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VFORK(2) Linux Programmer’s Manual VFORK(2)
NAME
vfork - create a child process and block parent
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
pid_t vfork(void);
STANDARD DESCRIPTION
(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.
ERRORS
EAGAIN Too many processes - try again.
ENOMEM There is insufficient swap space for the new process.
LINUX DESCRIPTION
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.
HISTORIC DESCRIPTION
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 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 chil-
dren 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."
Currently (Linux 2.3.25), strace(1) cannot follow vfork() and requires a kernel
patch.
HISTORY
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/Documenta-
tion/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.
CONFORMING TO
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 partic-
ular, 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.
SEE ALSO
clone(2), execve(2), fork(2), wait(2)
Linux 2.2.0 1999-11-01 VFORK(2)
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