fork(2) - NetBSD Manual Pages

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FORK(2)                   NetBSD 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 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-
               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.


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.

NetBSD 5.1                       June 10, 2004                      NetBSD 5.1