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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(): _BSD_SOURCE || _XOPEN_SOURCE >= 500

DESCRIPTION
   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 func-
       tion before successfully 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 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(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, including 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  par-
       ent'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 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 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 pro-
       cess  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.

CONFORMING TO
       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 synonymous is com-
       pliant.  In particular, the programmer cannot rely on the parent remaining blocked until the child either  ter-
       minates or calls execve(2), and cannot rely on any specific behavior with respect to shared memory.

NOTES
   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  Linux-
       Threads threading library.  (See pthreads(7) for a description of Linux threading libraries.)

   History
       The  vfork() system call appeared in 3.0BSD.  In 4.4BSD it was made synonymous 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
       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, 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."

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

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-06-21                          VFORK(2)