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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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fork(2) System Calls Manual fork(2)
fork - create a child process
Standard C library (libc, -lc)
#include <unistd.h>
pid_t fork(void);
fork() creates a new process by duplicating the calling process.
The new process is referred to as the child process. The calling
process is referred to as the parent process.
The child process and the parent process run in separate memory
spaces. At the time of fork() both memory spaces have the same
content. Memory writes, file mappings (mmap(2)), and unmappings
(munmap(2)) performed by one of the processes do not affect the
other.
The child process is an exact duplicate of the parent process
except for the following points:
• The child has its own unique process ID, and this PID does not
match the ID of any existing process group (setpgid(2)) or
session.
• The child's parent process ID is the same as the parent's
process ID.
• The child does not inherit its parent's memory locks (mlock(2),
mlockall(2)).
• Process resource utilizations (getrusage(2)) and CPU time
counters (times(2)) are reset to zero in the child.
• The child's set of pending signals is initially empty
(sigpending(2)).
• The child does not inherit semaphore adjustments from its
parent (semop(2)).
• The child does not inherit process-associated record locks from
its parent (fcntl(2)). (On the other hand, it does inherit
fcntl(2) open file description locks and flock(2) locks from
its parent.)
• The child does not inherit timers from its parent
(setitimer(2), alarm(2), timer_create(2)).
• The child does not inherit outstanding asynchronous I/O
operations from its parent (aio_read(3), aio_write(3)), nor
does it inherit any asynchronous I/O contexts from its parent
(see io_setup(2)).
The process attributes in the preceding list are all specified in
POSIX.1. The parent and child also differ with respect to the
following Linux-specific process attributes:
• The child does not inherit directory change notifications
(dnotify) from its parent (see the description of F_NOTIFY in
fcntl(2)).
• The prctl(2) PR_SET_PDEATHSIG setting is reset so that the
child does not receive a signal when its parent terminates.
• The default timer slack value is set to the parent's current
timer slack value. See the description of PR_SET_TIMERSLACK in
prctl(2).
• Memory mappings that have been marked with the madvise(2)
MADV_DONTFORK flag are not inherited across a fork().
• Memory in address ranges that have been marked with the
madvise(2) MADV_WIPEONFORK flag is zeroed in the child after a
fork(). (The MADV_WIPEONFORK setting remains in place for
those address ranges in the child.)
• The termination signal of the child is always SIGCHLD (see
clone(2)).
• The port access permission bits set by ioperm(2) are not
inherited by the child; the child must turn on any bits that it
requires using ioperm(2).
Note the following further points:
• The child process is created with a single thread—the one that
called fork(). The entire virtual address space of the parent
is replicated in the child, including the states of mutexes,
condition variables, and other pthreads objects; the use of
pthread_atfork(3) may be helpful for dealing with problems that
this can cause.
• After a fork() in a multithreaded program, the child can safely
call only async-signal-safe functions (see signal-safety(7))
until such time as it calls execve(2).
• The child inherits copies of the parent's set of open file
descriptors. Each file descriptor in the child refers to the
same open file description (see open(2)) as the corresponding
file descriptor in the parent. This means that the two file
descriptors share open file status flags, file offset, and
signal-driven I/O attributes (see the description of F_SETOWN
and F_SETSIG in fcntl(2)).
• The child inherits copies of the parent's set of open message
queue descriptors (see mq_overview(7)). Each file descriptor
in the child refers to the same open message queue description
as the corresponding file descriptor in the parent. This means
that the two file descriptors share the same flags (mq_flags).
• The child inherits copies of the parent's set of open directory
streams (see opendir(3)). POSIX.1 says that the corresponding
directory streams in the parent and child may share the
directory stream positioning; on Linux/glibc they do not.
On success, the PID of the child process is returned in the
parent, and 0 is returned in the child. On failure, -1 is
returned in the parent, no child process is created, and errno is
set to indicate the error.
EAGAIN A system-imposed limit on the number of threads was
encountered. There are a number of limits that may trigger
this error:
• the RLIMIT_NPROC soft resource limit (set via
setrlimit(2)), which limits the number of processes and
threads for a real user ID, was reached;
• the kernel's system-wide limit on the number of
processes and threads, /proc/sys/kernel/threads-max, was
reached (see proc(5));
• the maximum number of PIDs, /proc/sys/kernel/pid_max,
was reached (see proc(5)); or
• the PID limit (pids.max) imposed by the cgroup "process
number" (PIDs) controller was reached.
EAGAIN The caller is operating under the SCHED_DEADLINE scheduling
policy and does not have the reset-on-fork flag set. See
sched(7).
ENOMEM fork() failed to allocate the necessary kernel structures
because memory is tight.
ENOMEM An attempt was made to create a child process in a PID
namespace whose "init" process has terminated. See
pid_namespaces(7).
ENOSYS fork() is not supported on this platform (for example,
hardware without a Memory-Management Unit).
ERESTARTNOINTR (since Linux 2.6.17)
System call was interrupted by a signal and will be
restarted. (This can be seen only during a trace.)
C library/kernel differences
Since glibc 2.3.3, rather than invoking the kernel's fork() system
call, the glibc fork() wrapper that is provided as part of the
NPTL threading implementation invokes clone(2) with flags that
provide the same effect as the traditional system call. (A call
to fork() is equivalent to a call to clone(2) specifying flags as
just SIGCHLD.) The glibc wrapper invokes any fork handlers that
have been established using pthread_atfork(3).
POSIX.1-2008.
POSIX.1-2001, SVr4, 4.3BSD.
Under Linux, fork() is implemented using copy-on-write pages, so
the only penalty that it incurs is the time and memory required to
duplicate the parent's page tables, and to create a unique task
structure for the child.
See pipe(2) and wait(2) for more examples.
#include <signal.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
int
main(void)
{
pid_t pid;
if (signal(SIGCHLD, SIG_IGN) == SIG_ERR) {
perror("signal");
exit(EXIT_FAILURE);
}
pid = fork();
switch (pid) {
case -1:
perror("fork");
exit(EXIT_FAILURE);
case 0:
puts("Child exiting.");
fflush(stdout);
_exit(EXIT_SUCCESS);
default:
printf("Child is PID %jd\n", (intmax_t) pid);
puts("Parent exiting.");
exit(EXIT_SUCCESS);
}
}
clone(2), execve(2), exit(2), _exit(2), setrlimit(2), unshare(2),
vfork(2), wait(2), daemon(3), pthread_atfork(3), capabilities(7),
credentials(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 fork(2)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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