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]
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fork(2) System Calls Manual fork(2)
Name
fork - create a new process
Syntax
#include <sys/types.h>
#include <unistd.h>
pid = fork()
pid_t pid;
Description
The system call causes creation of a new process. The new process (child process) is an exact copy of the calling process except for the
following:
o The child process has a unique process ID.
o The child process has a different parent process ID (that is, the process ID of the parent process).
o The child process has its own copy of the parent's descriptors. These descriptors reference the same underlying objects, so that, for
instance, file pointers in file objects are shared between the child and the parent, so that a on a descriptor in the child process
can affect a subsequent read or write by the parent. This descriptor copying is also used by the shell to establish standard input
and output for newly created processes as well as to set up pipes.
o The child processes resource utilizations are set to 0. For further information, see
Return Values
Upon successful completion, 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.
Diagnostics
The system call fails and no child process are created under the following conditions:
[EAGAIN] The system-imposed limit {PROC_MAX} on the total number of processes under execution would be exceeded.
[EAGAIN] The system-imposed limit {CHILD_MAX} on the total number of processes under execution by a single user would be exceeded.
[ENOMEM] There is insufficient swap space for the new process.
See Also
execve(2), wait(2)
fork(2)