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VFORK(2)                    BSD System Calls Manual                   VFORK(2)

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

     #include <unistd.h>


     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.  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

     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
     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 rather than exit if you
     can't 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.)

     execve(2), fork(2), sigaction(2), wait(2)

     The vfork() system call will fail for any of the reasons described in the fork man page.  In addition,
     it will fail if:

     [EINVAL]           A system call other than _exit() or execve() (or libc functions that make no system
                        calls other than those) is called following calling a vfork() call.

     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.

     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.

     The vfork() function call appeared in 3.0BSD.

4th Berkeley Distribution        June 4, 1993        4th Berkeley Distribution

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