System Calls                                             vfork(2)



NAME
     vfork, vforkx - spawn new process in a virtual memory efficient way

SYNOPSIS
     #include <unistd.h>

     pid_t vfork(void);

     #include <sys/fork.h>

     pid_t vforkx(int flags);


DESCRIPTION
     The vfork() and vforkx() functions create a new process
     without fully
     copying  the address space of the old process. These functions
     are useful in instances where the purpose of a fork(2) opera-
     tion  is  to  create  a  new  system context for an execve()
     operation (see exec(2)).



     Unlike with the fork() function, the child  process  borrows
     the  parent's  memory  and thread of control until a call to
     execve() or an exit (either  abnormally  or  by  a  call  to
     _exit()  (see  exit(2)).  Any  modification made during this
     time to any part of memory in the child process is reflected
     in the parent process on return from vfork() or vforkx().
     The parent
     process is suspended while the child is using its resources.



     In a multithreaded application, vfork() and vforkx() borrow only the
     thread of control that called vfork() or vforkx() in the parent; that
     is, the child contains only one thread. The use of vfork() or vforkx()
     in  multithreaded  applications,  however,  is unsafe due to
     race conditons that can cause the child  process  to  become
     deadlocked  and consequently block both the child and parent
     process from execution indefinitely.



     The vfork() and vforkx() functions can normally be used the
     same way as fork() and forkx(), respectively.  The calling
     procedure however, should
     not return while running in the child's context,  since  the
     eventual return from vfork() or vforkx() in the parent would be to a
     stack frame that no  longer  exists.  The  _exit()  function
     should  be used in favor of exit(3C) if unable to perform an
     execve() operation, since exit() will invoke  all  functions
     registered  by  atexit(3C) and will flush and close standard
     I/O channels, thereby corrupting the parent process's  stan-
     dard  I/O  data  structures. Care must be taken in the child
     process not to modify any global or local data that  affects
     the behavior of the parent process on return from vfork()
     or vforkx(),
     unless such an effect is intentional.



     Unlike fork() and forkx(), fork handlers are not run when
     vfork() and vforkx() are called.



     The vfork() and vforkx() functions are deprecated. Their sole
     legitimate use
     as  a  prelude  to  an immediate call to a function from the
     exec family can be achieved  safely  by  posix_spawn(3C)  or
     posix_spawnp(3C).



  Fork extensions

     The vforkx() function accepts a flags argument consisting of
     a bitwise inclusive OR of zero or more of the following flags,
     which are defined in the header <sys/fork.h>:

     FORK_NOSIGCHLD

     FORK_WAITPID

     See fork(2) for an explanation of these flags.  If the flags
     argument is zero, vforkx() is identical to vfork().


RETURN VALUES
     Upon successful completion, vfork() and vforkx() return 0 to the child
     process and return the process ID of the child process to
     the parent process. Otherwise, -1 is returned to the  parent
     process,  no  child  process is created, and errno is set to
     indicate the error.



ERRORS
     The vfork() and vforkx() functions will fail if:



     EAGAIN  The system-imposed limit  on  the  total  number  of
             processes  under execution (either system-quality or
             by a single user) would be exceeded. This  limit  is
             determined when the system is generated.




     ENOMEM  There is insufficient swap space for  the  new  pro-
             cess.


     The vforkx() function will fail if:

     EINVAL  The flags argument is invalid.




ATTRIBUTES
     See attributes(5) for descriptions of the  following  attri-
     butes:

     ____________________________________________________________
    |       ATTRIBUTE TYPE        |       ATTRIBUTE VALUE       |
    |_____________________________|_____________________________|
    | Interface Stability         | Obsolete                    |
    |_____________________________|_____________________________|
    | MT-Level                    | Unsafe                      |
    |_____________________________|_____________________________|


SEE ALSO
     exec(2), exit(2), fork(2), ioctl(2),  atexit(3C),  exit(3C),
     posix_spawn(3C),      posix_spawnp(3C),     signal.h(3HEAD),
     wait(3C), attributes(5), standards(5)



NOTES
     To avoid a possible deadlock situation, processes  that  are
     children  in  the middle of a vfork() or vforkx() are never sent SIGTTOU
     or SIGTTIN signals; rather, output or ioctls are allowed and
     input attempts result in an EOF indication.



     To forstall parent memory corruption due to race  conditions
     with  signal handling, vfork() and vforkx() treat signal handlers in the
     child process in the same manner as the  exec(2)  functions:
     signals  set  to  be caught by the parent process are set to
     the default action  (SIG_DFL)  in  the  child  process  (see
     signal.h(3HEAD)).   Any  attempt  to set a signal handler in
     the child before execve() to anything other than SIG_DFL  or
     SIG_IGN  is disallowed and results in setting the handler to
     SIG_DFL.



     On some systems, the implementation of vfork() and vforkx() causes the
     parent  to  inherit register values from the child. This can
     create  problems  for  certain   optimizing   compilers   if
     <unistd.h> is not included in the source calling vfork() or
     if <sys/fork.h> is not included in the source calling vforkx().