char pointer initialization in C
c, char, pointers, string
Solution
You have allocated no memory to use with `ab`.
The first assignment works because you are assigning to `ab` a string constant: `"abc123"`. Memory for constant strings are provided by the compiler on your behalf: you don't need to allocate this memory.
Before you can use `ab` with e.g. `sprintf`, you'll need to allocate some memory using `malloc`, and assign that space to `ab`:
ab = malloc(sizeof(char) * (NUM_CHARS + 1));
Then your `sprintf` will work so long as you've made enough space using `malloc`. Note: the `+ 1` is for the null terminator.
Alternately you can make some memory for `ab` by declaring it as an array:
char ab[NUM_CHARS + 1];
Without allocating memory somehow for `ab`, the `sprintf` call will try to write to `NULL`, which is undefined behavior; this is the cause of your crash.
Problem
I am not so clear on character pointer and how they work. The program builds, but crashes when I run it. ``` char *ab = NULL; //ab = "abc123"; // works fine sprintf(ab, "abc%d", 123); // this line seems to crash the program ``` I don't get how this can be wrong, when sprintf takes in a (char * str) as a first argument. Can anyone please explain this to me?