FORK(2) System Calls Manual FORK(2)

fork — create a new process

#include <unistd.h>

pid_t
fork(void);

() 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, which also does not match any existing process group ID.
  • The child process has a different parent process ID (i.e., the process ID of the parent process).
  • The child process has a single thread.
  • 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 has no fcntl(2)-style file locks.
  • The child process' resource utilizations are set to 0; see getrusage(2).
  • All interval timers are cleared; see setitimer(2).
  • The child process' semaphore undo values are set to 0; see semop(2).
  • The child process' pending signals set is empty.
  • The child process has no memory locks; see mlock(2) and mlockall(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).

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:

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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.
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The limit RLIMIT_NPROC on the total number of processes under execution by the user ID would be exceeded.
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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.

September 10, 2015 OpenBSD 7.6