fork()

fork()

[Ronald Landheer]

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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Re: fork()

[Corinna Vinschen]

On Mon, Jul 16, 2001 at 02:59:18PM +0200, Ronald Landheer wrote:
> 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..

Your example is wrong. The output to stdout is line buffered. That means,
the actual output is generated only after the last `printf("\n");

Add a `setbuf(stdout, NULL)' right before the fork() and your
little application will suddenly behave as you've expected...

Corinna

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Red Hat, Inc.

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Re: fork()

[Warren Young]

Corinna Vinschen wrote:
> 
> Your example is wrong. The output to stdout is line buffered. That means,

Another problem is, the code depends on the OS's multitasking
characteristics.  There's nothing explicit in the code that says the OS
has to take the time slice away from the parent or the child.  If you
threw in a line like this:

	if (rand() % 2) sleep(0);

after each print statement, you might reasonably expect randomness in
the output.
-- 
= Warren -- ICBM Address: 36.8274040 N, 108.0204086 W, alt. 1714m

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fork()

[unknown user]

I'm trying to compile a Linux program that uses fork() ... it just crashes 
if i compile it with fork() so i have to comment out fork().. are there 
anyway to simulate fork() function under Windows 95/98 so it runs in 
background as it's suppose to like on Linux? Any help would be thankful.


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Re: fork()

[Chris Faylor]

On Sat, Jul 31, 1999 at 12:28:20AM -0500, Mumit Khan wrote:
>"unknown user" <jwl1980@hotmail.com> writes:
>> I'm trying to compile a Linux program that uses fork() ... it just crashes 
>> if i compile it with fork() so i have to comment out fork().. are there 
>> anyway to simulate fork() function under Windows 95/98 so it runs in 
>> background as it's suppose to like on Linux? Any help would be thankful.
>
>Why don't show us what you're trying to do? fork() in general does work
>fine under Cygwin b20.1; there're certainly bugs, but you need to come up
>with a testcase shows that problem. 

There are bugs in fork?  Hmm.

cgf

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Re: fork()

[Mumit Khan]

"unknown user" <jwl1980@hotmail.com> writes:
> I'm trying to compile a Linux program that uses fork() ... it just crashes 
> if i compile it with fork() so i have to comment out fork().. are there 
> anyway to simulate fork() function under Windows 95/98 so it runs in 
> background as it's suppose to like on Linux? Any help would be thankful.

Why don't show us what you're trying to do? fork() in general does work
fine under Cygwin b20.1; there're certainly bugs, but you need to come up
with a testcase shows that problem. 

Regards,
Mumit


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Re: fork()

[Chris Faylor]

On Sat, Jul 31, 1999 at 12:28:20AM -0500, Mumit Khan wrote:
>"unknown user" <jwl1980@hotmail.com> writes:
>> I'm trying to compile a Linux program that uses fork() ... it just crashes 
>> if i compile it with fork() so i have to comment out fork().. are there 
>> anyway to simulate fork() function under Windows 95/98 so it runs in 
>> background as it's suppose to like on Linux? Any help would be thankful.
>
>Why don't show us what you're trying to do? fork() in general does work
>fine under Cygwin b20.1; there're certainly bugs, but you need to come up
>with a testcase shows that problem. 

There are bugs in fork?  Hmm.

cgf

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Re: fork()

[Mumit Khan]

"unknown user" <jwl1980@hotmail.com> writes:
> I'm trying to compile a Linux program that uses fork() ... it just crashes 
> if i compile it with fork() so i have to comment out fork().. are there 
> anyway to simulate fork() function under Windows 95/98 so it runs in 
> background as it's suppose to like on Linux? Any help would be thankful.

Why don't show us what you're trying to do? fork() in general does work
fine under Cygwin b20.1; there're certainly bugs, but you need to come up
with a testcase shows that problem. 

Regards,
Mumit


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fork()

[unknown user]

I'm trying to compile a Linux program that uses fork() ... it just crashes 
if i compile it with fork() so i have to comment out fork().. are there 
anyway to simulate fork() function under Windows 95/98 so it runs in 
background as it's suppose to like on Linux? Any help would be thankful.


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fork()

[A Dark Elf]

I have a problem with threading. I use fork() so my app listens for
connections from a remote host, accept() it, forks, the parent loops and
closes the sockfd from the child and listens again:

  for(;;)
  {
   if(sockfd!=(int)NULL) close(sockfd);
   sockfd = accept(sockfd2, (struct sockaddr *)&address, &len);
   if(pid=fork()) break;
  }

Now it's telling me this error: child died before initialization..
Am I doing something wrong here ?


 




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