尝试从子标准输出读取时，简单的多子执行器执行的segfaulting

Question

I set up a simple fork example to fork and exec 2 children then read their output via pipes however I have a segfault (occuring at close(piping[i][1]);) and cannot find out why. See Code Below:

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

int reader(int *piping[]) {
    char readstring[50];
    char readstring2[50];
    FILE *outPut;
    FILE *outPut2;
    outPut = fdopen(piping[0][0], "r");
    outPut2 = fdopen(piping[1][0], "r");
    fgets(readstring, sizeof(readstring), outPut);
    fgets(readstring2, sizeof(readstring2), outPut2);
    printf("%s\n", readstring);
    printf("%s\n", readstring2);
    return 0;
}

int main(void) {
    int i;
    int j;
    int pid;
    int **piping = malloc(sizeof(int*) *2);
    for(i = 0; i < 2; ++i) {
        piping[i] = malloc(sizeof(int) *2); 
    }   
    for(j = 0; j < 2; ++j) {
        pipe(piping[j]);
        if((pid = fork()) == -1)   
            fprintf(stderr, "error reading pipe\n");
            exit(1);
        } else if(pid == 0) {
            //child
            //close read pipes dup2 write pipes to stdout
            //then close old write pipes
            close(piping[j][0]);
            if(dup2(piping[j][1], 1) == -1) {
                fprintf(stderr, "error dup2");
                exit(2);
            }
            close(piping[j][0]);
            close(1);
            if(execlp("./playex", "playex", NULL)) {
                fprintf(stderr, "exec error\n");
            }
        } else {
            //parent
            close(piping[j][1]);
        }
    }
    reader(piping);
}

Above is the main function that pipes and execs then should go to read, below I have the basic program it runs.

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

int main(void) {
    fprintf(stdout, "go\n");
}

I've tried to fix the segfault with no luck, please help with locating and fixing the issue.

Answer 1

Your main problem was (before you edited the question to hide what it originally showed) that you were writing outside the bounds of your array of pipes:

for(i = 0; i < 2; ++i) {
    piping[i] = malloc(sizeof(int) *2); 
}   
for(j = 0; j < 2; ++j) {
    pipe(piping[i]);

You were indexing the second loop on j , not i , so you were using piping[2] which is not allocated. Rinse and repeat.

You could have avoided the error by removing the current definitions of i and j and using:

for (int i = 0; i < 2; ++i) {
    piping[i] = malloc(sizeof(int) *2); 
}   
for (int j = 0; j < 2; ++j) {
    pipe(piping[i]);

Now i would be undefined in the second loop.