create a new process
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, which also does not match any
existing process group ID.
- The child process has a different parent process ID (i.e., the process ID
of the parent process).
- The child process has a single thread.
- 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
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 has no
- The child process' resource utilizations are set to 0; see
- All interval timers are cleared; see
- The child process' semaphore undo values are set to 0; see
- The child process' pending signals set is empty.
- The child process has no memory locks; 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
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
- The system-imposed limits on the total number of processes or total number
of threads under execution would be exceeded. These limits are
- The limit
RLIMIT_NPROC on the total number of
processes under execution by the user ID would be exceeded.
- There is insufficient swap space for the new process.
fork() function conforms to IEEE
Std 1003.1-2008 (“POSIX.1”).
fork() system call first appeared in
Version 1 AT&T UNIX.