cprintfunsigned-charconversion-specifier

Printf performing implicit casting

In the below code snippet, why do we get an output of 600? Two questions will help me understand the behavior.

  1. I specified the type for variable b to be uint8_t, why doesn't the math operation restrict to the same type?
  2. I thought PRIu8 is the right type to print uint8_t, why isn't this working?

I was hoping to get the answer 88 which results from looping beyond the range of uint8_t

#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>

void main()
{

    uint8_t b = 200;
    printf("%" PRIu8 "\n",b+b+b);
    printf("%" PRIu8 "\n",3*b);

}

gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.1)

Solution

  • In my opinion it is a library bug.

    In MS Visual Studio the macro PRIu8 is expanded to hhu as it should be and you get the expected result.

    It is interesting to note that if to use clang then if you will write for example

    printf("%" PRIu8 "\n",( uint8_t)(b+b+b));

    when again you will get the expected result.