xenial (2) vfork.2.gz

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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 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 parent, 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 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
       variables 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 remaining 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 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 provides 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 specified as:

            CLONE_VM | CLONE_VFORK | SIGCHLD

   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

       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

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