fork(2) BSD System Calls Manual fork(2)
NAME
fork -- create a new process
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:
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 or
write by the parent. This descriptor copying is also used by
the shell to establish standard input and output for newly cre-
ated processes as well as to set up pipes.
o The child processes resource utilizations are set to 0; see
setrlimit(2).
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 pro-
cesses under execution would be exceeded. This limit
is configuration-dependent.
[EAGAIN] The system-imposed limit MAXUPRC (<sys/param.h>) on
the total number of processes under execution by a
single user would be exceeded.
[ENOMEM] There is insufficient swap space for the new process.
LEGACY SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
The include file <sys/types.h> is necessary.
SEE ALSO
execve(2), sigaction(2), wait(2), compat(5)
HISTORY
A fork() function call appeared in Version 6 AT&T UNIX.
CAVEATS
There are limits to what you can do in the child process. To be totally
safe you should restrict yourself to only executing async-signal safe
operations until such time as one of the exec functions is called. All
APIs, including global data symbols, in any framework or library should
be assumed to be unsafe after a fork() unless explicitly documented to be
safe or async-signal safe. If you need to use these frameworks in the
child process, you must exec. In this situation it is reasonable to exec
yourself.
4th Berkeley Distribution June 4, 1993 4th Berkeley Distribution
Mac OS X 10.9.1 - Generated Sun Jan 5 20:46:47 CST 2014