FORK(2) | 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);

# DESCRIPTION

`fork`() 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. 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).

In general, the child process should call _exit(2) rather than exit(3). Otherwise, any stdio buffers that exist both in the parent and child will be flushed twice. Similarly, _exit(2) should be used to prevent atexit(3) routines from being called twice (once in the parent and once in the child).

In case of a threaded program, only the thread calling `fork`() is still running in the child processes.

Child processes of a threaded program have additional restrictions, a child must only call functions that are async-signal-safe. Very few functions are asynchronously safe (the list may be found in sigaction(2)) and applications should make sure they call exec(3) as soon as possible.

# 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

`fork`() 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. This limit is configuration-dependent; or the limit `RLIMIT_NPROC` on the total number of processes under execution by this user id would be exceeded.
[`ENOMEM`]
  There is insufficient swap space for the new process.

# SEE ALSO

execve(2), setrlimit(2), vfork(2), wait(2), pthread_atfork(3)

# STANDARDS

The `fork`() function conforms to IEEE Std 1003.1-1990 (“POSIX.1”).

# HISTORY

A `fork`() system call appeared in Version 1 AT&T UNIX.

September 1, 2019 | NetBSD 9.2