FORK(2) FreeBSD System Calls Manual FORK(2)
NAME
fork - create a new process
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
pid_t
fork(void);
pid_t
_Fork(void);
DESCRIPTION
The fork() function 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,
except for descriptors returned by kqueue(2), which are not
inherited from the parent process. 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).
• All interval timers are cleared; see setitimer(2).
• The robust mutexes list (see pthread_mutexattr_setrobust(3)) is
cleared for the child.
• The atfork handlers established with the pthread_atfork(3)
function are called as appropriate before fork in the parent
process, and after the child is created, in parent and child.
• The child process has only one thread, corresponding to the
calling thread in the parent process. If the process has more
than one thread, locks and other resources held by the other
threads are not released and therefore only async-signal-safe
functions (see sigaction(2)) are guaranteed to work in the
child process until a call to execve(2) or a similar function.
The FreeBSD implementation of fork() provides a usable
malloc(3), and rtld(1) services in the child process.
The fork() function is not async-signal safe and creates a cancellation
point in the parent process. It cannot be safely used from signal
handlers, and the atfork handlers established by pthread_atfork(3) do not
need to be async-signal safe either.
The _Fork() function creates a new process, similarly to fork(), but it
is async-signal safe. _Fork() does not call atfork handlers, and does
not create a cancellation point. It can be used safely from signal
handlers, but then no userspace services ( malloc(3) or rtld(1)) are
available in the child if forked from multi-threaded parent. In
particular, if using dynamic linking, all dynamic symbols used by the
child after _Fork() must be pre-resolved. Note: resolving can be done
globally by specifying the LD_BIND_NOW environment variable to the
dynamic linker, or per-binary by passing the -z now option to the static
linker ld(1), or by using each symbol before the _Fork() call to force
the binding.
RETURN VALUES
Upon successful completion, fork() and _Fork() return a value of 0 to the
child process and return 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.
EXAMPLES
The following example shows a common pattern of how fork() is used in
practice.
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int
main(void)
{
pid_t pid;
/*
* If child is expected to use stdio(3), state of
* the reused io streams must be synchronized between
* parent and child, to avoid double output and other
* possible issues.
*/
fflush(stdout);
switch (pid = fork()) {
case -1:
err(1, "Failed to fork");
case 0:
printf("Hello from child process!\n");
/*
* Since we wrote into stdout, child needs to use
* exit(3) and not _exit(2). This causes handlers
* registered with atexit(3) to be called twice,
* once in parent, and once in the child. If such
* behavior is undesirable, consider
* terminating child with _exit(2) or _Exit(3).
*/
exit(0);
default:
break;
}
printf("Hello from parent process (child's PID: %d)!\n", pid);
return (0);
}
The output of such a program is along the lines of:
Hello from parent process (child's PID: 27804)!
Hello from child process!
ERRORS
The fork() system call 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. The
limit is given by the sysctl(3) MIB variable
KERN_MAXPROC. (The limit is actually ten less than
this except for the super user).
[EAGAIN] The user is not the super user, and the system-imposed
limit on the total number of processes under execution
by a single user would be exceeded. The limit is
given by the sysctl(3) MIB variable
KERN_MAXPROCPERUID.
[EAGAIN] The user is not the super user, and the soft resource
limit corresponding to the resource argument
RLIMIT_NPROC would be exceeded (see getrlimit(2)).
[ENOMEM] There is insufficient swap space for the new process.
SEE ALSO
execve(2), rfork(2), setitimer(2), setrlimit(2), sigaction(2), vfork(2),
wait(2), pthread_atfork(3)
HISTORY
The fork() function appeared in Version 1 AT&T UNIX.
The _Fork() function was defined by Austin Group together with the
removal of a requirement that the fork() implementation must be async-
signal safe. The _Fork() function appeared in FreeBSD 14.0.
FreeBSD 13.1-RELEASE-p6 August 5, 2021 FreeBSD 13.1-RELEASE-p6
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