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NetBSD 6.1.5 - man page for fork (netbsd section 2)

FORK(2) 			     BSD System Calls Manual				  FORK(2)

     fork -- create a new process

     Standard C Library (libc, -lc)

     #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:

	   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
	       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.

	   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

     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 appli-
     cations should make sure they call exec(3) as soon as possible.

     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:

     [EAGAIN]  The system-imposed limit on the total number of processes under execution would be
	       exceeded.  This limit is configuration-dependent.

     [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.

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

     The fork() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1'').

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

BSD					  June 10, 2004 				      BSD

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