vfork(2)                  SYSTEM CALLS                   vfork(2)



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

SYNOPSIS
     #include <unistd.h>
     pidt vfork (void);

DESCRIPTION
     vfork can be used to  create  new  processes  without  fully
     copying  the  address  space  of  the  old process, which is
     horrendously inefficient in a paged environment.  It is use-
     ful when the purpose of fork would have been to create a new
     system context for an execve.  vfork differs  from  fork  in
     that  the  child  borrows  the parent's memory and thread of
     control until a call to execve or an exit (either by a  call
     to  exit  or  abnormally.)   The parent process is suspended
     while the child is using its resources.  vfork returns 0  in
     the  child's context and (later) the process ID (PID) of the
     child in the parent's context.  vfork can normally  be  used
     just  like fork.  It does not work, however, to return while
     running in the child's  context  from  the  procedure  which
     called vfork since the eventual return from vfork would then
     return to a no longer existent  stack  frame.   Be  careful,
     also,  to  call exit rather than exit if you cannot execve,
     since exit will flush and close standard I/O  channels,  and
     thereby  mess  up  the  parent  processes  standard I/O data
     structures.  Even with fork it is wrong to call  exit  since
     buffered data would then be flushed twice.

DIAGNOSTICS
     Upon successful completion, vfork returns a value  of  0  to
     the  child  process  and returns the process ID of the child
     process to the parent process.  Otherwise, a value of -1  is
     returned to the parent process, no child process is created,
     and the global variable errno is set to indicate the error.

     vfork will fail and no child process will be created if  one
     or more of the following are true:

     EAGAIN         The system-imposed limit on the total  number
                    of   processes   under   execution  would  be
                    exceeded.  This limit is determined when  the
                    system is generated.

     EAGAIN         The system-imposed limit on the total  number
                    of processes under execution 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
                    process.




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vfork(2)                  SYSTEM CALLS                   vfork(2)



SEE ALSO
     exec(2), exit(2), fork(2), ioctl(2), wait(2).

NOTES
     This system call will be eliminated  in  a  future  release.
     System implementation changes are making the efficiency gain
     of vfork over fork smaller.  The memory sharing semantics of
     vfork  can be obtained through other mechanisms.  To avoid a
     possible deadlock situation, processes that are children  in
     the middle of a vfork are never sent SIGTTOU or SIGTTIN sig-
     nals;  rather,  output  or  ioctls  are  allowed  and  input
     attempts  result in an EOF indication.  On some systems, the
     implementation of vfork causes the parent to inherit  regis-
     ter  values  from  the  child.  This can create problems for
     certain optimizing compilers if <unistd.h> is  not  included
     in the source calling vfork.







































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