Comparison to -1 of unsigned integer.

[ghost]

Hello. I am reading through the code and noticed the following branch condition:

raylib/src/rlgl.h

Line 2616 in 2a6bd97

if (glInternalFormat != -1)

While glInternalFormat is defined here as unsigned int. It should never be -1. Does this check work as intended, or does C perform some implicit conversion here?

[raysan5]

@Hexaflex Actually, it looks like a bug! Good catch! Reviewed!

[ghost]

@raysan5 This seems to happen again at these locations:

• raylib/src/rlgl.h

  Line 2764 in 2a6bd97

  if (glInternalFormat != -1)

• raylib/src/rlgl.h

  Line 2836 in 2a6bd97

  if ((glInternalFormat != -1) && (format < RL_PIXELFORMAT_COMPRESSED_DXT1_RGB))

[orcmid]

This looks like a tricky way to avoid UINT_MAX (from limits.h) when glInternalFormat might be a different unsigned size than unsigned int and the max value is some sort of flag case.

I just compiled a small program using cl.exe with no options and that is exactly what happens. I assigned -1 to unsigned int glInternalFormat and the comparison != -1 failed and comparison with UINT_MAX succeeded.

That's a beautiful hack. A better solution might be to declare a const (better than #define) with the intended value and a big XXX comment with it, all in rlgl.h

[orcmid]

/* umax.c 0.0.1                    UTF-8                      dh:2021-10-15
*---|----1----|----2----|----3----|----4----|----5----|----6----|----7----|--*

                     Check Unsigned Casting Trick
*/

#include <stdio.h>
#include <limits.h>

int main(void)
{
    unsigned int glInternalFormat;

    glInternalFormat = -1;

    if (glInternalFormat != -1)
         puts("-1 didn't work");
    else puts("-1 worked");
    if (glInternalFormat != UINT_MAX)
         puts("not UINT_MAX though");
    else puts("and it's UINT_MAX");
    }

/* 0.0.1 a simple check
   */

[orcmid]

[ghost]

Hah, interesting. Thanks for the input.

[orcmid]

It does depend on unsigned int data and operations being implemented as 2's-complement binary arithmetic. It gets easy though.

The C99 Standard has signed int be converted to unsigned int in a comparison or other operation) with an unsigned int. Assignment works the same way. In the case of unsigned int values it is all modular arithmetic with modulus UINT_MAX + 1. My copy of Harbison & Steele's C A Reference Manual (ed.5) has this example on p.198,

unsigned int UI = -1;

I would not have seen that except this case in raylib had me look things up.

Funny. Today I saw a thread with folks talking about undefined situations in C and C++. This is clearly not one of them, as surprising as it might be.