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Low-Level C & Bit Tricksfoundational

Show how to declare a pointer to constant data in C. Show how to declare a function pointer in C.

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Pointer to constant data — the pointee is read-only through the pointer, but the pointer itself may be reassigned. Read declarations right-to-left:

c
const int *p;          /* p points to a const int: *p is read-only, p can change   */
int const *p2;         /* identical meaning to the line above                       */

Contrast with a constant pointer (the pointer is fixed, the data is writable), and a constant pointer to constant data:

c
int x = 5, y = 6;
int * const cp = &x;        /* cp is const: cannot repoint, but *cp = 10 is OK      */
const int * const cpc = &x; /* neither the pointer nor the pointee may change       */


const int *p;
p = &x;        /* OK: repointing the pointer is allowed   */
/* *p = 7;     // ERROR: cannot write through pointer-to-const */
/* *cp = 7;    is OK; cp = &y; would be an ERROR             */

A very common, correct use is a function parameter promising not to modify the caller's buffer: size_t strlen(const char *s);.

Function pointer — a variable that holds the address of a function with a specific signature. The parentheses around *fp are required, otherwise it parses as a function returning a pointer:

c
int add(int a, int b) { return a + b; }


int (*fp)(int, int);   /* fp: pointer to a function taking (int,int) returning int */


fp = &add;             /* or simply fp = add;  (function name decays to address)    */
int r = fp(2, 3);      /* call: r == 5;  (*fp)(2,3) is equivalent                   */

A typedef makes function-pointer code far more readable (useful for callback tables / jump tables):

c
typedef int (*binop_t)(int, int);
binop_t op = add;
int r2 = op(4, 5);     /* r2 == 9 */