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Full Discussion: Fork
Top Forums UNIX for Dummies Questions & Answers Fork Post 1632 by Neo on Tuesday 20th of March 2001 06:24:47 PM
Old 03-20-2001
"To Fork" is like "a fork in your path" when you are walking in the forest. The concept is similar in UNIX processes, however the fork occurs with the logic flow of the program and processes.

Consider that your process is running as process ID 100. The process has an open file descriptor listening for incoming TCP/IP connections. When an incoming connection occurs, the process could "fork" another process to handle the incoming connection. The process ID, for example, of the new process that was "forked" could be 132 (whatever the next available process ID was).

When a new process is forked it has a few interesting options. One is to take the content of the global variables and other data structures and make a copies in the new fork. Another option is to fork and completely replace the original process.

So, forking is just something that occurs to a process in its "path of execution". The main difference between forking processes and your fork as your walk down the road is that you can't physically walk down two paths. Software processes can and do spawn many processes by forking.

Hope this helps.

[Edited by Neo on 03-20-2001 at 06:27 PM]
This User Gave Thanks to Neo For This Post:
shekhar_4_u
 

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LEARN ABOUT OPENDARWIN

vfork

VFORK(2)						      BSD System Calls Manual							  VFORK(2)

NAME

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

SYNOPSIS

     #include <unistd.h>

     pid_t
     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(2) 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(2) or an exit (either by a call to
     exit(2) 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 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 childs context from the procedure
     that 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 can't 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.)

SEE ALSO

     fork(2), execve(2), sigaction(2), wait(2),

DIAGNOSTICS

     Same as for fork.

BUGS

     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.

     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 ioctl(2) calls are allowed and input attempts result in an end-of-file indication.

HISTORY

     The vfork() function call appeared in 3.0BSD.

4th Berkeley Distribution					   June 4, 1993 					 4th Berkeley Distribution
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