- NetBSD Manual Pages
FORK(2) NetBSD System Calls Manual FORK(2)
Powered by man-cgi (2020-09-24).
Maintained for NetBSD
by Kimmo Suominen.
Based on man-cgi by Panagiotis Christias.
fork -- create a new process
Standard C Library (libc, -lc)
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 indi-
cate 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 configura-
tion-dependent; or the limit RLIMIT_NPROC on the total number
of processes under execution by this user id would be
[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 1 AT&T UNIX.
NetBSD 9.0 September 1, 2019 NetBSD 9.0