vfork(2)                                                           vfork(2)

NAME
     vfork - spawn new process in virtual memory

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. 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, how-
     ever, to return while running in the child's context from the pro-
     cedure 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.

RESULT
     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.

ERRORS
     The following error code descriptions are function-specific. You will
     find a general description in introprm2(2) or in errno(5).

     vfork() will fail and no child process will be created if one or more
     of the following apply:

     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)                                                           vfork(2)

     ENOMEM    There is insufficient swap space for the new process.

APPLICATION USAGE
     On some systems, vfork() is the same as fork().

     The vfork() function differs from fork() only in that the child pro-
     cess can share code and data with the calling process (parent pro-
     cess). This speeds cloning activity significantly at a risk to the
     integrity of the parent process if vfork() is misused.

     The use of vfork() for any purpose except as a prelude to an immediate
     call to a function from the exec family, or to exit(), is not
     advised.

     The vfork() function can be used to create new processes without fully
     copying the address space of the old process. If a forked process is
     simply going to call exec the data space copied from the parent to the
     child by fork() is not used. This is particularly inefficient in a
     paged environment, making vfork() particularly useful. Depending upon
     the size of the parent's data space, vfork() can give a significant
     performance improvement over fork().

     If signal handlers are invoked in the child process after vfork(),
     they must follow the same rules as other code in the child process.

     The [vfork, exec] window begins at the vfork() call and ends when the
     child completes its exec call.

SEE ALSO
     exec(2), exit(2), fork(2), ioctl(2), wait(2), unistd(4).
























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