fork

Arguments

none

Returns

A positive task id that identifies the child process, and 0 that identifies the parent process to the child; or -1 if an error occurred. The errno is set if an error occurs.

Description

The fork function creates a new process identical to the calling (parent) process except for a unique process ID. The child process has a different parent process ID and its own copy of the parent file descriptors. The child process does not inherit outstanding signals.

Example

The following example illustrates using the fork function with if syntax. This is a useful way of separating the two, identical processes produced from fork. The first block of code applies to the parent process, while the second block applies to the child.

    #!/usr/cogent/bin/gamma
    
    if ((childID = fork()) > 0)
    {
      princ("P> My ID is: ", getpid(),"\n");
      princ("P> My child's ID is: ", childID, "\n");
      signal(SIGCHLD, #princ("P> Signal received that my child -- ",
                             childID, " -- has died.\n"));
      princ("P> Waiting for my child.\n");
      w = wait(childID);
      princ("P> wait() returned this: ", w, "\n");
    }
    else
    {
      sleep(2);
      if (childID == -1)
        error("C> An error occurred.\n");
      else
        {
          princ("C> I am the child process.\nC> My process ID is: ",
                getpid(), "\n");
          sleep(2);
          princ("C> Time to exit.\n");
          exit_program(3);
        }
    }

Will produce these results:

    P> My ID is: 1225
    P> My child's ID is: 1226
    P> Waiting for my child.

(after 2 seconds)

    C> I am the child process.
    C> My process ID is: 1226

(after 2 more seconds)

    C> Time to exit.
    P> Signal received that my child -- 1226 -- has died.
    P> wait() returned this: (1226 3 nil nil)

See the Starting Programs section of the Common Functions chapter (Tutorial One), or the Start a Process section of the Common Functions for Any Program chapter (Tutorial Two) in the Cogent Tools Demo and Tutorials book for examples of this function used in context.

See Also

exec, wait