Question

I have to code a simple C application that creates a process and a child (fork()) and I have to do an operation. Parent initializes the values and child calculates. I write this:

#include <stdlib.h>
#include <signal.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

typedef struct {
    int op1;
    char op;
    int op2;
} Operation;

Operation *varOP;

void finalResult()
{
    float result = 0;
    if(varOP->op == '+') result = (varOP->op1 + varOP->op2);
    if(varOP->op == '-') result = (varOP->op1 - varOP->op2);
    if(varOP->op == '*') result = (varOP->op1 * varOP->op2);
    if(varOP->op == '+') result = (varOP->op1 / varOP->op2)
    printf("%f",result);
}

int main () {
    int p;
    varOP  = (Operation *)malloc(sizeof(Operation));
    p = fork();
    if(p == 0) // If child
    {
        signal(SIGUSR1, finalResult );
        pause();
    }

    if(p > 0) // If parent
    {
        varOP->op = '+';
        varOP->op1 = 2;
        varOP->op2 = 3;
        kill(p, SIGUSR1);
        wait(NULL);
    }
    return 0;
}

But my child is never called. Is there something wrong with my code?
Thanks for your help!

Answer 1

Your sample code also has a more fundamental problem: each process has its own data space, and so your technique of sending information to the child via the heap will not work. One solution is to use a pipe. This adds only four more lines to your code:

typedef struct {
    int op1;
    char op;
    int op2;
}Operation;

Operation *varOP;

static int pipe_fds[2]; /* <-- added */

static void finalResult(void)
{
    float result = 0;
    read(pipe_fds[0], varOP, sizeof(Operation)); /* <-- added */
    if(varOP->op == '+') result = (varOP->op1 + varOP->op2);
    if(varOP->op == '-') result = (varOP->op1 - varOP->op2);
    if(varOP->op == '*') result = (varOP->op1 * varOP->op2);
    if(varOP->op == '/') result = (varOP->op1 / varOP->op2); /* <-- typo */

    printf("%f\n",result);
}

int main (void) 
{
    int p;
    pipe(pipe_fds); /* <-- added */
    varOP = (Operation *)malloc(sizeof(Operation)); 
    p = fork();

    if(p == 0) // If child
    {
        signal(SIGUSR1, finalResult );
        pause();
    }

    if(p > 0) // If parent
    {
        varOP->op = '+';
        varOP->op1 = 2;
        varOP->op2 = 3;
        write(pipe_fds[1], varOP, sizeof(Operation)); /* <-- added */
        kill(p, SIGUSR1);
        wait(NULL);
    }

    return 0;
}