Freeing dynamically allocated memory

I found this code on the internet. Its purpose is to allocate a contiguous memory block for a two-dimensional dynamic array.

Code:

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int** x;
int* temp;

x = (int**)malloc(dimension1_max * sizeof(int*));
temp = (int*)malloc(dimension1_max * dimension2_max * sizeof(int));
for (int i = 0; i < dimension1_max; i++) {
  x[i] = temp + (i * dimension2_max);
}

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How should I free the memory allocated with such code?
I'd go for:

Code:

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free(x[0]);
free(x);

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Is my assumption correct? I also think I can "forget" the temp pointer because it points to the same memory block as x[0].

Yes.

Quote:

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Originally Posted by laserlight

Yes.

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Thanks. Is the temp pointer actually needed at all? Can I replace it with x[0]?

You can replace temp with x[0] if you wish, as long as you only use x[0] after x has been given an initial value.

Incidentally, I suggest writing:

Code:

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x = malloc(dimension1_max * sizeof(x[0]));

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There is no need to cast the return value of malloc, and then you might as well make sizeof use x in some way so that if the type of x changes, the code would remain correct.

Yes, you can remove temp and work with x[0] directly:

Code:

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

#define TEST
#define N 10
#define M 5

int main()
{
    int** p = (int**)malloc(N * sizeof(int*));
    int k = 0;

    if (p)
    {
        // Allocate one master block
        p[0] = (int*)malloc((N * M) * sizeof(int));

        // Split up the master block
        for (int x = 1; x < N; ++x)
        {
            p[x] = p[0] + (x * M);
        }

        // Initialize the array
        for (int x = 0; x < N; ++x)
        {
            for (int y = 0; y < M; ++y)
            {
                p[x][y] = k++;
            }
        }

#if defined(TEST)
        // Print for testing
        for (int x = 0; x < N; ++x)
        {
            for (int y = 0; y < M; ++y)
            {
                printf("%3d", p[x][y]);
            }

            putchar('\n');
        }
#endif

        // Clean up the mess
        free(p[0]);
        free(p);
    }
}

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