使用scanf读取unsigned char

Question

I'm trying to use this code to read values between 0 to 255 ( unsigned char ).

#include<stdio.h>
int main(void)
{
    unsigned char value;

    /* To read the numbers between 0 to 255 */
    printf("Please enter a number between 0 and 255 \n");
    scanf("%u",&value);
    printf("The value is %u \n",value);

    return 0;
}

I do get the following compiler warning as expected.

warning: format ‘%u’ expects type ‘unsigned int *’, but argument 2 has type ‘unsigned char *’

And this is my output for this program.

Please enter a number between 0 and 255
45
The value is 45 
Segmentation fault

I do get the segmentation fault while running this code.

What is the best way to read unsigned char values using scanf ?

Answer 1

The %u specifier expects an integer which would cause undefined behavior when reading that into a unsigned char . You will need to use the unsigned char specifier %hhu .

Answer 2

For pre C99 I would consider writing an extra function for this just alone to avoid that segmentation fault due to undefined behaviour of scanf.

Approach:

#include<stdio.h>
int my_scanf_to_uchar(unsigned char *puchar)
{
  int retval;
  unsigned int uiTemp;
  retval = scanf("%u", &uiTemp);
  if (retval == 1)   
  {
    if (uiTemp < 256) {
      *puchar = uiTemp;
    }
    else {
      retval = 0; //maybe better something like EINVAL
    }
  }
  return retval; 
}

Then replace scanf("%u", with my_scanf_to_uchar(

Hope this is not off topic as I still used scanf and not another function like getchar :)

Another approach (without extra function)

if (scanf("%u", &uiTemp) == 1 && uiTemp < 256) { value = uitemp; }
else {/* Do something for conversion error */}