trusty (2) vfork.2freebsd.gz

NAME

     vfork — create a new process without copying the address space

LIBRARY

     Standard C Library (libc, -lc)

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.