segmentation fault in simple code

Hi,
I wrote this simple program to time a program. I wanted to time the writes to a matrix. but for some reason the program gives me a segmentation fault. can anyone please tell me why.

Code:

---

/* Program to time matrix writes */

#include <stdio.h>
#include <time.h>

#define SIZE 100

void MatrixWrite(int matrix[SIZE][SIZE]){
        int i = 0;
        int j = 0;

        for(i = 0; i < SIZE; i++){
                for(j = 0; j < SIZE; j++){
                        matrix[i][j] = i+j;  /* initialize matrix[i][j] to i+j */
                        }
                }
}

int main()
{
        clock_t start = clock();
        int matrix[SIZE][SIZE] = {};      /* initialize a matrix to NULL*/
        MatrixWrite(matrix[SIZE][SIZE]);
        printf("Time elapsed: %f\n", ((double)clock() - start) / CLOCKS_PER_SEC);
        return 0;

}

---

Line 8 ... the function should prototype as..

Code:

---

void MatrixWrite(int *Matrix)
  {...

---

Line 22 ... the correct initializer is...

Code:

---

int matrix[SIZE][SIZE] = {0};  // initialize to 0

---

Line 23 ... the correct call to the function is...

Code:

---

MatrixWrite(matrix);

---

Also Line 21... depending on your version of C, you may not be able to place active code before variable declarations. AFIK, that's only supported in C-99.

I tried the changes you suggested..gave me more warnings

matrix.c: In function ‘MatrixWrite’:
matrix.c:14: error: subscripted value is neither array nor pointer
matrix.c: In function ‘main’:
matrix.c:22: warning: missing braces around initializer
matrix.c:22: warning: (near initialization for ‘matrix[0]’)
matrix.c:23: warning: passing argument 1 of ‘MatrixWrite’ from incompatible pointer type

Code:

---

/* Program to time matrix writes */

#include <stdio.h>
#include <time.h>

#define SIZE 100

void MatrixWrite(int *matrix){
        int i = 0;
        int j = 0;

        for(i = 0; i < SIZE; i++){
                for(j = 0; j < SIZE; j++){
                        matrix[i][j] = i+j;  /* initialize matrix[i][j] to i+j */
                        }
                }
}

int main()
{
        clock_t start = clock();
        int matrix[SIZE][SIZE] = {0};      /* initialize a matrix to NULL*/
        MatrixWrite(matrix);
        printf("Time elapsed: %f\n", ((double)clock() - start) / CLOCKS_PER_SEC);
        return 0;

}

---

recanted... see next post

Ok, having lead you down he wrong path, here's a working version of your code... with my apologies.

Code:

---

/* Program to time matrix writes */

#include <stdio.h>
#include <time.h>

#define SIZE 100

void MatrixWrite(int matrix[SIZE][SIZE]){
        int i = 0;
        int j = 0;

        for(i = 0; i < SIZE; i++){
                for(j = 0; j < SIZE; j++){
                        matrix[i][j] = i+j;  /* initialize matrix[i][j] to i+j */
                        }
                }
}

int main( void )
{
        int matrix[SIZE][SIZE] = {0};      /* initialize a matrix to NULL*/
        clock_t start = clock();
        MatrixWrite(matrix);
        printf("Time elapsed: %f\n", ((double)clock() - start) / CLOCKS_PER_SEC);
        return 0;
}

---

Do note that your elapsed time is almost certainly going to return 0 since this is a very small task that can likely be done in less than a millisecond on most machienes (well, except an IBM XT, maybe)

yeah that works. A couple of warnings though. but it compiles and runs..ya I would slowly increase the matrix size and see when it will start showing me some real execution time. Thanks for your help tater.

Hmm actually now theres another problem. I tried to scale the size of matrix to 2000 and now the program crashes with a segmentation fault. I observed (using gdb) that it crashes at the initializing line

Code:

---

int matrix[SIZE][SIZE] = {0};

---

What I dont understand is how can a program behavior change with the size of its parameters while the compiler has no problems at all. could this be cos of my system limitations?

You're probably crashing the program's stack... Do the math... 2000 x 2000 x sizeof(int) = 16,000,000 bytes.

You're going to have to create an array that big on the heap, using malloc() or calloc()...