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VFORK(2) Linux Programmer's Manual VFORK(2)
vfork - create a child process and block parent
(From SUSv2 / POSIX draft.) The vfork() function has the same effect
as fork(), except that the behaviour is undefined if the process cre-
ated 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
vfork(), just like fork(2), creates a child process of the calling pro-
cess. For details and return value and errors, see fork(2).
vfork() is a special case of clone(2). It is used to create new pro-
cesses 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 mem-
ory 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's memory.
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 struc-
ture for the child. However, in the bad old days a fork() would
require making a complete copy of the caller's data space, often need-
lessly, 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 bor-
rowed 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, pro-
cesses 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 syn-
onymous 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 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. shared memory.
clone(2), execve(2), fork(2), unshare(2), wait(2)
Linux 2.2.0 1999-11-01 VFORK(2)
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