Array elements are "lost" outside the function

Question

I have a matrix in a file like:

3
1 2 3
4 5 6
7 8 -9

where the first line indicates the square matrix order. I'm using the following code to read the file and store it into a vector (I have removed all if checks for sake of simplicity):

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

int read_matrix_file(const char *fname, double *vector)
{
    /* Try to open file */
    FILE *fd = fopen(fname, "r");
    char line[BUFSIZ];
    fgets(line, sizeof line, fd);

    int n;
    sscanf(line, "%d", &n)

    vector = realloc(vector, n * n * sizeof *vector);
    memset(vector, 0, n * n * sizeof *vector);

    /* Reads the elements */
    int b;
    for(int i=0; i < n; i++) {
        // Read the i-th line into line
        if (fgets(line, sizeof line, fd) == NULL) {
            perror("fgets");
            return(-1);
        }

        /* Reads th j-th element of i-th line into the vector */
        char *elem_ptr = line;
        for (int j=0; j < n; j++) {
            if(sscanf(elem_ptr, "%lf%n", &vector[n*i+j] , &b) != 1) {
                perror("sscanf");
                return(1);
            }
            elem_ptr += b;
        }
    }
    fclose(fd);

    /* HERE PRINTS OK */
    for(int i=0; i<n*n; i++)
        printf("%i %f\n",i, vector[i]);

    return n;
}

The read_matrix_file receives a filename and an array of doubles and fill the array, returning the matrix order. The expected usage can be seen in this code block.

int main(void)
{

    const char *fname = "matrix.txt";
    double *vector = malloc(sizeof * vector);

    int n = read_matrix_file(fname, vector);

    /* Here prints junk */
    for(int i=0; i<n*n; i++)
        printf("%i %f\n",i, vector[i]);

    free(vector);
}

The issue is, the printf works fine inside the read_matrix_file but seems invalid in main.

I'm allocating the array outside the function and passing it by "reference", but I'm very suspecious of realloc, unfortunatelly I don't know how to fix or a better approach.

Answer 1

You are reallocating memory inside read_matrix_file() and storing the elements of the matrix in that memory region. But when you come out of the function, since the pointer vectoris a local variable, its new value is lost when you leave the function.

When you come back inside main() vector still points to the (now likely invalid) memory region that you had previously allocated with malloc().

You should either allocate large enough memory before calling read_matrix_file or pass a double pointer (**) if you want to modify the pointer and see the change reflected back in main()

What I meant is something like this:

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

int read_matrix_file(const char *fname, double **p_vector)
{
    /* Try to open file */
    FILE *fd = fopen(fname, "r");
    char line[BUFSIZ];
    fgets(line, sizeof line, fd);

    int n;
    sscanf(line, "%d", &n);

    *p_vector = realloc(*p_vector, n * n * sizeof **p_vector);
    double *vector = *p_vector;

    memset(vector, 0, n * n * sizeof *vector);

    /* Reads the elements */
    int b;
    for(int i=0; i < n; i++) {
        // Read the i-th line into line
        if (fgets(line, sizeof line, fd) == NULL) {
            perror("fgets");
            return(-1);
        }

        /* Reads th j-th element of i-th line into the vector */
        char *elem_ptr = line;
        for (int j=0; j < n; j++) {
            if(sscanf(elem_ptr, "%lf%n", &vector[n*i+j] , &b) != 1) {
                perror("sscanf");
                return(1);
            }
            elem_ptr += b;
        }
    }
    fclose(fd);

    /* HERE PRINTS OK */
    for(int i=0; i<n*n; i++)
        printf("%i %f\n",i, vector[i]);

    return n;
}

In main, call it with:

int n = read_matrix_file(fname, &vector);

EDIT: Please note that this code does not handle the failure of realloc() properly.