Using pipes and structures with dynamically allocated variables in C

Question

I'm writting a program that needs to pass a matrix from a parent process to its child (that's why I'm using the fork() instruction). I've just read this and this to solve the problem myself, but I still can't understand how to use the read() and write() instructions with the pipe I've created so far. I know these instructions write series of bytes, but I'm not sure about using them with structures or dynamically allocated variables (like a matrix).

Here is the code I used to test (note the comments I put):

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

// Structure definition (Matrix)
typedef struct {
    int **mat;
    int rows;
    int cols;
} Matrix;

int main() {
    // Create the pipe
    int file_desc[2];
    if (pipe(file_desc) != 0) exit(1);

    // Create two processes
    if (fork() == 0) {
        /** Instructions for the child process */
        // Read the matrix structure from the pipe
        Matrix *received = NULL;
        read(file_desc[0], received, sizeof *received);
        if (received != NULL) {
            // Print the received matrix
            int i, j;
            printf("The matrix I've just *received* from the parent is:\n");
            for (i = 0; i < received->cols; i++) {
                for (j = 0; j < received->rows; j++) printf("%d\t", received->mat[i][j]);
                printf("\n");
            }
        } else printf("received = NULL :'(\n");
    } else {
        /** Instructions for the parent process */
        /* Create a matrix dinamically.
         * In fact, in the real program I have a function to create a matrix given the
         * rows and columns, and fill it with random values, so it returns a Matrix *
         * (pointer to Matrix), but for testing purposes I've only written this
         * (also useful if I need an array of Matrix elements, for example)
         * */
        Matrix *myMatrix = calloc(1, sizeof *myMatrix);

        // Put the contents into the variable
        myMatrix->rows = 2;
        myMatrix->cols = 2;
        myMatrix->mat = calloc(myMatrix->rows, sizeof *(myMatrix->mat));
        int i, j;
        for (i = 0; i < myMatrix->cols; i++)
            (myMatrix->mat)[i] = calloc(myMatrix->cols, sizeof **(myMatrix->mat));

        // Fill the matrix with some values (testing)
        (myMatrix->mat)[0][0] = 4;
        (myMatrix->mat)[0][1] = 2;
        (myMatrix->mat)[1][0] = 1;
        (myMatrix->mat)[1][1] = 3;

        // Print the matrix
        printf("The matrix I've just filled in the parent is:\n");
        for (i = 0; i < myMatrix->cols; i++) {
            for (j = 0; j < myMatrix->rows; j++) printf("%d\t", myMatrix->mat[i][j]);
            printf("\n");
        }

        // Write the matrix structure to the pipe (here is where I have the problem!)
        write(file_desc[1], myMatrix, sizeof *myMatrix);

        // Wait for the child process to terminate
        wait(0);
        printf("The child process has just finished, the parent process continues.\n");
    }
    return 0;
}

In fact, I tried first with a pointer to an int and it worked. But when I run this program, I receive this output:

The matrix I've just filled in the parent is:
4   2   
1   3   
received = NULL :'(
The child process has just finished, the parent process continues.

And I don't know why I get the NULL -- I'm almost sure I'm using the write() instruction incorrectly. Any help about this will be appreciated =)

EDIT: I think the matrix should be converted to text, for example, and then pass the string to the child, parse it and convert it to a Matrix structure again. I don't know if this approach is the best. Is there another approach besides this one?

EDIT: I tried the same code with a static variable (changing int **mat; to int mat[2][2]; inside the structure declaration) but the user should change the matrix size.

Answer 1

This is a serious problem:

Matrix *received = NULL;
read(file_desc[0], received, sizeof *received);

Received is a null pointer. That read is going to try to write data to NULL, which is an invalid address. It would be much simpler to write:

Matrix received;
read(file_desc[0], &received, sizeof received);