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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)):

       vfork(): _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 will be
       created which then immediately issues an execve(2).

       vfork() differs from fork(2) in that the parent 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 parent, includ‐
       ing the stack.  The child must not return from the current function  or
       call exit(3), but may call _exit(2).

       Signal  handlers	 are inherited, but not shared.	 Signals 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 afterwards 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 are held in a register.

       4.3BSD,	 POSIX.1-2001.	 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 syn‐
       onymous 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.

   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.)

       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.net‐
       bsd.org/Documentation/kernel/vfork.html .  In Linux, it has been equiv‐
       alent 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.

       It is rather unfortunate that Linux revived this specter from the past.
       The  BSD	 man  page  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(2)."

       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 3.22 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2009-06-21			      VFORK(2)

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