Segmentation Fault when adding value into dynamic array-CodePudding

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

CodePudding user response：

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).

CodePudding user response：

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;

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));