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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);

   Feature Test	Macro Requirements for glibc (see feature_test_macros(7)):

       vfork():
	   Since glibc 2.12:
	       _BSD_SOURCE ||
		   (_XOPEN_SOURCE >= 500 ||
		       _XOPEN_SOURCE &&	_XOPEN_SOURCE_EXTENDED)	&&
		   !(_POSIX_C_SOURCE >=	200809L	|| _XOPEN_SOURCE >= 700)
	   Before glibc	2.12:
	       _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
	       _XOPEN_SOURCE &&	_XOPEN_SOURCE_EXTENDED

DESCRIPTION
   Standard description
       (From POSIX.1) The vfork() function has the same	effect as fork(2), ex-
       cept  that  the behavior	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  success-
       fully calling _exit(2) or one of	the exec(3) family of functions.

   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  pro-
       cesses  without	copying	the page tables	of the parent process.	It may
       be useful in performance-sensitive applications where a child  is  cre-
       ated which then immediately issues an execve(2).

       vfork()	differs	 from  fork(2) in that the calling thread is suspended
       until the child terminates (either normally, by	calling	 _exit(2),  or
       abnormally,  after  delivery  of	a fatal	signal), or it makes a call to
       execve(2).  Until that point, the child shares all memory with its par-
       ent,  including	the stack.  The	child must not return from the current
       function	or call	exit(3), but may call _exit(2).

       As with fork(2),	the child process created by vfork()  inherits	copies
       of  various of the caller's process attributes (e.g., file descriptors,
       signal dispositions, and	current	working	directory); the	 vfork()  call
       differs	only  in  the  treatment  of the virtual address space,	as de-
       scribed above.

       Signals sent to the parent arrive after the child releases the parent's
       memory (i.e., after the child terminates	or calls execve(2)).

   Historic description
       Under  Linux,  fork(2) is implemented using copy-on-write pages,	so the
       only penalty incurred by	fork(2)	is the time and	memory required	to du-
       plicate the parent's page tables, and to	create a unique	task structure
       for the child.  However,	in the bad old days a  fork(2)	would  require
       making  a  complete  copy of the	caller's data space, often needlessly,
       since usually immediately afterward an  exec(3)	is  done.   Thus,  for
       greater	efficiency,  BSD introduced the	vfork()	system call, which 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(2) 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	 vari-
       ables were held in a register.

CONFORMING TO
       4.3BSD;	POSIX.1-2001  (but marked OBSOLETE).  POSIX.1-2008 removes the
       specification of	vfork().

       The requirements	put on vfork() by the standards	are weaker than	 those
       put  on	fork(2),  so an	implementation where the two are synonymous is
       compliant.  In particular, the programmer cannot	rely on	the parent re-
       maining	blocked	 until the child either	terminates or calls execve(2),
       and cannot rely on any specific behavior	with respect to	shared memory.

NOTES
       Some consider the semantics of vfork() to be an architectural  blemish,
       and  the	 4.2BSD	 man page stated: "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(2)."  However, even though	modern
       memory management hardware has decreased	the performance	difference be-
       tween fork(2) and vfork(), there	are  various  reasons  why  Linux  and
       other systems have retained vfork():

       *  Some performance-critical applications require the small performance
	  advantage conferred by vfork().

       *  vfork() can be implemented on	systems	that lack a  memory-management
	  unit	(MMU),	but  fork(2)  can't  be	 implemented  on such systems.
	  (POSIX.1-2008	removed	vfork()	from the standard; the POSIX rationale
	  for the posix_spawn(3) function notes	that that function, which pro-
	  vides	functionality equivalent to fork(2)+exec(3), is	designed to be
	  implementable	on systems that	lack an	MMU.)

   Linux notes
       Fork handlers established using pthread_atfork(3) are not called	when a
       multithreaded  program  employing  the  NPTL  threading	library	 calls
       vfork().	  Fork handlers	are called in this case	in a program using the
       LinuxThreads threading library.	(See pthreads(7) for a description  of
       Linux threading libraries.)

       A  call	to vfork() is equivalent to calling clone(2) with flags	speci-
       fied as:

	    CLONE_VM | CLONE_VFORK | SIGCHLD

   History
       The vfork() system call appeared	in 3.0BSD.  In 4.4BSD it was made syn-
       onymous	  to   fork(2)	 but   NetBSD	introduced   it	  again,   cf.
       <http://www.netbsd.org/Documentation/kernel/vfork.html>.	 In Linux,  it
       has   been  equivalent  to  fork(2)  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.

BUGS
       Details	of the signal handling are obscure and differ between systems.
       The BSD man page	states:	"To avoid a possible deadlock situation,  pro-
       cesses  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."

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

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2012-08-05			      VFORK(2)

NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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