NAME
fork —
create a new process
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
pid_t
fork(
void);
DESCRIPTION
fork() causes creation of a new process. The new process
(child process) is an exact copy of the calling process (parent process)
except for the following:
- The child process has a unique process ID.
- The child process has a different parent process ID
(i.e., the process ID of the parent process).
- The child process has its own copy of the parent's
descriptors. These descriptors reference the same underlying objects, so
that, for instance, file pointers in file objects are shared between the
child and the parent, so that an
lseek(2) on a descriptor in
the child process can affect a subsequent
read(2) or
write(2) by the parent. This
descriptor copying is also used by the shell to establish standard input
and output for newly created processes as well as to set up pipes.
- The child process' resource utilizations are set to 0;
see setrlimit(2).
In general, the child process should call
_exit(2) rather than
exit(3). Otherwise, any stdio
buffers that exist both in the parent and child will be flushed twice.
Similarly,
_exit(2) should be
used to prevent
atexit(3)
routines from being called twice (once in the parent and once in the child).
In case of a threaded program, only the thread calling
fork()
is still running in the child processes.
Child processes of a threaded program have additional restrictions, a child must
only call functions that are async-signal-safe. Very few functions are
asynchronously safe (the list may be found in
sigaction(2)) and
applications should make sure they call
exec(3) as soon as possible.
RETURN VALUES
Upon successful completion,
fork() returns a value of 0 to the
child process and returns the process ID of the child process to the parent
process. Otherwise, a value of -1 is returned to the parent process, no child
process is created, and the global variable
errno is set
to indicate the error.
ERRORS
fork() will fail and no child process will be created if:
-
-
- [
EAGAIN
]
- The system-imposed limit on the total number of processes
under execution would be exceeded. This limit is configuration-dependent;
or the limit
RLIMIT_NPROC
on the total number of
processes under execution by this user id would be exceeded.
-
-
- [
ENOMEM
]
- There is insufficient swap space for the new process.
SEE ALSO
execve(2),
setrlimit(2),
vfork(2),
wait(2),
pthread_atfork(3)
STANDARDS
The
fork() function conforms to
IEEE Std
1003.1-1990 (“POSIX.1”).
HISTORY
A
fork() system call appeared in
Version 6 AT&T UNIX.