C - fork() & execl() & loop + "half pyramid output"

Question

I have a troubles with the following. Let's say, I have a two programs, one is "input.c" and second is "output.c". Output is a simple one and looks like this (I will paste only the most important passage).

outputbin.c

//
char buffer[512];
strncpy(buffer, argv[1], sizeof(buffer));
printf("Your output is: %s\n", buffer);
//

And this is the main passage from my input.c:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(int argc, char *argv[])
{
    pid_t pid;
    char *charchar = "\x41";
    int status;
    char *outputbin;
    int i, j, iterations;

    if(argc < 2)
    {
        fprintf(stderr, "Usage: %s <iterations> <outputbin>\n", argv[0]);
        exit(0);
    }

    iterations = atoi(argv[1]);
    outputbin = argv[2];

    pid = fork();

    if(pid != 0)
    {       
        waitpid(-1, &status, 0);
    }

    if(pid == 0)
    {
        for(i=0; i < iterations; ++i)
        {
            for(j = 0; j <= i; ++j)
            {   
                printf("%s", charchar);                         
                //execl(outputbin, outputbin, charchar, NULL);              
            }
            printf("\n");
        }
    }
    return 0;
}

When I compile this program using gcc and do this (without argv[2]): ./input 10 I get this:

A
AA
AAA
AAAA
AAAAA
AAAAAA
AAAAAAA
AAAAAAAA
AAAAAAAAA
AAAAAAAAAA

It's okay, but only till I remove this piece of code - "printf("%s", input);" and uncomment "execl", so:

for(j = 0; j <= i; ++j)
                {                                                   
                    execl(output, output, input, NULL);             
                }

And run: ./a.out 10 ./outputbin

I get only this - Your output is: A

Only first char and that's all. How to let is execute whole "half pyramid" of strings line by line? No matter what I tried, everything end with the same result.

Your code and question is a little confusing. I suggest post the working code in block and post the problematic code in another block. Also, please include the entire `main` function. That make it simpler to follow what your code is doing. — R Sahu, Feb 15 '15 at 19:37
What's the `while` loop supposed to be doing? It's useless for 2 completely separate reasons (you have a nested loop incrementing `i` and using the same termination test, and the `exit` at the end of the loop prevents the `while` condition from being tested again anyway) — , Feb 15 '15 at 19:38
@RSahu Okay, sorry. I edited it and added whole code of input.c file. — Yeez, Feb 15 '15 at 19:44
@WumpusQ.Wumbley I just tried something with the while loop, but still, it doesn't matter so much. Because it's the same result - with and without while. — Yeez, Feb 15 '15 at 19:46

score 0 · Answer 1 · answered Feb 15 '15 at 19:42

0

Assuming it executes successfully, execl does not return - it replaces the program running in the process with another program. So when calling:

for(j = 0; j <= i; ++j)
{                                                   
    execl(output, output, input, NULL);             
}

The loop will only iterate one time - after execl is called, outputbin will start executing. If you want to keep the loop logic, you should move it to outputbin.c so that it'll be executed there.

answered Feb 15 '15 at 19:42

Daniel Kleinstein

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It looks like a good idea, I will definitely try it. But for now, I am looking for solution, how to execute it directly into "outputbin" from "input", if it's possible. – Yeez Feb 15 '15 at 19:49
@Yeez No, it's not possible - once you call `execl`, execution will *not* return to the program. You should move the loop logic to `outputbin.c` (you have all the necessary parameters). – Daniel Kleinstein Feb 15 '15 at 19:50
As I see now, it's not the real solution I am looking for... What about some other solution with a another function instead of execl? – Yeez Feb 15 '15 at 20:01
@Yeez It's kind of hard to answer that without knowing exactly what you're trying to do (why are you even calling `outputbin`?).. You could fork a process from inside the loop, have that process run `outputbin` with appropriate parameters, and then wait until the forked process finishes before continuing. – Daniel Kleinstein Feb 15 '15 at 20:04
Ok, I don't know, if I understand how you mean it or honestly - I don't know how to implement it. I'm calling "outputbin", because I want to send whole strings into some external C binary, in this case outputbin, but it doesn't matter so much, it can be any other. – Yeez Feb 15 '15 at 20:50
1

@Yeez Using `exec` is the wrong way to go if you want to run a program and then resume execution in your main program. Fork a process, use `exec` in the forked process to run your external binary, and wait for the forked process to finish (using the `wait()` function) before resuming your main program's execution. – Daniel Kleinstein Feb 15 '15 at 20:59
1

Ah, yes.. I checked today some more informations about fork(), now I think, I know how you mean it. Thanks a lot for showing me the right way. – Yeez Feb 16 '15 at 17:09

C - fork() & execl() & loop + "half pyramid output"

1 Answers1