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FORK(2)			    BSD	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:

	   +o   The child process has a unique process ID.

	   +o   The child process has a different parent	process	ID (i.e., the
	       process ID of the parent	process).

	   +o   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 de-
	       scriptor	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.

	   +o   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 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 indi-
     cate 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-de-
	       pendent.

     [EAGAIN]  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 ISO/IEC 9945-1:1990 ("POSIX.1").

HISTORY
     A fork() system call appeared in Version 6	AT&T UNIX.

BSD				 June 10, 2004				   BSD

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY

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