c - when to carefully use free() to free up malloc() used memory?

Question

I read from many question here in SO and some other articles regarding free() function in c that frees the memory of unused variables. In my case, I have the following code block.

char *injectStrAt(char *str, char *strToIn, int pos)
{
    char *strC = malloc(strlen(str) + strlen(strToIn) + 1);
    strncpy(strC, str, pos);
    strC[pos] = '';
    strcat(strC, strToIn);
    strcat(strC, str + pos);
    return strC;
}

The above function I use to inject a string block in to an array. I am using malloc to create a new char*. In the above case do I need to do free(strC) ? advice pls.

Answer 1

strC is the return value of this function, so you don't call free(strC) inside the function itself. You need to free it outside the function, the time when this string is not used anymore.