stb_image.flip_vertically.h

// Old code which could only have one value for all threads:

static int stbi__vertically_flip_on_load = 0;

STBIDEF void stbi_set_flip_vertically_on_load(int flag_true_if_should_flip)
{
   stbi__vertically_flip_on_load = flag_true_if_should_flip;
}

// New code which allows per-thread values without breaking old behavior:

static int stbi__vertically_flip_on_load_global = 0;

STBIDEF void stbi_set_flip_vertically_on_load(int flag_true_if_should_flip)
{
   stbi__vertically_flip_on_load_global = flag_true_if_should_flip;
}

#ifndef STBI_NO_THREAD_LOCALS
   #if __cplusplus >= 201103L
      #define STBI_THREAD_LOCAL       thread_local
   #elif __STDC_VERSION_ >= 201112L
      #define STBI_THREAD_LOCAL       _Thread_local
   #elif defined(__GNUC__)
      #define STBI_THREAD_LOCAL       __thread
   #elif _MSC_VER
      #define STBI_THREAD_LOCAL       __declspec(thread)
#endif
#endif

#ifndef STBI_THREAD_LOCAL
#define stbi__vertically_flip_on_load  stbi__vertically_flip_on_load_global
#else
static STBI_THREAD_LOCAL int stbi__vertically_flip_on_load_local, stbi__vertically_flip_on_load_set;

STBIDEF void stbi_set_flip_vertically_on_load_thread(int flag_true_if_should_flip)
{
   stbi__vertically_flip_on_load_local = flag_true_if_should_flip;
   stbi__vertically_flip_on_load_set = 1;
}

#define stbi__vertically_flip_on_load  (stbi__vertically_flip_on_load_set       \
                                         ? stbi__vertically_flip_on_load_local  \
                                         : stbi__vertically_flip_on_load_global)
#endif // STBI_THREAD_LOCAL