vfork manpage

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VFORK(2)		   Linux Programmer's Manual		      VFORK(2)

       vfork - create a child process and block parent

       #include <sys/types.h>
       #include <unistd.h>

       pid_t vfork(void);

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

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

   Standard description
       (From POSIX.1) The vfork() function has the  same  effect  as  fork(2),
       except 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
       described 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
       duplicate  the parent's page tables, and to create a unique task struc-
       ture 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 need-
       lessly, 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  bor-
       rowed  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 variables were held in a register.

       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
       remaining blocked until the child either terminates or calls execve(2),
       and cannot rely on any specific behavior with respect to shared memory.

       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
       between 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:


       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.

       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."

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

       This  page  is  part of release 4.04 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

Linux				  2012-08-05			      VFORK(2)