vfork


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  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
       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 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:

            CLONE_VM | CLONE_VFORK | SIGCHLD

   History
       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.

BUGS
       Details  of the signal handling are obscure and differ between systems.



Linux                             2012-08-05                          VFORK(2)
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