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FORK(2)                       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

              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

     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 (the list may be found in sigaction(2))
     and applications 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; or 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.

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

     The fork() function conforms to IEEE Std 1003.1-1990 ("POSIX.1").

     A fork() system call appeared in Version 1 AT&T UNIX.

NetBSD 9.99                    September 1, 2019                   NetBSD 9.99