why only my first x forks make the job (gcc)

Question

before you ask... this is from my study guide. From my perspective this is almost done but I can't put it working in the way I want. the exercise is: given an string fork X times and print one character per child until the the string finish.

this is the code and compiles:

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>
#include <dirent.h>
#include <string.h>
#include <sys/wait.h>
#include <fcntl.h>



// SOME PROGRAM CONSTANTS

#define CHILD_QUANTITY 5
#define FIFO_FILE "/tmp/printer.fifo"
#define STRING_TO_PRINT "hola mundo como estas!!!!"
#define DELAY_TIME 2 // two seconds

// SOME GLOBAL VARIABLES
int fdfifo, next_printer = 0, next_char = 0;
pid_t printers[CHILD_QUANTITY];



void process_call_printer(int sig) {
    char char_to_print;

    if (read(fdfifo, &char_to_print, sizeof(char)) == -1) {
     perror(__FUNCTION__);
    }
     // print the char
    printf("[%d] -> %c\n", getpid(), char_to_print);


    // alert the parent process about it
    kill(getppid(), SIGUSR2);

    // just wait for another signal
    while(1) {
     pause();
    } 

}

void process_call_hub(int sig) {
    pid_t printer;
    if (next_char < strlen(STRING_TO_PRINT)) {
     if (write(fdfifo, &STRING_TO_PRINT[next_char], sizeof(char)) == -1) {
      perror(__FUNCTION__);
     }
     alarm(DELAY_TIME);

     if (next_printer >= CHILD_QUANTITY) {
      next_printer = 0;
     }

     printer = printers[next_printer]; 
     next_printer++;
     next_char++;
     printf("sending char %c to the printer %d\n", STRING_TO_PRINT[next_char - 1], next_printer - 1);
     kill(printer, SIGUSR1);
    }
    else {
     kill(getpid(), SIGQUIT);
    }

}

void process_callback(int sig) {
//  alarm(0);
    printf("a callback function call\n");
//  kill(getpid(), SIGALRM);

//  while(1) {
//   pause();
//  }
}

void system_shutdown(int sig) {
    printf("SIGQUIT recived...terminating\n");

    if (unlink(FIFO_FILE) == -1) {
     perror(__FUNCTION__);
    }

    close(fdfifo);
}


int main(void) {
    pid_t pid;

    int i;

    if (mkfifo(FIFO_FILE, 0777) == -1) {
     perror("pipe()");
     return -1;
    }

    fdfifo = open(FIFO_FILE, O_RDWR, 0777);

    if (fdfifo == -1) {
     perror("open()");
     return -2;
    }


    for (i = 0; i < CHILD_QUANTITY; i++) {
     pid = fork();
     printers[i] = pid;

     switch(pid) {
      case -1:
       perror("Error\n");
      break;
      case 0:
       // printer
       signal(SIGUSR1, process_call_printer);
       while(1) {
        pause();
       }

      break;
      default:
       // hub
       // do nothing.. we will figure out later...
      break;
     }

    }

    signal(SIGALRM, process_call_hub);
    signal(SIGQUIT, system_shutdown);
    signal(SIGUSR2, process_callback);

    alarm(DELAY_TIME);

    while(1) {
     pause();
    }


}

and this is the output I got

gabriel@GaboMac:20090918$ ./threaded_printer 
sending char h to the printer 0
[1397] -> h
a callback function call
sending char o to the printer 1
[1398] -> o
a callback function call
sending char l to the printer 2
[1399] -> l
a callback function call
sending char a to the printer 3
[1400] -> a
a callback function call
sending char   to the printer 4
[1401] ->  
a callback function call
sending char m to the printer 0
sending char u to the printer 1
sending char n to the printer 2
sending char d to the printer 3
sending char o to the printer 4
sending char   to the printer 0
sending char c to the printer 1
sending char o to the printer 2
sending char m to the printer 3
sending char o to the printer 4
sending char   to the printer 0
sending char e to the printer 1
sending char s to the printer 2
sending char t to the printer 3
sending char a to the printer 4
sending char s to the printer 0
sending char ! to the printer 1
sending char ! to the printer 2
sending char ! to the printer 3
sending char ! to the printer 4
SIGQUIT recived...terminating

all echos should be like the five first ones..

any idea ?

thank you

Answer 1

Firstly, this is terrible code. You should never do real work in a signal handler. This program has its main loop in a signal handler - not good!

The problem is that your child processes do their work in the process_call_printer() signal handler, but that function never returns. It ends with

// just wait for another signal
while(1) {
    pause();
}

Well, it's going to wait forever for another signal because a signal is blocked while it is being handled. So your child is not going to receive any more SIGUSR1s until it's finished processing the first one - and it never does.

That's why your child processes stop responding after they've handled the first signal.

Now, seriously. Go and rewrite this with the bare minimum of signal handling. It's usually done something like this...

int got_signal;

void handler(int) {
    got_signal = 1;
}


int main() {
    ...
    /* Wait for signal */
    got_signal = 0;
    while(!got_signal) {
        sleep(1);
    }
    /* Signal has arrived - do something... */
    ...
}