including FTP, require you to keep reusing the same port number.
The value has type int; a nonzero value means "yes".
SO_KEEPALIVE
This option controls whether the underlying protocol should
periodically transmit messages on a connected socket. If the peer
fails to respond to these messages, the connection is considered
broken. The value has type int; a nonzero value means
"yes".
SO_DONTROUTE
This option controls whether outgoing messages bypass the normal
message routing facilities. If set, messages are sent directly to the
network interface instead. The value has type int; a nonzero
value means "yes".
SO_LINGER
This option specifies what should happen when the socket of a type
that promises reliable delivery still has untransmitted messages when
it is closed; see 16.8.2 Closing a Socket. The value has type
struct linger.
Data Type:struct linger
This structure type has the following members:
int l_onoff
This field is interpreted as a boolean. If nonzero, close
blocks until the data are transmitted or the timeout period has expired.
int l_linger
This specifies the timeout period, in seconds.
SO_BROADCAST
This option controls whether datagrams may be broadcast from the socket.
The value has type int; a nonzero value means "yes".
SO_OOBINLINE
If this option is set, out-of-band data received on the socket is
placed in the normal input queue. This permits it to be read using
read or recv without specifying the MSG_OOB
flag. See section 16.9.8 Out-of-Band Data. The value has type int; a
nonzero value means "yes".
SO_SNDBUF
This option gets or sets the size of the output buffer. The value is a
size_t, which is the size in bytes.
SO_RCVBUF
This option gets or sets the size of the input buffer. The value is a
size_t, which is the size in bytes.
SO_STYLE
SO_TYPE
This option can be used with getsockopt only. It is used to
get the socket's communication style. SO_TYPE is the
historical name, and SO_STYLE is the preferred name in GNU.
The value has type int and its value designates a communication
style; see 16.2 Communication Styles.
SO_ERROR
This option can be used with getsockopt only. It is used to reset
the error status of the socket. The value is an int, which represents
the previous error status.
Many systems come with a database that records a list of networks known
to the system developer. This is usually kept either in the file
`/etc/networks' or in an equivalent from a name server. This data
base is useful for routing programs such as route, but it is not
useful for programs that simply communicate over the network. We
provide functions to access this database, which are declared in
`netdb.h'.
Data Type:struct netent
This data type is used to represent information about entries in the
networks database. It has the following members:
char *n_name
This is the "official" name of the network.
char **n_aliases
These are alternative names for the network, represented as a vector
of strings. A null pointer terminates the array.
int n_addrtype
This is the type of the network number; this is always equal to
AF_INET for Internet networks.
Use the getnetbyname or getnetbyaddr functions to search
the networks database for information about a specific network. The
information is returned in a statically-allocated structure; you must
copy the information if you need to save it.
The getnetbyname function returns information about the network
named name. It returns a null pointer if there is no such
network.
Function: struct netent * getnetbyaddr(unsigned long int net, int type)
The getnetbyaddr function returns information about the network
of type type with number net. You should specify a value of
AF_INET for the type argument for Internet networks.
getnetbyaddr returns a null pointer if there is no such
network.
You can also scan the networks database using setnetent,
getnetent and endnetent. Be careful when using these
functions because they are not reentrant.
Function: void setnetent(int stayopen)
This function opens and rewinds the networks database.
If the stayopen argument is nonzero, this sets a flag so that
subsequent calls to getnetbyname or getnetbyaddr will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
Function: struct netent * getnetent(void)
This function returns the next entry in the networks database. It
returns a null pointer if there are no more entries.