The vfork() function shall be equivalent to fork(), except that
the behavior is undefined if the process created by vfork() either
modifies any data other than a variable of type pid_t used to store the
return value from vfork(), or returns from the function in which
vfork() was called, or calls any other function before successfully
calling _exit() or one of the exec family of functions.
Upon successful completion, vfork() shall return 0 to the child process
and return the process ID of the child process to the parent process.
Otherwise, -1 shall be returned to the parent, no child process shall be
created, and errno shall be set to indicate the error.
The system-wide limit on the total number of processes
under execution would be exceeded, or the system-imposed limit on the
total number of processes under execution by a single user would be
There is insufficient swap space for the new process.
Conforming applications are recommended not to depend on vfork(), but to
use fork() instead. The vfork() function may be withdrawn in a
On some implementations, vfork() is equivalent to fork().
The vfork() function differs from fork() only in that the child
process can share code and data with the calling process (parent process).
This speeds cloning activity significantly at a risk to the integrity of the
parent process if vfork() is misused.
The use of vfork() for any purpose except as a prelude to an immediate
call to a function from the exec family, or to _exit(), is not
The vfork() function can be used to create new processes without fully
copying the address space of the old process. If a forked process is simply
going to call exec, the data space copied from the parent to the child
by fork() is not used. This is particularly inefficient in a paged
environment, making vfork() particularly useful. Depending upon the
size of the parent's data space, vfork() can give a significant
performance improvement over fork().
The vfork() function can normally be used just like fork(). It
does not work, however, to return while running in the child's context from
the caller of vfork() since the eventual return from vfork()
would then return to a no longer existent stack frame. Care should be taken,
also, to call _exit() rather than exit() if exec cannot
be used, since exit() flushes and closes standard I/O channels, thereby
damaging the parent process' standard I/O data structures. (Even with
fork(), it is wrong to call exit(), since buffered data would
then be flushed twice.)
If signal handlers are invoked in the child process after vfork(), they
must follow the same rules as other code in the child process.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html