VFORK(2) Linux Programmer's Manual VFORK(2)
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 functions.
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 applica-
tions 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 func-
tion 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 BSD 4.4 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 sys-
tems. 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), wait(2)
Linux 2.2.0 1999-11-01 VFORK(2)