Segmentation fault error for a matrix assignment

Question

This is my code for the creation of a grid similar to that of the game "mines":

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

int main(){

    int nr, nc, ** mtx, i, j;

    //Matrix costruction

    FILE * inStr;

    inStr = fopen("brasa.dat","r");

    if(inStr == NULL){

        printf("\nProblem while opening the file.\n");

        return 1;

    }

    fscanf(inStr, "%d%d", &nr, &nc);

    mtx = (int**) malloc(nr * sizeof(int*));
    if(mtx == NULL){
        printf("\nMemory allocation error.\n");
        return 1;
    }
    else{
        for(i = 0; i < nr; i++)      
            mtx[i] = (int*) malloc(nc * sizeof(int));
            if(mtx[i] == NULL){
                printf("\nMemory allocation error.\n");
                return 1;
            }
    }

    //Filling matrix

    for(i = 0; i < nr; i++){
        for(j = 0; j < nc; j++)
            mtx[i][j] = 0;
    }

    while(!feof(inStr)){
        fscanf(inStr,"%d %d",&i,&j);
        mtx[i][j] = 9;
    }

    fclose(inStr);

    for(i = 0; i < nr; i++){
        for(j = 0; j < nc; j++){

            if(i == 0){
                if(j == 0){
                    if(mtx[i][j+1] > 8)
                        mtx[i][j]++;
                    else if(mtx[i+1][j] > 8)
                        mtx[i][j]++;
                    else if(mtx[i+1][j+1] > 8)
                        mtx[i][j]++;
                }
                else if(j == nc-1){
                    if(mtx[i][j-1] > 8)
                        mtx[i][j]++;
                    else if(mtx[i+1][j-1] > 8)
                        mtx[i][j]++;
                    else if(mtx[i+1][j] > 8)
                        mtx[i][j]++;
                }
                else{
                    if(mtx[i][j-1] > 8)
                        mtx[i][j]++;
                    else if(mtx[i][j+1] > 8)
                        mtx[i][j]++;
                    else if(mtx[i+1][j-1] > 8)
                        mtx[i][j]++;
                    else if(mtx[i+1][j] > 8)
                        mtx[i][j]++;
                    else if(mtx[i+1][j+1] > 8)
                        mtx[i][j]++;
                }
            }
            else if(i == nr-1){
                if(j == 0){
                    if(mtx[i+1][j] > 8)
                        mtx[i][j]++;
                    else if(mtx[i+1][j+1] > 8)
                        mtx[i][j]++;
                    else if(mtx[i][j+1] > 8)
                        mtx[i][j]++;
                }
                else if(j == nc-1){
                    if(mtx[i][j-1] > 8)
                        mtx[i][j]++;
                    else if(mtx[i+1][j-1] > 8)
                        mtx[i][j]++;
                    else if(mtx[i+1][j] > 8)
                        mtx[i][j]++;

                }   
                else{
                    if(mtx[i][j-1] > 8)
                        mtx[i][j]++;
                    else if(mtx[i+1][j-1] > 8)
                        mtx[i][j]++;
                    else if(mtx[i+1][j] > 8)
                        mtx[i][j]++;
                    else if(mtx[i+1][j+1] > 8)
                        mtx[i][j]++;
                    else if(mtx[i][j+1] > 8)
                        mtx[i][j]++;
                }
            }
            else if(j == 0){
                if(mtx[i+1][j] > 8)
                    mtx[i][j]++;
                else if(mtx[i+1][j+1] > 8)
                    mtx[i][j]++;
                else if(mtx[i][j+1] > 8)
                    mtx[i][j]++;
                else if(mtx[i-1][j+1] > 8)
                    mtx[i][j]++;
                else if(mtx[i-1][j] > 8)
                    mtx[i][j]++;
            }
            else if(j == nc-1){
                if(mtx[i+1][j] > 8)
                    mtx[i][j]++;
                else if(mtx[i+1][j-1] > 8)
                    mtx[i][j]++;
                else if(mtx[i][j-1] > 8)
                    mtx[i][j]++;
                else if(mtx[i-1][j-1] > 8)
                    mtx[i][j]++;
                else if(mtx[i-1][j] > 8)
                    mtx[i][j]++;

            }   
            else{
                if(mtx[i-1][j-1] > 8)
                    mtx[i][j]++;
                else if(mtx[i][j-1] > 8)
                    mtx[i][j]++;
                else if(mtx[i+1][j-1] > 8)
                    mtx[i][j]++;
                else if(mtx[i+1][j] > 8)
                    mtx[i][j]++;
                else if(mtx[i+1][j+1] > 8)
                    mtx[i][j]++;
                else if(mtx[i][j+1] > 8)
                    mtx[i][j]++;
                else if(mtx[i-1][j+1] > 8)
                    mtx[i][j]++;
                else if(mtx[i-1][j] > 8)
                    mtx[i][j]++;
            }
            if(mtx[i][j] > 8)
                printf("*\t");
            else
                printf("%d\t", mtx[i][j]);
        }
    }

    free(mtx);
    return 0;
}

During the execution of the program appears a segmentation fault. Doing several tests I noticed that the problem should be between line 85 and line 158. Can you help me?

Compile your code with `-g` and run it through a debugger, then you'll find out exactly where it crashes. — Tom Fenech, Apr 22 '14 at 11:46

score 0 · Accepted Answer · answered Apr 22 '14 at 11:49

0

You have an invalid access

        // ...
        else if(i == nr-1){               // i is nr - 1
            if(j == 0){
                // ...
            }
            else if(j == nc-1){
                // ...
                else if(mtx[i+1][j] > 8)  // i + 1 is nr (invalid access)
                // ...

answered Apr 22 '14 at 11:49

pmg

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Thank you very much for your reply! How can I fix it? Excuse me, but I'm a novice programmer! – user3482381 Apr 22 '14 at 13:13
1

At that particular place in the code you need `i - 1` to check the row below the current one. But check throughout the rest of the code for similar mistakes. Also [your use of feof() is wrong](http://stackoverflow.com/questions/5431941/while-feof-file-is-always-wrong) and you don't `free()` the `mtx[]` elements. – pmg Apr 22 '14 at 13:25

Segmentation fault error for a matrix assignment

1 Answers1