#include <unistd.h>#include <fcntl.h>int fcntl(int fd, int cmd);int fcntl(int fd, int cmd, long arg);int fcntl(int fd, int cmd, struct flock *lock);
performs one of various miscellaneous operations on
The operation in question is determined by
Find the lowest numbered available file descriptor
greater than or equal to
and make it be a copy of
This is different form
which uses exactly the descriptor specified.
The old and new descriptors may be used interchangeably. They share locks,
file position pointers and flags; for example, if the file position is
modified by using
on one of the descriptors, the position is also changed for the other.
The two descriptors do not share the close-on-exec flag, however.
The close-on-exec flag of the copy is off, meaning that it will
not be closed on exec.
On success, the new descriptor is returned.
Read the close-on-exec flag. If the
bit is 0, the file will remain open across
otherwise it will be closed.
Set the close-on-exec flag to the value specified by the
Read the descriptor's flags (all flags (as set by
Set the descriptor's flags to the value specified by
O_APPEND, O_NONBLOCK and O_ASYNC
may be set; the other flags are unaffected.
The flags are shared between copies (made with
etc.) of the same file descriptor.
The flags and their semantics are described in
F_GETLK, F_SETLK and F_SETLKW
are used to manage discretionary file locks.
The third argument
is a pointer to a struct flock
(that may be overwritten by this call).
Return the flock structure that prevents us from obtaining
the lock, or set the
field of the lock to
if there is no obstruction.
The lock is set (when
or cleared (when it is
If the lock is held by someone
else, this call returns -1 and sets
but instead of returning an error we wait for the lock to be released.
If a signal that is to be caught is received while
is waiting, it is interrupted and (after the signal handler has returned)
returns immediately (with return value -1 and
F_GETOWN, F_SETOWN, F_GETSIG and F_SETSIG
are used to manage I/O availability signals:
Get the process ID or process group currently receiving SIGIO
and SIGURG signals for events on file descriptor
Process groups are returned as negative values.
Set the process ID or process group that will receive SIGIO
and SIGURG signals for events on file descriptor
Process groups are specified using negative values.
can be used to specify a different signal instead of SIGIO).
If you set the
status flag on a file descriptor (either by providing this flag with the
call, or by using the
a SIGIO signal is sent whenever input or output becomes possible
on that file descriptor.
The process or process group to receive the signal can be selected by
command to the
function. If the file descriptor is a socket, this also selects
the recipient of SIGURG signals that are delivered when out-of-band
data arrives on that socket. (SIGURG is sent in any situation where
would report the socket as having an "exceptional condition".)
If the file descriptor corresponds to a terminal device, then SIGIO
signals are sent to the foreground process group of the terminal.
Get the signal sent when input or output becomes possible. A value of
zero means SIGIO is sent. Any other value (including SIGIO) is the
signal sent instead, and in this case additional info is available to
the signal handler if installed with SA_SIGINFO.
Sets the signal sent when input or output becomes possible. A value of
zero means to send the default SIGIO signal. Any other value (including
SIGIO) is the signal to send instead, and in this case additional info
is available to the signal handler if installed with SA_SIGINFO.
By using F_SETSIG with a non-zero value, and setting SA_SIGINFO for the
signal handler (see
extra information about I/O events is passed to
the handler in a
field indicates the source is SI_SIGIO, the
field gives the file descriptor associated with the event. Otherwise,
there is no indication which file descriptors are pending, and you
should use the usual mechanisms
set etc.) to determine which file descriptors are available for I/O.
By selecting a POSIX.1b real time signal (value >= SIGRTMIN), multiple
I/O events may be queued using the same signal numbers. (Queuing is
dependent on available memory). Extra information is available
if SA_SIGINFO is set for the signal handler, as above.
Using these mechanisms, a program can implement fully asynchronous I/O
most of the time.
The use of
is specific to BSD and Linux.
are Linux-specific. POSIX has asynchronous I/O and the
structure to achieve similar things; these are also available
in Linux as part of the GNU C Library (Glibc).
For a successful call, the return value depends on the operation:
The new descriptor.
Value of flag.
Value of flags.
Value of descriptor owner.
Value of signal sent when read or write becomes possible, or zero
for traditional SIGIO behaviour.
All other commands
On error, -1 is returned, and
is set appropriately.
Operation is prohibited by locks held by other processes.
Operation is prohibited because the file has been memory-mapped by
is not an open file descriptor or command was
file descriptor open mode doesn't match with type of lock requested (eg:
file descriptor was read only and the lock requested was
It was detected that the specified
command would cause a deadlock.
is outside your accessible address space.
the command was interrupted by a signal.
F_GETLK and F_SETLK,
the command was interrupted by a signal before the lock was checked or
acquired. Most likely when locking a remote file (e.g. locking over
NFS), but can sometimes happen locally.
is negative or is greater than the maximum allowable value. For
is not an allowable signal number.
the process already has the maximum number of file descriptors open.
Too many segment locks open, lock table is full, or a remote locking
protocol failed (e.g. locking over NFS).
Attempted to clear the
flag on a file that has the append-only attribute set.
The errors returned by
are different from those returned by
SVr4, SVID, POSIX, X/OPEN, BSD 4.3. Only the operations F_DUPFD,
F_GETFD, F_SETFD, F_GETFL, F_SETFL, F_GETLK, F_SETLK and F_SETLKW are
specified in POSIX.1. F_GETOWN and F_SETOWN are BSDisms not supported
in SVr4; F_GETSIG and F_SETSIG are specific to Linux. The flags
legal for F_GETFL/F_SETFL are those supported by
and vary between these systems; O_APPEND, O_NONBLOCK, O_RDONLY,
and O_RDWR are specified in POSIX.1. SVr4 supports several other
options and flags not documented here.
SVr4 documents additional EIO, ENOLINK and EOVERFLOW error conditions.