Provided by: manpages-dev_2.17-1_all
vfork - create a child process and block parent
(From XPG4 / SUSv2 / POSIX draft.) The vfork() function has the same
effect as fork(), except that the behaviour is undefined if the process
created by vfork() either modifies any data other than a variable of
type pid_t used to store the return value from vfork(), or returns from
the function in which vfork() was called, or calls any other function
before successfully calling _exit() or one of the exec() family of
EAGAIN Too many processes; try again.
ENOMEM There is insufficient swap space for the new process.
vfork(), just like fork(2), creates a child process of the calling
process. For details and return value and errors, see fork(2).
vfork() is a special case of clone(2). It is used to create new
processes without copying the page tables of the parent process. It
may be useful in performance sensitive applications where a child will
be created which then immediately issues an execve().
vfork() differs from fork() in that the parent is suspended until the
child makes a call to execve(2) or _exit(2). The child shares all
memory with its parent, including the stack, until execve() is issued
by the child. The child must not return from the current function or
call exit(), but may call _exit().
Signal handlers are inherited, but not shared. Signals to the parent
arrive after the child releases the parent.
Under Linux, fork() is implemented using copy-on-write pages, so the
only penalty incurred by fork() is the time and memory required to
duplicate the parent’s page tables, and to create a unique task
structure for the child. However, in the bad old days a fork() would
require making a complete copy of the caller’s data space, often
needlessly, since usually immediately afterwards an exec() is done.
Thus, for greater efficiency, BSD introduced the vfork() system call,
that did not fully copy the address space of the parent process, but
borrowed the parent’s memory and thread of control until a call to
execve() or an exit occurred. The parent process was suspended while
the child was using its resources. The use of vfork() was tricky: for
example, not modifying data in the parent process depended on knowing
which variables are held in a register.
It is rather unfortunate that Linux revived this spectre from the past.
The BSD manpage states: "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()."
Formally speaking, the standard description given above does not allow
one to use vfork() since a following exec() might fail, and then what
happens is undefined.
Details of the signal handling are obscure and differ between systems.
The BSD manpage states: "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 ioctls are allowed and
input attempts result in an end-of-file indication."
Currently (Linux 2.3.25), strace(1) cannot follow vfork() and requires
a kernel patch.
The vfork() system call appeared in 3.0BSD. In 4.4BSD it was made
synonymous to fork() but NetBSD introduced it again, cf.
http://www.netbsd.org/Documentation/kernel/vfork.html . In Linux, it
has been equivalent to fork() until 2.2.0-pre6 or so. Since 2.2.0-pre9
(on i386, somewhat later on other architectures) it is an independent
system call. Support was added in glibc 2.0.112.
The vfork() call may be a bit similar to calls with the same name in
other operating systems. The requirements put on vfork() by the
standards are weaker than those put on fork(), so an implementation
where the two are synonymous is compliant. In particular, the
programmer cannot rely on the parent remaining blocked until a call of
execve() or _exit() and cannot rely on any specific behaviour w.r.t.
clone(2), execve(2), fork(2), wait(2)