Stack error Smashing Detected in C

Question

Could someone help me on the following question:

Create a program that fills a 6x4 matrix with integers, calculate and show how many elements of this matrix are greater than 30, and then assemble a second matrix with the different elements of 30. In place of number 30, of the second matrix, put the number zero.

I made the following code but this giving the following error: stack Smashing Detected: terminated and I’m not getting to know why. I’m using Netbeans as an IDE.

#include <stdio.h>
#include <stdlib.h>
#define L 2
#define C 2

int main(int argc, char** argv) {
    int matriz[L][C], mat2[L][C];
    int i, j, pL = 0, pC = 0, contMaior30 = 0, contIgual30 = 0, contDif30 = 0;

    for (i = 0; i < L; i++) {
        for (j = 0; j < C; j++) {
            printf("M[%d][%d] = ", i, j);
            scanf("%d", &matriz[i][j]);
        }
    }

    //Laço para fazer a verificação dos valores digitados
    for (i = 0; i < L; i++) {
        for (j = 0; j < C; j++) {
            if (matriz[i][j] <= 30) {
                if (matriz[i][j] == 30) {
                    mat2[pL][pC] = 0;
                } else if (matriz[i][j] < 30) {
                    mat2[pL][pC] = matriz[i][j];
                }
                contDif30++;
                pL++;
                pC++;
            }

           //Contar a quantidade de números maiores que 30 
            else {
                contMaior30++;
            }
        }
    }

    //Não sei como controlar a impressão de segunda matriz
    printf("\nImpressão do segundo vetor: \n");
    for (i = 0; i < 2; i++) {
        for (j = 0; j < 2; j++) {
            printf("\tM2: %d \n", mat2[i][j]);
        }
    }

    return (EXIT_SUCCESS);
}

Answer 1

Error indicates you are crossing matrix boundaries, and write to memory zones that cross boundaries.

This happens in the for building the second matrix:

if (matriz[i][j] <= 30) {
    if (matriz[i][j] == 30) {
        mat2[pL][pC] = 0; // <--- aqui
    } else if (matriz[i][j] < 30) {
        mat2[pL][pC] = matriz[i][j]; // <-- e aqui
    }
    contDif30++;
    pL++;
    pC++;
}

First of all the comparison is strange because it starts by testing whether matriz[i][j] <= 30 And then inside it goes back to testing matriz[i][j] < 30. But if the element is <=30 increases the pL and the pC, and these two are used as matrix indices:

mat2[pL][pC] = matriz[i][j];
//    ^---^

Then in your matrix example 2x2, where the validated positions are 0 to 1, three elements smaller than 30 the pL and the pC are already in vain 3. This will result in an assignment equivalent to:

mat2[3][3] = matriz[i][j];

Which is incorrect because it crosses the boundaries of the matrix.

Instead I suggest you do it like this:

//Laço para fazer a verificação dos valores digitados
for (i = 0; i < L; i++) {
    for (j = 0; j < C; j++) {
        if (matriz[i][j] == 30){
            mat2[i][j] = 0; //atribui em mat2 com base no i, j também
            contIgual30++;
        }
        else {
            contDif30++;
            mat2[i][j] = matriz[i][j]; //atribui em mat2 com base no i, j também
            if (matriz[i][j] > 30){
                contMaior30++;
            }
        }
    }
}

The assignments in mat2 are made with [i][j] to place the element in the same position as the resulting matrix.

See this example working on Ideone

Edit:

To show output only values less than or equal to 30 needs to add a if in the part showing:

for (i = 0; i < 2; i++) {
    for (j = 0; j < 2; j++) {
        if (mat2[i][j] <= 30){ // <--este
            printf("\tM2: %d \n", mat2[i][j]);
        }
    }
}

See also this example in Ideone