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 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 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 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. shared memory.
clone(2), execve(2), fork(2), wait(2)
Linux 2.2.0 1999-11-01 VFORK(2)
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