C - program crashes when using free function

Question

first, sorry for my bad english jeje. I don't speak it.

I'm having a problem when using free() function to free memory, this is what I have :

Using free() function in C

I'm using codeblocks on Windows 7. I have around 3 years using dinamic memory allocation and this is the first time I get this problem. There are people that say that the problem is gcc compiler

equal Directions

Directions are equal in and out the function, so I don't think I'm trying to free an invalid pointer

Do not post your code as image. Please post your code as text directly in your question. Also please don't forget to post a [Minimal, Complete, and Verifiable example](http://stackoverflow.com/help/mcve). — MikeCAT, Jun 01 '16 at 16:35
They say [you shouldn't cast the result of `malloc()` in C](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc). — MikeCAT, Jun 01 '16 at 16:36
Checking if `M == NULL` after assigning some value to `*M` isn't good. See [EXP34-C. Do not dereference null pointers - CERT C Coding Standard - CERT Secure Coding Standards](https://www.securecoding.cert.org/confluence/display/c/EXP34-C.+Do+not+dereference+null+pointers) — MikeCAT, Jun 01 '16 at 16:37

score 1 · Answer 1 · answered Jun 01 '16 at 16:44

1

Your for loop should be using n not m on your second round of allocation.

for (i=0; i < n; i++) // You're using m here: use n
{
    ...
    malloc(m...) // Here m is correct

answered Jun 01 '16 at 16:44

noelicus

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MikeCAT · Accepted Answer · 2016-06-01T17:07:00.680

The errors in the code is:

M == NULL is not a valid way to check if malloc is succeeded. You should use (*M) == NULL because the value is assigned to (*M), not M.
The loop condition in ReservarMemoriaMatriz i<=m seems wrong. It should be i<=n.
You forgot to free (*M)[n] in LiberarMemoriaMatriz.

Corrected code:

void LiberarMemoriaMatriz(double*** M,int n){
    int i = 0;
    if(M == NULL) return;
    for(i=0; i<=n; i++){
        free((*M)[i]);
    }
    free(*M);
}

int ReservarMemoriaMatriz(double*** M, int n, int ,){
    int i = 0;
    if(M == NULL) return 0;
    (*M) = malloc((n+1)*sizeof(double*));
    if((*M) == NULL){
        return 0;
    }
    for(i = 0; i<=n; i++){
        (*M)[i] = malloc(m*sizeof(double));
        if((*M)[i] == NULL){
            /* free what is allocated before failing */
            for(i--; i>=0; i--) free((*M)[i]);
            free(*M);
            return 0;
        }
    }
    return 1;
}

Lines 2-4 of ReservarMemoriaMatriz are still wrong, Presumably the caller is passing `&M` as the first parameter, and that can never be NULL. so the second line is meaningless. Line 4 should check *M for NULL, not M. And assuming the caller also passes &M into LiberarMemoriaMatriz, the same applies there, too. — FredK, Jun 01 '16 at 17:00

score 0 · Answer 3 · edited Jun 03 '16 at 04:19

0

I was looking line-by-line and finally found the mistake:

I like what you do that says:

free what is allocated before failing

Another thing that I changed is when allocating memory I use:

(*M) = (double**)malloc(n * sizeof(double*));

and the loop condition, i < n, and everything is working fine.

edited Jun 03 '16 at 04:19

Jamal

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answered Jun 03 '16 at 03:36

Carlos Obregon

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C - program crashes when using free function

3 Answers3