const question

Hi, my attention was directed at the gprs_shift_v_fixed() function: int gprs_shift_v_fixed(uint8_t **data, size_t *data_len, size_t len, uint8_t **value) It advances the *data pointer and returns a *value pointer. So it could technically be consting much more aggressively. However, the compiler warnings confuse me. I've created this test code to illustrate; the ints should be treated read-only by func(): void func(const int **yyy) { (*yyy) ++; } int main(void) { int y[2] = {23, 42}; // mutable "by coincidence" int *yy = y; func(&yy); return *yy; // returns 42 } The idea is that yy is a pointer to a mutable int. func() expects a const int, but what harm could be done by passing a mutable-int pointer as an immutable-int pointer. Apparently this harm: check.c: In function ‘main’: check.c:10:14: warning: passing argument 1 of ‘func’ from incompatible pointer type func(&yy); ^ check.c:1:6: note: expected ‘const int **’ but argument is of type ‘int **’ void func(const int **yyy) Changing yy to a pointer-to-const-int clears the warning: const int *yy = y; func(&yy); I'm expecting the const to apply to the int pointed at, so I should be able to modify the pointer while the compiler shouldn't waste a thought at whether the int pointed at is const or not. func() is stricter, not less strict than y[]. To further illustrate, this would make the pointer address itself immutable, which is not what I want: int * const yy = y; I just want to declare the underlying int data to be unchangeable by func(), not caring whether the caller passes a const int or a mutable int buffer. Is the compiler merely nitpicking or am I getting something wrong fundamentally? Thanks! ~Neels -- - Neels Hofmeyr <nhofmeyr(a)sysmocom.de> http://www.sysmocom.de/ ======================================================================= * sysmocom - systems for mobile communications GmbH * Alt-Moabit 93 * 10559 Berlin, Germany * Sitz / Registered office: Berlin, HRB 134158 B * Geschäftsführer / Managing Directors: Holger Freyther, Harald Welte