Segmentation Fault when adding value into dynamic array

Question

I wanted to add values into my. array

typedef struct teleporter {
    int start;
    int end;
}teleporters;



int main(int argc, char **argv) {
    int lenght = 2;
    teleporters *teleportPlaces;
    teleporters mytele;
    mytele.start = 0;
    mytele.end = 0;
    teleportPlaces = calloc(2, sizeof(teleporters));//malloc (sizeof(teleporters) * (lenght));
    if (teleportPlaces != NULL) {
        teleportPlaces = NULL;
    }
    for (int i = 0; i < lenght; i++) {
        teleportPlaces[i] = mytele;
    }

    printf("Teleport END[0] = %d",teleportPlaces[0].end);
    free(teleportPlaces);
    return 0;
}

but everytime i add it, it gave me an segmentation error,

how do i solve this error? it'll be great if there's an article or answer about it, thanks

if (teleportPlaces != NULL) { teleportPlaces = NULL; } - what is this? — Eugene Sh.
– Eugene Sh., Commented Nov 17, 2021 at 17:11
Please explain your understanding of all lines between calloc and printf. They really need some explanation. Alternatively please explain why you are surprised by a segfault if you access via a pointer that is guaranteed by your explicit code to be NULL. — Yunnosch
– Yunnosch, Commented Nov 17, 2021 at 17:19

Viktor · Accepted Answer · 2021-11-17 18:14:41Z

4

The problem is that you are literally throwing away the address to your teleportPlaces right after allocating it.

Remove the if statement in which you point teleportPlaces to NULL.

In the for loop, the address teleportPlaces[i] is supposed to be the address to the beginning of your array (teleportPlaces) and an offset (i). But when you reassign it to rather point to NULL, the actual address to the array is lost, leaving you with memory leakage (since you can't free a calloc if you don't know the address to it).

edited Nov 17, 2021 at 18:14

answered Nov 17, 2021 at 17:27

Viktor

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Comments

Vlad from Moscow · Accepted Answer · 2021-11-17 19:11:14Z

This if statement

if (teleportPlaces != NULL) {
    teleportPlaces = NULL;
}

does not make a sense. It means that if the memory was allocated successfully you set the pointer teleportPlaces to the allocated memory to NULL, producing a memory leak.

After that you are using this null pointer in the following for loop.

Remove this if statement or for example write

if (teleportPlaces == NULL) return 0;

Or

if (teleportPlaces != NULL) {
    for (int i = 0; i < lenght; i++) {
        teleportPlaces[i] = mytele;
    }

    printf("Teleport END[0] = %d\n",teleportPlaces[0].end);
}

free( teleportPlaces );

Also you could simplify this code snippet

teleporters mytele;
mytele.start = 0;
mytele.end = 0;

the following way

teleporters mytele = { .start = 0, .end = 0 };

Also do not use magic numbers like 2. Instead of this statement

teleportPlaces = calloc(2, sizeof(teleporters));

you should write

teleportPlaces = calloc( length, sizeof(teleporters));

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