Function Pointers, Callbacks & String Memory Layouts

In C, code resides in the Text / Code Segment. Every function has an entry-point memory address that can be stored in a function pointer.

Declaration Syntax:

return_type (*pointer_name)(param_types);

- Example: int (*fp)(int, int); declares fp as a pointer to a function taking two int parameters and returning an int.

- Note: int fp(int, int); WITHOUT parentheses declares a function returning an int!

Invocation:

If int add(int a, int b) { return a + b; }, then fp = add; or fp = &add;. Invoking fp(3, 4) or (*fp)(3, 4) both return 7.

Strings in C are null-terminated ('\0') character sequences:

1. char s[] = "GATE";:

- Allocates an array of 5 characters ('G', 'A', 'T', 'E', '\0') on the Stack.

- The contents are MUTABLE: s[0] = 'L'; modifies the string to "LATE" safely.

2. char *s = "GATE";:

- Stores the string literal in the Read-Only Data Segment (.rodata).

- The pointer s points to read-only memory.

- Attempting s[0] = 'L'; causes a Segmentation Fault / Bus Error (Undefined Behavior)!