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), 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 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 is 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  abnor-
       mally,  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 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 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
       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  know-
       ing which variables were 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 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.

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 LinuxThreads 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 3.27 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								    2010-09-20								  VFORK(2)