Question

Currently, I need to strcat() 2 strings together. The catch is that I have to do this 3 times. (Total of 6 concatenations).
The procedure is this, repeated 3 times using loops:

1. Malloc a string
2. Using for loop, call strcat 2 times
3. Free the string

The problem is that even after I free the string and re-malloc, the strcat seems to keep on concatenating the previous string.

For example:

Expected Output from AA BB CC DD EE FF

• strcat string 1: AABB
• strcat string 2: CCDD
• strcat string 3: EEFF

Actual Output:

• strcat string 1: AABB
• strcat string 2: AABBCCDD
• strcat string 3: AABBCCDDEEFF

Does anyone know why it’s doing this?

void sendInitialHand(card * deck) {

    char * stringToSend;
    playerNode * curNode;
    curNode = housePlayers.head;

    for (int i=0; i<housePlayers.playerCount; i++) {

        stringToSend = malloc(sizeof(char)*6);

        for (int j=0; j<52; j++) {
            if (deck[j].inPlay == curNode->playerFD) {
                strcat(stringToSend, deck[j].identifier);
            }
        }

        for (int j=0; j<52; j++) {
            if (deck[j].inPlay == 10) {
                strcat(stringToSend, deck[j].identifier);
            }
        }    

        printf("[NETWORK] Send %d the following: %s\n", curNode->playerFD, stringToSend);
        //send(curNode->playerFD, stringToSend, 6, 0);
        free(stringToSend);
        curNode = curNode->next;
    }
}

Answer 1

After ptr=malloc(…), before strcat(), initialize the space with *ptr = '\0';. The memory returned by malloc() is usually not zeroed.