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       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)):

           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 ||


   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

   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.


       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.


       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:


       The  vfork()  system  call  appeared  in 3.0BSD.  In 4.4BSD it was made
       synonymous  to  fork(2)   but   NetBSD   introduced   it   again,   cf.
       ⟨⟩.   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.


       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."


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


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