VFORK(2)

NAME
vfork - spawn new process in a virtual memory efficient way

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 can't 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.)

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

RETURN VALUES
Same as for fork(2).

BUGS
This system call will be eliminated when proper system sharing mechanisms are implemented. Users should not depend on the memory sharing semantics of vfork() as it will, in that case, be made synonymous to fork(2).

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.

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