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FORK(2) System Calls Manual FORK(2)

create a new process

#include <unistd.h>

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:
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).

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.

fork() will fail and no child process will be created if:
The system-imposed limits on the total number of processes or total number of threads under execution would be exceeded. These limits are configuration dependent.
The limit RLIMIT_NPROC on the total number of processes under execution by the user ID would be exceeded.
There is insufficient swap space for the new process.

execve(2), getrusage(2), wait(2)

The fork() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

The fork() system call first appeared in Version 1 AT&T UNIX.
March 31, 2013 OpenBSD-5.8