Fix compilation error with armv7, x86

[lmarz]

Description

Fixes a compilation error, that appears on alpine linux.

 /builds/lmarz/aports/testing/openimageio/src/oiio-Release-2.1.18.0/src/libOpenImageIO/exif.cpp: In function 'void OpenImageIO_v2_1::read_exif_tag(OpenImageIO_v2_1::ImageSpec&, const TIFFDirEntry*, OpenImageIO_v2_1::cspan<unsigned char>, bool, int, std::set<unsigned int>&, const OpenImageIO_v2_1::pvt::TagMap&)':
 /builds/lmarz/aports/testing/openimageio/src/oiio-Release-2.1.18.0/src/libOpenImageIO/exif.cpp:789:20: error: comparison of integer expressions of different signedness: 'unsigned int' and 'OpenImageIO_v2_1::span<const unsigned char, -1>::index_type' {aka 'int'} [-Werror=sign-compare]
   789 |         if (offset >= buf.size()) {
       |

Checklist:

• I have read the contribution guidelines.
• If this is more extensive than a small change to existing code, I
  have previously submitted a Contributor License Agreement
  (individual, and if there is any way my
  employers might think my programming belongs to them, then also
  corporate).
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• My code follows the prevailing code style of this project.

[hobbes1069]

Interesting, it hasn't shown up in the Fedora armv7 builds.

[lgritz]

Wait a minute, the warning you quoted was about line 789, but your edit is to line 786. Did you change the wrong thing?

[lmarz]

I didn't. Alpine Linux is a bit ... weird. For instance, it uses musl libc instead of the GNU libc library. It quite often happens, that the compiler outputs some weird error messages. This error only appears on 2 architectures, the others are fine. It isn't even specifically arch related.
The error appears a few lines below my change. The compiler says that it's comparing an unsigned int against a signed int (i dont know why this is an error). So I simply changed the offset from unsigned int to int, which eliminates the error and doesn't produce a warning, so I guess it's fine.

[lgritz]

The error appears a few lines below my change. The compiler says that it's comparing an unsigned int against a signed int (i dont know why this is an error). So I simply changed the offset from unsigned int to int, which eliminates the error and doesn't produce a warning, so I guess it's fine.

Aah, no, that will just cause other warnings, maybe for people with different compiler versions.

I think the correct fix is to change

    if (offset >= buf.size()) {

to

    if (offset >= size_t(buf.size())) {

I understand why it's happening. It's because buf is a cspan, and its size() returns a signed value, because at the time I made our span class, the draft of C++20's span had its size returning a signed value, but it has since been changed to unsigned, much like the other STL types. We'll do this eventually, too, but it's a compatibility change, so I'm waiting until the kind of release where I'm allowed to introduce potential backwards incompatibilities.