VFORK(2) BSD System Calls Manual VFORK(2)NAMEvfork — spawn new process in a virtual memory efficient way
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
pid_t
vfork(void);
DESCRIPTION
The vfork system call creates a new process that does not have a new vir‐
tual address space, but rather shares address space with the parent, thus
avoiding potentially expensive copy-on-write operations normally associ‐
ated with creating a new process. It is useful when the purpose of
fork(2) would have been to create a new system context for an execve(2).
The vfork system call differs from fork(2) in that the child borrows the
parent's memory and thread of control until a call to execve(2) or an
exit (either by a call to _exit(2) or abnormally). The parent process is
suspended while the child is using its resources.
The vfork system call returns 0 in the child's context and (later) the
pid of the child in the parent's context.
The vfork system call can normally be used just like fork(2). It does
not work, however, to return while running in the childs context from the
procedure that called vfork() since the eventual return from vfork()
would then return to a no longer existent stack frame. Be careful, also,
to call _exit(2) rather than exit(3) if you can't execve(2), since
exit(3) will flush and close standard I/O channels, and thereby mess up
the standard I/O data structures in the parent process. (Even with
fork(2) it is wrong to call exit(3) since buffered data would then be
flushed twice.)
RETURN VALUES
Same as for fork(2).
ERRORS
Same as for fork(2).
SEE ALSOexecve(2), fork(2), sigaction(2), wait(2)HISTORY
The vfork() function call appeared in 3.0BSD. In 4.4BSD, the semantics
were changed to only suspend the parent. The original semantics were
reintroduced in NetBSD 1.4.
BUGS
Users should not depend on the memory sharing semantics of vfork() as
other ways of speeding up the fork process may be developed in the
future.
To avoid a possible deadlock situation, processes that are children in
the middle of a vfork() are never sent SIGTTOU or SIGTTIN signals;
rather, output or ioctl(2) calls are allowed and input attempts result in
an end-of-file indication.
BSD January 3, 1998 BSD
