VFORK(2)

NAME
vfork - create a new process without copying the address space

LIBRARY
libc

SYNOPSIS
#include <unistd.h>
pid_t
vfork(void);

DESCRIPTION
The vfork() system call can be used to create new processes without fully copying the address space of the old process, which is horrendously inefficient in a paged environment. 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 child's 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 cannot execve(2), since exit(3) will flush and close standard I/O channels, and thereby mess up the parent processes standard I/O data structures. (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).

SEE ALSO
execve(2), _exit(2), fork(2), rfork(2), sigaction(2), wait(2), exit(3)

HISTORY
The vfork() system call appeared in 2.9BSD.

BUGS
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.