vfork(2)                      (UNIX System V)                      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 useful 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.





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   vfork(2)                      (UNIX System V)                      vfork(2)


         ENOMEM         There is insufficient swap space for the new process.

   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 signals;
         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 register 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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