C malloc of struct execution error

Question

I do not understand why I can use malloc with struct var **tableauVariables, but I receve an error with struct zone **tableauZonesMemoireLibres.

When debuging I receve :

At C:\Users\Onel\Desktop\test\test.c:33 Program received signal SIGSEGV, Segmentation fault.

My code :

#include <stdlib.h>

void tabInit(), tabDelete();

struct zone {
    int pos;
    int taille;
};

struct var {
    char id[10];
    struct zone  tableau;
};

struct var **tableauVariables;
struct zone **tableauZonesMemoireLibres;
int tailleMemoire= 10;

int main () {

    tabInit();

    tabDelete();

    return 0;
}

void tabInit() {
    tableauVariables = (struct var**) malloc(sizeof(struct var) * tailleMemoire);
    tableauVariables[0] = NULL;

    tableauZonesMemoireLibres = (struct zone**) malloc(sizeof(struct zone) * tailleMemoire);
    tableauZonesMemoireLibres[0]->pos = 0; //Line 33 segmentation fault
    tableauZonesMemoireLibres[0]->taille = tailleMemoire;
    tableauZonesMemoireLibres[1] = NULL;

    return;
}

void tabDelete() {
    int i;

    for(i=0; tableauZonesMemoireLibres[i] != NULL; i++)
        free(tableauZonesMemoireLibres[i]);
    free(tableauZonesMemoireLibres);

    for(i=0; tableauVariables[i] != NULL; i++)
        free(tableauVariables[i]);
    free(tableauVariables);
}

tableauVariables = malloc(sizeof(struct var *) * tailleMemoire); or simpler and more robust: tableauVariables = malloc(tailleMemoire * sizeof *tableauVariables ); — wildplasser
– wildplasser, Commented Nov 24, 2013 at 23:30
Learn the language before even touching a debugger. Debuggers will teach you nothing. BTW: tableauZonesMemoireLibres : similar error. — wildplasser
– wildplasser, Commented Nov 24, 2013 at 23:34

Charlie Burns · Accepted Answer · 2013-11-24 23:41:03Z

1

You allocate an array, and cast it to a **.

This allocates an array of struct zone of size tailleMemoire.

tableauZonesMemoireLibres = (struct zone**) malloc(sizeof(struct zone) * tailleMemoire);

Maybe what you intended is

struct zone *tableauZonesMemoireLibres;
tableauZonesMemoireLibres = malloc(sizeof(struct zone) * tailleMemoire);
tableauZonesMemoireLibres[0].pos = 0; //Line 33 segmentation fault
tableauZonesMemoireLibres[0].taille = tailleMemoire;

Also, your tabDelete is totally wrong. You want one free foreach malloc:

void tabDelete() {
    free(tableauZonesMemoireLibres);
    free(tableauVariables);
}

On the other hand, if you really did want an array of pointers, then each element of that array needs to malloc'ed by itself.

edited Nov 24, 2013 at 23:41

answered Nov 24, 2013 at 23:35

Charlie Burns

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1 Comment

Ionut Over a year ago

Yes, I do want an array of pointers and you are right I forgot to malloc the first element before using it. Thanks

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