Child-parent process can't operate with signals

Question

I have a problems to communicate a signal from parent process to child, cause its like the child didnt open correctly cause when i do if(oct_pid == 0) i cant enter in.

calculator.c

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


void octal(int fd){
    int valor;
    while (1)
    {
        pause();
        read(fd,&valor,sizeof(int));
        printf("%o",valor);
    }
}

void hexa(int fd){
    int valor;
    while (1)
    {
        pause();
        read(fd,&valor,sizeof(int));
        printf("%X",valor);
    }
}


int main(){
    int canal,pipe_fd[2],octal_pid,hexa_pid;

    unlink("canal");

    if(mkfifo("./canal", 00660)!=0){
        printf("Error al crear la pipe");
        exit(EXIT_FAILURE);
    }

    canal = open("./canal",O_RDONLY);
    if(canal < 0){
        printf("Error al obrir la pipe.");
        exit(EXIT_FAILURE);
    }
    
    pipe(pipe_fd);

    octal_pid = fork();

    if(octal_pid == -1){
        printf("Error al crear el proceso octal.\n");
        exit(EXIT_FAILURE);
    } else if(octal_pid == 0){
        close(pipe_fd[1]);
        octal(pipe_fd[0]);
        signal(SIGUSR1,octal);
    } else {
        hexa_pid = fork();
        if(hexa_pid == -1){
            printf("Error al crear el proceso hexadecimal.\n");
            kill(octal_pid, SIGTERM);
            exit(EXIT_FAILURE);
        } else if(hexa_pid == 0){
            close(pipe_fd[1]);
            hexa(pipe_fd[0]);
            signal(SIGUSR1,hexa);
        } else {
            close(pipe_fd[0]);
            while(1){
                char c;
                int valor;
                read(canal,&c,sizeof(char));
                read(canal,&valor,sizeof(int));
                if(c == 'o'){
                    printf("Calculant en octal ... \n");
                    write(pipe_fd[1], &valor, sizeof(int));
                    kill(octal_pid,SIGUSR1);
                }
                else if(c == 'h'){
                    printf("Calculant en hexadecimal ... \n");
                    write(pipe_fd[1], &valor, sizeof(int));
                    kill(hexa_pid,SIGUSR1);
                }
                else if( c == 'x'){
                    printf("Sortint...\n");
                    kill(octal_pid, SIGTERM);
                    kill(hexa_pid, SIGTERM);
                    break;
                }   
            }
            close(canal);
            close(pipe_fd[1]);
            waitpid(octal_pid, NULL, 0);
            waitpid(hexa_pid, NULL, 0);
        }
    }
    return 0;
}

client.c

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

int fd, valor;
char operacio;

int llegir_numero()
{
    char buff [10]; 
    int llegits;
    llegits=read(0, buff, 10); //llegir del teclat un màxim de 10 chars
    buff[llegits-1] = (char)0; //canvi del \n per un \0
    return(atoi(buff)); //es converteix a enter
}

int main(){

    fd = open("./canal",O_WRONLY);

    if(fd < 0){
        printf("La pipe no s'ha obert correctament.");
        exit(EXIT_FAILURE);
    }

    do{
        printf("Introdueix 'o' o 'h', x per acabar\n");
        scanf(" %c",&operacio);

        if(operacio == 'o' || operacio == 'h'){
            printf("Introdueix un valor enter positiu:\n");
            valor = llegir_numero();
            write(fd,&operacio,sizeof(char));
            write(fd,&valor,sizeof(int));
        }
        else if(operacio != 'x'){
            printf("Lletra incorrecte.");
        }
    }
    while(operacio != 'x');
    close(fd);
    return 0;
}

I have this code where I have a problem with kill(octal_pid,SIGUSR1), when, in theory, I send the signal to the child process octal_pid, it didnt work cause I cant enter in this pipe and idk why. Thanks for the help

Answer 1

Besides the problems Andrew Henle notes, your two children that you fork will exit immediately after registering a signal handler. Timing is uncertain, but it is likely that they'll be dead by the time the signal is sent to them, so the signal will do nothing.

Aside from that, the signal handler will call pause and wait for a second signal before actually doing anything.

To avoid these problems, you should have the while (1) pause(); loop in main for the children after calling signal, and should have neither a loop nor a pause call in the signal handlers.

Answer 2

First, read this:

What is the difference between sigaction and signal?

You really need to use sigaction() on POSIX systems instead of signal().

Second, given

void octal(int fd){
    int valor;
    while (1)
    {
        pause();
        read(fd,&valor,sizeof(int));
        printf("%o",valor);
    }
}

this is wrong:

    signal(SIGUSR1,octal);

because the int argument that gets passed to a signal handler is the value if the signal - not some file descriptor. You need to find another way to pass the proper file descriptor values to your signal handler.

Also, you need to install the signal handler (using sigaction()) before you call fork(), or your child process could very well send signals and even complete running before the parent process gets its signal handler(s) installed.

Finally, multiple signals of the same type can be coalesced into one signal, so if your child process sends, for example, 8 SIGUSR1 signals, the parent process might only get one of them. Note also this question and answer: Blocking signals in C