fork(2) - Linux manual page (original) (raw)
fork(2) System Calls Manual fork(2)
NAME top
fork - create a child processLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <unistd.h>**
**pid_t fork(void);**DESCRIPTION top
**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)](../man2/mmap.2.html)), and unmappings
([munmap(2)](../man2/munmap.2.html)) 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)](../man2/setpgid.2.html)) 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)](../man2/mlock.2.html),
[mlockall(2)](../man2/mlockall.2.html)).
• Process resource utilizations ([getrusage(2)](../man2/getrusage.2.html)) and CPU time
counters ([times(2)](../man2/times.2.html)) are reset to zero in the child.
• The child's set of pending signals is initially empty
([sigpending(2)](../man2/sigpending.2.html)).
• The child does not inherit semaphore adjustments from its
parent ([semop(2)](../man2/semop.2.html)).
• The child does not inherit process-associated record locks from
its parent ([fcntl(2)](../man2/fcntl.2.html)). (On the other hand, it does inherit
[fcntl(2)](../man2/fcntl.2.html) open file description locks and [flock(2)](../man2/flock.2.html) locks from
its parent.)
• The child does not inherit timers from its parent
([setitimer(2)](../man2/setitimer.2.html), [alarm(2)](../man2/alarm.2.html), [timer_create(2)](../man2/timer%5Fcreate.2.html)).
• The child does not inherit outstanding asynchronous I/O
operations from its parent ([aio_read(3)](../man3/aio%5Fread.3.html), [aio_write(3)](../man3/aio%5Fwrite.3.html)), nor
does it inherit any asynchronous I/O contexts from its parent
(see [io_setup(2)](../man2/io%5Fsetup.2.html)).
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)](../man2/fcntl.2.html)).
• The [prctl(2)](../man2/prctl.2.html) **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)](../man2/prctl.2.html).
• Memory mappings that have been marked with the [madvise(2)](../man2/madvise.2.html)
**MADV_DONTFORK** flag are not inherited across a **fork**().
• Memory in address ranges that have been marked with the
[madvise(2)](../man2/madvise.2.html) **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)](../man2/clone.2.html)).
• The port access permission bits set by [ioperm(2)](../man2/ioperm.2.html) are not
inherited by the child; the child must turn on any bits that it
requires using [ioperm(2)](../man2/ioperm.2.html).
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)](../man3/pthread%5Fatfork.3.html) 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)](../man7/signal-safety.7.html))
until such time as it calls [execve(2)](../man2/execve.2.html).
• 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)](../man2/open.2.html)) 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)](../man2/fcntl.2.html)).
• The child inherits copies of the parent's set of open message
queue descriptors (see [mq_overview(7)](../man7/mq%5Foverview.7.html)). 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 (_mqflags_).
• The child inherits copies of the parent's set of open directory
streams (see [opendir(3)](../man3/opendir.3.html)). 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.RETURN VALUE top
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](../man3/errno.3.html)_ is
set to indicate the error.ERRORS top
**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)](../man2/setrlimit.2.html)), 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)](../man5/proc.5.html));
• the maximum number of PIDs, _/proc/sys/kernel/pidmax_,
was reached (see [proc(5)](../man5/proc.5.html)); 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)](../man7/sched.7.html).
**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)](../man7/pid%5Fnamespaces.7.html).
**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.)VERSIONS top
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).
STANDARDS top
POSIX.1-2008.HISTORY top
POSIX.1-2001, SVr4, 4.3BSD.NOTES top
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.EXAMPLES top
See [pipe(2)](../man2/pipe.2.html) and [wait(2)](../man2/wait.2.html) 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);
}
}SEE ALSO top
[clone(2)](../man2/clone.2.html), [execve(2)](../man2/execve.2.html), [exit(2)](../man2/exit.2.html), [_exit(2)](../man2/%5Fexit.2.html), [setrlimit(2)](../man2/setrlimit.2.html), [unshare(2)](../man2/unshare.2.html),
[vfork(2)](../man2/vfork.2.html), [wait(2)](../man2/wait.2.html), [daemon(3)](../man3/daemon.3.html), [pthread_atfork(3)](../man3/pthread%5Fatfork.3.html), [capabilities(7)](../man7/capabilities.7.html),
[credentials(7)](../man7/credentials.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2025-01-14 fork(2)
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