NAME
vfork —
spawn new process in a virtual memory
efficient way
SYNOPSIS
#include
<unistd.h>
int
vfork(void);
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.
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 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.)
SEE ALSO
DIAGNOSTICS
Same as for fork.
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.
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 function call appeared in
3.0BSD.