segmentation fault on simple loop - c

Question

I'm trying to write a program that calculates the number of elements of given matrix,for example M[2][3] = [1,2,3][3,4,5]
should give the number of elements from 0 trough 9 in a nxm array.
The algorithm is correct up until line 34(M[i][j] = i + 2;) in which I'm having a segmentation fault.
What am I doing wrong?
#include<stdio.h>
#include<stdlib.h>
#define ROW 10
#define COL 10
#define MAX 10
void Print_Matrix(int **M,int row,int col);
int MallocX(int **M,int row,int col);
int main(void)
{
int **M = 0x0;
int count[MAX] = {0};
int i,j;
if(MallocX(M,ROW,COL)){
fprintf(stdout,"Could not allocate memory\n");
exit(1);
}
for(i = 0;i<ROW;i++){
for(j = 0;j<COL;j++){
M[i][j] = i + 2;
}
}
Print_Matrix(M,ROW,COL);
for(i = 0;i<ROW;i++){
for(j = 0;j<COL;j++){
++count[M[i][j]];
}
}
for(j = 0;j<MAX;j++){
if(count[j]){
printf("%d %d\n",j,count[j]);
}
}
for(i = 0;i<ROW;i++){
free(M[i]);
}
free(M);
}
int MallocX(int **M,int row,int col)
{
int i;
M = (int **) malloc(row * sizeof(int *));
if(M == NULL){
fprintf(stderr,"Error allocating memory\n");
free(M);
return 1;
}
for(i = 0;i<row;i++){
M[i] = (int *) malloc(col * sizeof(int));
if(M[i] == NULL){
fprintf(stderr,"Error allocating memory\n");
free(M[i]);
return 1;
}
}
return 0;
}
void Print_Matrix(int **M,int row,int col)
{
int i,j;
for(i = 0;i<row;i++){
for(j = 0;j<col;j++){
printf("%d ",M[i][j]);
}
printf("\n");
}
}

Answer 1

This is because you are passing M by value, not reference. Your MallocX allocates memory for your matrix, but those allocations are orphaned when you return to the main program, where M is still 0x0 (or NULL), which is why allocation functions typically return a pointer. Perhaps you want something like this:
int **MallocX(int row,int col)
{
int **Matrix, i;
Matrix = malloc(row * sizeof(int*));
if(Matrix == NULL) {
fprintf(stderr,"Error allocating memory\n");
return NULL;
}
for (i = 0; i < row; i++) {
Matrix[i] = (int*) malloc(col * sizeof(int));
if(Matrix[i] == NULL){
fprintf(stderr,"Error allocating memory (%d)\n",i);
for (int j = 0; j < i; ++j)
free(Matrix[j]);
free(Matrix);
return NULL;
}
}
return Matrix;
}
Then in main, call it with:
if (!(M = MallocX(ROW,COL)) {
fprintf(stdout,"Could not allocate memory\n");
exit(1);
}
Note that in your original code, you were calling free(M) when M was NULL, which will itself cause a segfault. So I've tidied up your garbage collection a bit too.