000602 - I get incompatible pointer types warning when I use const for pointers

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GCC

When making the following code:

#include <general.dep>

typedef struct {
} obj;

void f1(obj **ppo) {}
void f2(const obj **ppo) {}
void f3(obj *const *ppo) {}
void f4(obj **const ppo) {}
void f5(const obj *const *ppo) {}
void f6(const obj **const ppo) {}
void f7(obj *const *const ppo) {}
void f8(const obj *const *const ppo) {}

void g1(obj *po) {}
void g2(const obj *po) {}
void g3(obj *const po) {}
void g4(const obj *const po) {}

int main() {
  obj o, *po, **ppo;

  f1(&po);
  f2(&po);
  f3(&po);
  f4(&po);
  f5(&po);
  f6(&po);
  f7(&po);
  f8(&po);

  f1(ppo);
  f2(ppo);
  f3(ppo);
  f4(ppo);
  f5(&po);
  f6(&po);
  f7(&po);
  f8(&po);

  g1(&o);
  g2(&o);
  g3(&o);
  g4(&o);

  g1(po);
  g2(po);
  g3(po);
  g4(po);

  return 0;
}

I get the following warning messages:

Quote:

gcc t1.c -I/home/hamidi/code/lib/general/inc -fmax-errors=1 -lgeneral -llzma -pthread -o t1
t1.c: In function ‘main’:
t1.c:24:6: warning: passing argument 1 of ‘f2’ from incompatible pointer type [-Wincompatible-pointer-types]
24 | f2(&po);
| ^~~
| |
| obj ** {aka struct <anonymous> **}
t1.c:7:21: note: expected ‘const obj **’ {aka ‘const struct <anonymous> **’} but argument is of type ‘obj **’ {aka ‘struct <anonymous> **’}
7 | void f2(const obj **ppo) {}
| ~~~~~~~~~~~~^~~
t1.c:27:6: warning: passing argument 1 of ‘f5’ from incompatible pointer type [-Wincompatible-pointer-types]
27 | f5(&po);
| ^~~
| |
| obj ** {aka struct <anonymous> **}
t1.c:10:27: note: expected ‘const obj * const*’ {aka ‘const struct <anonymous> * const*’} but argument is of type ‘obj **’ {aka ‘struct <anonymous> **’}
10 | void f5(const obj *const *ppo) {}
| ~~~~~~~~~~~~~~~~~~^~~
t1.c:28:6: warning: passing argument 1 of ‘f6’ from incompatible pointer type [-Wincompatible-pointer-types]
28 | f6(&po);
| ^~~
| |
| obj ** {aka struct <anonymous> **}
t1.c:11:27: note: expected ‘const obj ** const’ {aka ‘const struct <anonymous> ** const’} but argument is of type ‘obj **’ {aka ‘struct <anonymous> **’}
11 | void f6(const obj **const ppo) {}
| ~~~~~~~~~~~~~~~~~~^~~
t1.c:30:6: warning: passing argument 1 of ‘f8’ from incompatible pointer type [-Wincompatible-pointer-types]
30 | f8(&po);
| ^~~
| |
| obj ** {aka struct <anonymous> **}
t1.c:13:33: note: expected ‘const obj * const* const’ {aka ‘const struct <anonymous> * const* const’} but argument is of type ‘obj **’ {aka ‘struct <anonymous> **’}
13 | void f8(const obj *const *const ppo) {}
| ~~~~~~~~~~~~~~~~~~~~~~~~^~~
t1.c:33:6: warning: passing argument 1 of ‘f2’ from incompatible pointer type [-Wincompatible-pointer-types]
33 | f2(ppo);
| ^~~
| |
| obj ** {aka struct <anonymous> **}
t1.c:7:21: note: expected ‘const obj **’ {aka ‘const struct <anonymous> **’} but argument is of type ‘obj **’ {aka ‘struct <anonymous> **’}
7 | void f2(const obj **ppo) {}
| ~~~~~~~~~~~~^~~
t1.c:36:6: warning: passing argument 1 of ‘f5’ from incompatible pointer type [-Wincompatible-pointer-types]
36 | f5(&po);
| ^~~
| |
| obj ** {aka struct <anonymous> **}
t1.c:10:27: note: expected ‘const obj * const*’ {aka ‘const struct <anonymous> * const*’} but argument is of type ‘obj **’ {aka ‘struct <anonymous> **’}
10 | void f5(const obj *const *ppo) {}
| ~~~~~~~~~~~~~~~~~~^~~
t1.c:37:6: warning: passing argument 1 of ‘f6’ from incompatible pointer type [-Wincompatible-pointer-types]
37 | f6(&po);
| ^~~
| |
| obj ** {aka struct <anonymous> **}
t1.c:11:27: note: expected ‘const obj ** const’ {aka ‘const struct <anonymous> ** const’} but argument is of type ‘obj **’ {aka ‘struct <anonymous> **’}
11 | void f6(const obj **const ppo) {}
| ~~~~~~~~~~~~~~~~~~^~~
t1.c:39:6: warning: passing argument 1 of ‘f8’ from incompatible pointer type [-Wincompatible-pointer-types]
39 | f8(&po);
| ^~~
| |
| obj ** {aka struct <anonymous> **}
t1.c:13:33: note: expected ‘const obj * const* const’ {aka ‘const struct <anonymous> * const* const’} but argument is of type ‘obj **’ {aka ‘struct <anonymous> **’}
13 | void f8(const obj *const *const ppo) {}
| ~~~~~~~~~~~~~~~~~~~~~~~~^~~

How can I resolve these warning issues?

Update:

Let's discuss about these prototypes. What are their meanings and what are their differences?

void f0(obj **ppo) {}
void f1(const obj **ppo) {}	 //
void f2(obj const **ppo) {}	 //
void f3(obj *const *ppo) {}
void f4(obj **const ppo) {}
void f5(const obj const **ppo) {}  //
void f6(const obj *const *ppo) {}  //
void f7(const obj **const ppo) {}  //
void f8(obj const *const *ppo) {}  //
void f9(obj const **const ppo) {}  //
void fa(obj *const *const ppo) {}
void fb(const obj const *const *ppo) {}		   //
void fc(const obj const **const ppo) {}		   //
void fd(const obj *const *const ppo) {}		   //
void fe(obj const *const *const ppo) {}		   //
void ff(const obj const *const *const ppo) {}  //

The ones tagged with // cause the warning to be issued.

What I have tried:

Essentially, what does

const obj **p

mean? 'p' is a variable of type pointer. It addresses a variable of type pointer which addresses a variable of type obj. I want to ensure that the function doesn't alter any of these. Can't I? What's the problem?