NAME

vfork − spawn new process in a virtual memory efficient way

SYNOPSIS

#include <vfork.h>

pid = vfork()

int pid;

DESCRIPTION

vfork() 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).  vfork() differs from fork() 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. 

vfork() returns 0 in the child’s context and (later) the pid of the child in the parent’s context. 

vfork() can normally be used just like fork.  It does not work, however, to return while running in the child’s context from the procedure which called vfork() since the eventual return from vfork() would then return to a no longer existent stack frame.  Be careful, also, to call _exit() rather than exit() if you cannot execve, since exit() will flush and close standard I/O channels, and thereby mess up the parent processes standard I/O data structures.  (Even with fork() it is wrong to call exit() since buffered data would then be flushed twice.) 

On Sun-4 machines, the parent inherits the values of local and incoming argument registers from the child.  Since this violates the usual data flow properties of procedure calls, the file <vfork.h> must be included in programs that are compiled using global optimization. 

SEE ALSO

execve(2), exit(2), fork(2), ioctl(2), sigvec(2), wait(2),

DIAGNOSTICS

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 ioctls are allowed and input attempts result in an EOF indication. 

Sun Release 4.0  —  Last change: 14 November 1987