Question

I having few questions about typed in ANSI C:
1. what’s the difference between “\x” in the beginning of a char to 0x in the beginning of char (or in any other case for this matter). AFAIK, they both means that this is hexadecimal.. so what’s the difference.

2. when casting char to (unsigned), not (unsigned char) – what does it mean? why (unsigned)’\xFF’ != 0xFF?

Thanks!

Answer 1

what’s the difference between “\x” in
the beginning of a char to 0x in the
beginning of char

The difference is that 0x12 is used for specifying an integer in hexadecimal, while “\x” is used for string literals. An example:

#include <stdio.h>

int main(){
  int ten = 0xA;
  char* tenString = "1\x30";
  printf("ten as integer: %d\n", ten);
  printf("ten as string: %s\n", tenString);
  return 0;
}

The printf’s should both output a “10” (try to understand why).

when casting char to (unsigned), not
(unsigned char) – what does it mean?
why (unsigned)’\xFF’ != 0xFF?

“unsigned” is just an abbreviation for “unsigned int”. So you’re casting from char to int. This will give you the numeric representation of the character in the character set your platform uses. Note that the value you get for a character is platform-dependent (typically depending on the default character encoding). For ASCII characters you will (usually) get the ASCII code, but anything beyond that will depend on platform and runtime configuration.

Understanding what a cast from one typ to another does is very complicated (and often, though not always, platform-dependent), so avoid it if you can. Sometimes it is necessary, though. See e.g. need-some-clarification-regarding-casting-in-c