creates a new process, just like
is a library function layered on top
of the underlying
system call, hereinafter referred to as
A description of
is given towards the end of this page.
allow the child process to share parts of its execution context with
the calling process, such as the memory space, the table of file
descriptors, and the table of signal handlers. (Note that on this manual
page, "calling process" normally corresponds to "parent process". But see
the description of
The main use of
is to implement threads: multiple threads of control in a program that
run concurrently in a shared memory space.
When the child process is created with
it executes the function
(This differs from
where execution continues in the child from the point
argument is a pointer to a function that is called by the child
process at the beginning of its execution.
argument is passed to the
function application returns, the child process terminates. The
integer returned by
is the exit code for the child process. The child process may also
terminate explicitely by calling
or after receiving a fatal signal.
argument specifies the location of the stack used by the child
process. Since the child and calling process may share memory,
it is not possible for the child process to execute in the
same stack as the calling process. The calling process must therefore
set up memory space for the child stack and pass a pointer to this
Stacks grow downwards on all processors that run Linux
(except the HP PA processors), so
usually points to the topmost address of the memory space set up for
the child stack.
The low byte of
contains the number of the signal sent to the parent when the child
dies. If this signal is specified as anything other than
then the parent process must specify the
options when waiting for the child with
If no signal is specified, then the parent process is not signaled
when the child terminates.
may also be bitwise-or'ed with one or several of the following
constants, in order to specify what is shared between the calling process
and the child process:
(Linux 2.4 onwards) If
is set, then the parent of the new child (as returned by
will be the same as that of the calling process.
is not set, then (as with
the child's parent is the calling process.
Note that it is the parent process, as returned by
which is signaled when the child terminates, so that
is set, then the parent of the calling process, rather than the
calling process itself, will be signaled.
is set, the caller and the child processes share the same file system
information. This includes the root of the file system, the current
working directory, and the umask. Any call to
performed by the callng process or the child process also takes effect in the
is not set, the child process works on a copy of the file system
information of the calling process at the time of the
performed later by one of the processes do not affect the other process.
is set, the calling process and the child processes share the same file
descriptor table. File descriptors always refer to the same files in
the calling process and in the child process. Any file descriptor created by
the calling process or by the child process is also valid in the other
process. Similarly, if one of the processes closes a file descriptor,
or changes its associated flags, the other process is also affected.
is not set, the child process inherits a copy of all file descriptors
opened in the calling process at the time of
Operations on file descriptors performed later by either the calling process or
the child process do not affect the other process.
is set, the calling process and the child processes share the same table of
signal handlers. If the calling process or child process calls
to change the behavior associated with a signal, the behavior is
changed in the other process as well. However, the calling process and child
processes still have distinct signal masks and sets of pending
signals. So, one of them may block or unblock some signals using
without affecting the other process.
is not set, the child process inherits a copy of the signal handlers
of the calling process at the time
is called. Calls to
performed later by one of the processes have no effect on the other
is specified, and the calling process is being traced, then trace the child also (see
is set, the execution of the calling process is suspended
until the child releases its virtual memory
resources via a call to
is not set then both the calling process and the child are schedulable
after the call, and an application should not rely on execution occurring
in any particular order.
is set, the calling process and the child processes run in the same memory
space. In particular, memory writes performed by the calling process
or by the child process are also visible in the other process.
Moreover, any memory mapping or unmapping performed with
by the child or calling process also affects the other process.
is not set, the child process runs in a separate copy of the memory
space of the calling process at the time of
Memory writes or file mappings/unmappings performed by one of the
processes do not affect the other, as with
is set, the child process is created with the same process ID as
the calling process.
is not set, the child process possesses a unique process ID, distinct
from that of the calling process.
This flag can only be specified by the system boot process (PID 0).
(Linux 2.4 onwards)
is set, the child is placed in the same thread group as the calling process.
is not set, then the child is placed in its own (new)
thread group, whose ID is the same as the process ID.
(Thread groups are feature added in Linux 2.4 to support the
POSIX threads notion of a set of threads sharing a single PID. In Linux
2.4, calls to
return the thread group ID of the caller.)
system call corresponds more closely to
in that execution in the child continues from the point of the
only requires the
arguments, which have the same meaning as for
(Note that the order of these arguments differs from
Another difference for
is that the
argument may be zero, in which case copy-on-write semantics ensure that the
child gets separate copies of stack pages when either process modifies
the stack. In this case, for correct operation, the
option should not be specified.
On success, the PID of the child process is returned in the caller's thread
of execution. On failure, a -1 will be returned in the caller's
context, no child process will be created, and
will be set appropriately.
Too many processes are already running.
Cannot allocate sufficient memory to allocate a task structure for the
child, or to copy those parts of the caller's context that need to be
when a zero value is specified for
was specified by a process with a non-zero PID.
As of version 2.1.97 of the kernel,
flag should not be used, since other parts of the kernel and most system
software still assume that process IDs are unique.
There is no entry for
in libc version 5. libc 6 (a.k.a. glibc 2) provides
as described in this manual page.
calls are Linux-specific and should not be used in programs
intended to be portable. For programming threaded applications
(multiple threads of control in the same memory space), it is better
to use a library implementing the POSIX 1003.1c thread API, such as
the LinuxThreads library (included in glibc2 (libc6) ). See
This manual page corresponds to kernels 2.0.x, 2.1.x, 2.2.x, 2.4.x,
and to glibc 2.0.x and 2.1.x.