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SubjectRe: vfork
People tell me that vfork() no longer is equivalent to fork()
as the manpage states. Unfortunately, they are right, so I
wrote a new page - see below.

I consider the introduction of vfork into Linux a very bad move
(as will be clear from the text I wrote), but since there were
people writing code and submitting patches there must be some
positive side to this horrible kludge.

If nobody corrects me, this will be the vfork man page in man-pages-1.27.
(The title is borrowed from the BSD title:
`vfork - spawn new process in a virtual memory-efficient way'
)

Andries - aeb@cwi.nl

-------------------------------------------------------------------------
VFORK(2) Linux Programmer's Manual VFORK(2)


NAME
vfork - create a child process in a broken way

SYNOPSIS
#include <unistd.h>

pid_t vfork(void);

POSIX DESCRIPTION
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 func­
tion in which vfork() was called, or calls any other function
before successfully calling _exit() or one of the exec family of
functions.

ERRORS
EAGAIN Too many processes - try again.

ENOMEM There is insufficient swap space for the new process.

DESCRIPTION
vfork, just like fork(2), creates a child process of the calling
process. For details and return value and errors, see fork(2).

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.

BUGS
It is rather unfortunate that Linux revived this spectre from the
past. The BSD manpage states: "This system call will be elimi­
nated 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 POSIX 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 sys­
tems. The BSD manpage states: "To avoid a possible deadlock situ­
ation, 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 indica­
tion."

HISTORY
The vfork() system call occurs in the BSD 2.9.1 (but not in the
2.8) manual pages. 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.

CONFORMING TO
The vfork call may be a bit similar to calls with the same name in
other operating systems. They all resemble fork(), have obscure
semantics, and are not really faster today than fork().

SEE ALSO
clone(2), execve(2), fork(2), wait(2)


Linux 2.2.0 1 Nov 1999 1

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