LinuxQuestions.org - Fork() problem

- Programming (https://www.linuxquestions.org/questions/programming-9/)

- - Fork() problem (https://www.linuxquestions.org/questions/programming-9/fork-problem-241079/)

Hello,
I'm writing an daemon C-program for linux but I got a strange problem with fork(). When I call an function located in another c-file the program just dissappears...

This is the way I've done it:

in main.c I fork() and if PID > 0 I exit
after some system-logging and SID recieving i start a while(1) loop
in the loop I call for action(&value) which is in action.c
the action() is ran but when it should return to the main while(1) loop it just dies, like it is running exit(0) instead of return(0) :scratch:

I define prototypes of all functions in a common header file (header.h) which I include in all c-files. If i comment the action(&value) part, it will run on and on in the while(1) loop until I kill it with a SIGKILL.

Am I using fork wrong here or is fork messing with me? As far as I understand with my scills in C this should work just fine since I'm including this header file with prototypes an link the compiled files as supposed to. The compiler is not complaining about anything...

I cannot find anything about this kind of problems anywhere :study:

What exactly does action() do? Can you post the code?

The reason I ask is because weird things seem to happen with I/O in fork()'d processes. Like this:

Code:

itsme@itsme:~/C$ cat fork.c

#include <stdio.h>

#include <string.h>

#include <stdlib.h>

#include <unistd.h>

#include <errno.h>



void action(int *num)

{

  int ch;



  printf("Call #%d. Press ENTER to continue.\n", *num);

  ch = getchar();

  if(ch == EOF)

  {

    printf("Error! %s\n", strerror(errno));

    *num = -1;

  }

  else

    (*num)++;

}



int main(void)

{

  int ctr = 0;



  if(fork())

    exit(EXIT_SUCCESS);



  while(1)

  {

    action(&ctr);

    if(ctr == -1)

      break;

  }



  return EXIT_SUCCESS;

}

This is what I get when I run it:

Code:

itsme@itsme:~/C$ ./fork

Call #0. Press ENTER to continue.

itsme@itsme:~/C$ Call #1. Press ENTER to continue.

Error! Input/output error

I press ENTER after the Call #0 line and then the Call #1 line comes up but then displays that error right away. However, if I comment out the 2 lines that deal with fork()'ing in main() the loop keeps happening as expected.

Instead of this:

Code:

 if (fork() >0) {

  exit();

}

Try this:

Code:

if (daemon(0,0) < 0) {

    perror("daemon: ");

    exit(1);

}

Or this:

Code:

if ((fork()) != 0)

        exit(0);

setsid();

if ((fork()) != 0)

    exit(0);

Thanx for the reply, I was going to post the code when I noticed an declaration I hade

Code:

FILE *fp;

and later in the code

Code:

fclose(fp);

Some old garbage that had left there when I copy-pasted the structure of my function from an old program :tisk:

My bad :)

I would have expected some complains from the gcc though..