VFORK(2) | System Calls Manual | VFORK(2) |
vfork
—
#include <unistd.h>
pid_t
vfork
(void);
vfork
system call creates a new process that does
not have a new virtual address space, but rather shares address space with the
parent, thus avoiding potentially expensive copy-on-write operations normally
associated with creating a new process. 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
existing stack frame. Be careful, also, to call
_exit(2) rather than
exit(3) if you can't
execve(2), since
exit(3) will flush and close
standard I/O channels, and thereby mess up the standard I/O data structures
in the parent process. (Even with
fork(2) it is wrong to call
exit(3) since buffered data
would then be flushed twice.)
vfork
() function call appeared in
3.0BSD. In 4.4BSD, the
semantics were changed to only suspend the parent and not share the address
space. The original semantics were reintroduced in NetBSD
1.4.
vfork
() as implementations exist that implement
vfork
() as plain
fork(2).
To avoid a possible deadlock situation, processes that are
children in the middle of a vfork
() are never sent
SIGTSTP
, SIGTTOU
or
SIGTTIN
signals; rather, output or
ioctl(2) calls are allowed and
input attempts result in an end-of-file indication.
May 19, 2018 | NetBSD 9.2 |