Tuesday, August 5, 2008

What's the difference between fork() and vfork()?

Some systems have a system call `vfork()', which was originally designed as
a lower-overhead version of `fork()'. Since `fork()' involved copying the
entire address space of the process, and was therefore quite expensive, the
`vfork()' function was introduced (in 3.0BSD).

*However*, since `vfork()' was introduced, the implementation of `fork()'
has improved drastically, most notably with the introduction of
`copy-on-write', where the copying of the process address space is
transparently faked by allowing both processes to refer to the same
physical memory until either of them modify it. This largely removes the
justification for `vfork()'; indeed, a large proportion of systems now lack
the original functionality of `vfork()' completely. For compatibility,
though, there may still be a `vfork()' call present, that simply calls
`fork()' without attempting to emulate all of the `vfork()' semantics.

As a result, it is *very* unwise to actually make use of any of the
differences between `fork()' and `vfork()'. Indeed, it is probably unwise
to use `vfork()' at all, unless you know exactly *why* you want to.

The basic difference between the two is that when a new process is created
with `vfork()', the parent process is temporarily suspended, and the child
process might borrow the parent's address space. This strange state of
affairs continues until the child process either exits, or calls
`execve()', at which point the parent process continues.

This means that the child process of a `vfork()' must be careful to avoid
unexpectedly modifying variables of the parent process. In particular, the
child process must *not* return from the function containing the `vfork()'
call, and it must *not* call `exit()' (if it needs to exit, it should use
`_exit()'; actually, this is also true for the child of a normal `fork()').

1.1.3 Why use _exit rather than exit in the child branch of a fork?

There are a few differences between `exit()' and `_exit()' that become
significant when `fork()', and especially `vfork()', is used.

The basic difference between `exit()' and `_exit()' is that the former
performs clean-up related to user-mode constructs in the library, and calls
user-supplied cleanup functions, whereas the latter performs only the
kernel cleanup for the process.

In the child branch of a `fork()', it is normally incorrect to use
`exit()', because that can lead to stdio buffers being flushed twice, and
temporary files being unexpectedly removed. In C++ code the situation is
worse, because destructors for static objects may be run incorrectly.
(There are some unusual cases, like daemons, where the *parent* should call
`_exit()' rather than the child; the basic rule, applicable in the
overwhelming majority of cases, is that `exit()' should be called only once
for each entry into `main'.)

In the child branch of a `vfork()', the use of `exit()' is even more
dangerous, since it will affect the state of the *parent* process.

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