fork()

["Ronald Landheer" <ronald at landheer dot com>]

Hello all,

Being used to DJGPP - not CygWin - I may have lost my hand on the forking
idea a while back, so I thought I'd try the following piece of code:

-- BEGIN CODE SNIPLET --
#include <stdio.h>
#include <unistd.h>

int main(void) {
  int rVal, i;
  rVal = fork();
  if (!rVal) {
    for (i = 0; i < 100; i++) printf("0");
  } else {
    for (i = 0; i < 100; i++) printf("1");
  } // if
  printf("\n");

  return(0);
} // main()
--- END CODE SNIPLET ---
This compiles without warnings or errors (phew!) but produces an output
somewhat different than I expected:
-- BEGIN OUTPUT SNIPLET --
0000000000000000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000001111111111111111111111111111111111111111111111111111
111111111111111111111111111111111111111111111111
--- END OUTPUT SNIPLET ---
I'd expected the 0's and 1's to be jumbled..
I tried the same with the following Perl script: (please don't mind the
syntax - I'm not a Perl programmer..)
-- BEGIN CODE SNIPLET --
#!/usr/bin/perl
unless (defined($pid = fork)) {
  die("Cannot fork: $!");
}
$i = 0;
unless ($pid) {
  while ($i < 100) {
    printf("0");
    $i = $i + 1;
  }
  exit;
}
while ($i < 100) {
  printf("1");
  $i = $i + 1;
}
--- END CODE SNIPLET ---
expecting the same output as the C thingy, but no:
-- BEGIN OUTPUT SNIPLET --
1111111111111111111111111111111111111111111111111111111111111111111111111111
1111111111111111111111110000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000
--- END OUTPUT SNIPLET ---

So here's my questions:
  1. does fork() really make a new process?
  2. does the new process (if made at all) run simultaniously with the
     old one?
  3. if so, why this output?
  4. why is there a difference between the Perl and C programs'
     outputs? (please don't laugh too loud if it's because of my Perl
     inabilities..)

Thanx!

Ronald



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