Questions on -Wtype-limits

Questions on -Wtype-limits

[William Roberts]

When I compile the below program, I expect the -Wtype-limits warning
to be displayed for all 4 if conditions, but I only get uint8_t and
uint16_t conditions, why?

#include <stdint.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
(void)argv;

uint8_t   i8 = (uint8_t)  argc;
uint16_t i16 = (uint16_t) argc;
uint32_t i32 = (uint32_t) argc;
uint64_t i64 = (uint64_t) argc;

if (i8 > UINT8_MAX) {
return 8;
}
if (i16 > UINT16_MAX) {
return 16;
}
if (i32 > UINT32_MAX) {
return 32;
}
if (i64 > UINT64_MAX) {
return 64;
}

        return 0;
}

gcc -Wall -Wextra -o go a.c
a.c: In function ‘main’:
a.c:13:9: warning: comparison is always false due to limited range of
data type [-Wtype-limits]
   13 |  if (i8 > UINT8_MAX) {
      |         ^
a.c:17:10: warning: comparison is always false due to limited range of
data type [-Wtype-limits]
   17 |  if (i16 > UINT16_MAX) {

gcc --version
gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Thanks,
Bill

Re: Questions on -Wtype-limits

[Kewen.Lin]

Hi,

on 2021/1/4 上午2:53, William Roberts via Gcc-help wrote:
> When I compile the below program, I expect the -Wtype-limits warning
> to be displayed for all 4 if conditions, but I only get uint8_t and
> uint16_t conditions, why?

This is a known issue, see below PR.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89126

BR,
Kewen

> 
> #include <stdint.h>
> #include <stdlib.h>
> 
> int main(int argc, char *argv[]) {
> (void)argv;
> 
> uint8_t   i8 = (uint8_t)  argc;
> uint16_t i16 = (uint16_t) argc;
> uint32_t i32 = (uint32_t) argc;
> uint64_t i64 = (uint64_t) argc;
> 
> if (i8 > UINT8_MAX) {
> return 8;
> }
> if (i16 > UINT16_MAX) {
> return 16;
> }
> if (i32 > UINT32_MAX) {
> return 32;
> }
> if (i64 > UINT64_MAX) {
> return 64;
> }
> 
>         return 0;
> }
> 
> gcc -Wall -Wextra -o go a.c
> a.c: In function ‘main’:
> a.c:13:9: warning: comparison is always false due to limited range of
> data type [-Wtype-limits]
>    13 |  if (i8 > UINT8_MAX) {
>       |         ^
> a.c:17:10: warning: comparison is always false due to limited range of
> data type [-Wtype-limits]
>    17 |  if (i16 > UINT16_MAX) {
> 
> gcc --version
> gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
> Copyright (C) 2019 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> 
> Thanks,
> Bill
>