I'm trying to solve one of my excercise for an university exam but I'm stuck with a memory error that I can't really figure it out. I have to alloc and inizialise a matrix of float having as matrix a struct called Mat.

typedef struct {
  int rows; 
  int cols;
  float **row_ptrs;
} Mat; 

To do so I have to write a function who returns the address of the matrix. The matrix has to be an array of pointer to the rows of the matrix:

Mat* Mat_alloc(int rows, int cols){
        Mat* matrice = (Mat*)calloc(rows,sizeof(float**));
        float** righe = (float**)calloc(cols,sizeof(float*));
        (*matrice).row_ptrs=righe;
        (*matrice).rows=rows;
        (*matrice).cols=cols;
        *righe = (float*)malloc(sizeof(float));
        float* elem = *righe;
        Mat* sav_ptr = matrice; 
        int i,j;
        for(i=0;i<rows;i++){
            printf("questa e' la riga: %d con indirizzo: %x\n\n",i,righe);
            for(j=0;j<cols;j++){
                *elem = 0.0;
                printf("%f io sono l'elemento: %d con indirizzo: %x \n",*elem,j,elem);
                elem++;
                }
            printf("\n");
            righe++;
            j=0;
            }
        return sav_ptr;
    }

Also i have to free everything after the function is called and runned, I've wrote this function to do it:

void Mat_free(Mat *m){
    float** tofree = (*m).row_ptrs;
    free(*tofree);
    free(tofree);
    free(m);
    }

When I run the program under valgrind after the first print output i get this:

==6688== Memcheck, a memory error detector
==6688== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==6688== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
==6688== Command: ./e1
==6688== 
Avvio Mat_alloc...

questa e' la riga: 0 con indirizzo: 4a4c4f0

0.000000 io sono l'elemento: 0 con indirizzo: 4a4c560 
==6688== Invalid write of size 4
==6688==    at 0x109311: Mat_alloc (in /home/sam/Scrivania/esercitazione 4 TDP/e1)
==6688==    by 0x1091FE: main (in /home/sam/Scrivania/esercitazione 4 TDP/e1)
==6688==  Address 0x4a4c564 is 0 bytes after a block of size 4 alloc'd
==6688==    at 0x483874F: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==6688==    by 0x1092BC: Mat_alloc (in /home/sam/Scrivania/esercitazione 4 TDP/e1)
==6688==    by 0x1091FE: main (in /home/sam/Scrivania/esercitazione 4 TDP/e1)
==6688== 
==6688== Invalid read of size 4
==6688==    at 0x109319: Mat_alloc (in /home/sam/Scrivania/esercitazione 4 TDP/e1)
==6688==    by 0x1091FE: main (in /home/sam/Scrivania/esercitazione 4 TDP/e1)
==6688==  Address 0x4a4c564 is 0 bytes after a block of size 4 alloc'd
==6688==    at 0x483874F: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==6688==    by 0x1092BC: Mat_alloc (in /home/sam/Scrivania/esercitazione 4 TDP/e1)
==6688==    by 0x1091FE: main (in /home/sam/Scrivania/esercitazione 4 TDP/e1)
==6688== 

But then I do not get a sigsegv signal and the program keeps on running. What am I getting wrong? I found this even weirder because the free doesn't generate any error and all the block are free'd. Do you guys have any suggestion?

Her's the main function:

int main(int argc, char **argv) {
      /********************************************************
       *             TEST Mat_alloc/Mat_read                  *
       ********************************************************/
      printf("Avvio Mat_alloc...\n\n");
      Mat *m1 = Mat_alloc(5,5);
      float** try = (*m1).row_ptrs;
      printf("%f\n",**try);
      printf("fatto alloc\n");
      Mat_free(m1);
      printf("fatto free\n");

      return 0;
    }

1 Answers

0
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You should allocate memory for mat struct, because you return pointer to that struct.

Try this:

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

typedef struct {
    int rows;
    int cols;
    float **arr;
} Mat;

Mat* Mat_alloc(int rows, int cols){
    // memory allocation for struct Mat
    Mat* matrice = (Mat*)malloc(sizeof(Mat));
    // memory allocation for a given number of rows of our two-dimensional array
    matrice->arr = (float **)malloc(rows * sizeof(float *));
    int i = 0; int j = 0;
    // each row is a one-dimensional array, so we must allocate memory for each element
    for (i = 0; i < rows; ++i)
        matrice->arr[i] = (float *)malloc(cols * sizeof(float));

    (*matrice).rows=rows;
    (*matrice).cols=cols;

    for(i=0;i<rows; ++i){
        printf("questa e' la riga: %d con indirizzo: %p\n\n",i,&matrice->arr[i]);
        for(j=0;j<cols; ++j){
            matrice->arr[i][j] = j;
            printf("%f io sono l'elemento: %d con indirizzo: %p \n",matrice->arr[i][j],j,&matrice->arr[i][j]);
        }
        printf("\n");
    }
    return matrice;
}

void Mat_free(Mat *m){
    int i = 0;
    for(i=0;i< m->rows; ++i) {
        free(m->arr[i]);
    }

    free(m->arr);
    free(m);
}

int main(int argc, char **argv) {
    /********************************************************
     *             TEST Mat_alloc/Mat_read                  *
     ********************************************************/
    printf("Avvio Mat_alloc...\n\n");
    Mat *m1 = Mat_alloc(3,3);
    printf("fatto alloc\n");
    Mat_free(m1);
    printf("fatto free\n");

    return 0;
}