Question

For a problem at school, I need to convert a ASCII string of character digits to a decimal value. I wrote a function to do this and specified the return type to be an unsigned short as you can see in the code below.

#include <stdio.h>
unsigned short str_2_dec(char* input_string, int numel);
int main()
{
  short input;
  char input_string[6]= "65535";

  input = str_2_dec(input_string, 5);

  printf("Final Value: %d", input);
  return 0;
}
unsigned short str_2_dec(char* input_string, int numel)
{
  int factor = 1;
  unsigned short value = 0;
  int index;

  for(index=0; index <(numel-1); index++)
  {
    factor *= 10;
  }

  for(index = numel; index > 0; index--)
  {
    printf("Digit: %d; Factor: %d; ", *(input_string+(numel-index))-48, factor);
    value += factor * ((*(input_string+(numel - index))-48));

    printf("value: %d\n\n", value);

    factor /= 10;
  }
  return value;
}

When running this code, the program prints -1 as the final value instead of 65535. It seems it’s displaying the corresponding signed value anyway. Seems like something very simple, but I can’t find an answer. A response would be greatly appreciated.

Answer 1

The problem is that you are taking the unsigned short return value of the function and storing it in a (signed) short variable, input. Since the value is outside the range representable in short, and since short is signed, this results in either an implementation-defined result or an implementation-defined signal being raised.

Change the type of input to unsigned short and everything will be fine.