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 mod
ifies 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 pro
vfork, just like fork(2), creates a child process of the
calling process. For details and return value and errors,
vfork() is a special case of clone(2). It is used to cre
ate 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. How
ever, 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
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
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 pos
sible 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/ker
nel/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 indepen
dent 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 com
pliant. 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.
clone(2), execve(2), fork(2), wait(2)