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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 would have been to create a new system
context for an differs from in that the child borrows the parent’s memory
and thread of control until a call to or an exit (either by a call to
or abnormally.) The parent process is suspended while the child is using
its resources. returns 0 in the child’s context and (later) the pid of
the child in the parent’s context. can normally be used just like It does
not work, however, to return while running in the childs context from the
procedure that called since the eventual return from would then return
to a no longer existent stack frame. Be careful, also, to call rather than
if you can’t since will flush and close standard I/O channels, and thereby
mess up the parent processes standard I/O data structures. (Even with it
is wrong to call since buffered data would then be flushed twice.)
Same as for
This system call will be eliminated
when proper system sharing mechanisms are implemented. Users should not
depend on the memory sharing semantics of as it will, in that case, be
made synonymous to To avoid a possible deadlock situation, processes
that are children in the middle of a are never sent or signals; rather,
output or calls are allowed and input attempts result in an end-of-file
indication.
The function call appeared in
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