fork(2) System Calls Manual fork(2)
NAME
fork - create a child process
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
pid_t fork(void);
DESCRIPTION
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 (sigpend-
ing(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 follow-
ing 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_DONT-
FORK 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 descrip-
tors. 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 correspond-
ing file descriptor in the parent. This means that the two file de-
scriptors 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 di-
rectory streams in the parent and child may share the directory
stream positioning; on Linux/glibc they do not.
RETURN VALUE
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.
ERRORS
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 num-
ber" (PIDs) controller was reached.
EAGAIN The caller is operating under the SCHED_DEADLINE scheduling pol-
icy and does not have the reset-on-fork flag set. See sched(7).
ENOMEM fork() failed to allocate the necessary kernel structures be-
cause 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.)
VERSIONS
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
POSIX.1-2008.
HISTORY
POSIX.1-2001, SVr4, 4.3BSD.
NOTES
Under Linux, fork() is implemented using copy-on-write pages, so the
only penalty that it incurs is the time and memory required to dupli-
cate the parent's page tables, and to create a unique task structure
for the child.
EXAMPLES
See pipe(2) and wait(2) for more examples.
#include <signal.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.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.");
exit(EXIT_SUCCESS);
default:
printf("Child is PID %jd\n", (intmax_t) pid);
puts("Parent exiting.");
exit(EXIT_SUCCESS);
}
}
SEE ALSO
clone(2), execve(2), exit(2), setrlimit(2), unshare(2), vfork(2),
wait(2), daemon(3), pthread_atfork(3), capabilities(7), credentials(7)
Linux man-pages 6.7 2023-10-31 fork(2)
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