NAME
fork —
create a new process
LIBRARY
library “libc”
SYNOPSIS
#include
<unistd.h>
pid_t
fork(void);
DESCRIPTION
Thefork()
system call 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 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.
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
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_NPROCwould 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)
HISTORY
The fork() function appeared in
Version 1 AT&T UNIX.