Fix DOS platform comments and workarounds for DJGPP support

Author: AJenbo

CMakeLists.txt

@@ -3724,7 +3724,7 @@ if(NOT HAVE_SDL_THREADS)
   endif()
 endif()
 if(NOT HAVE_SDL_TIMERS)
-#  message(FATAL_ERROR "Timers are needed by many SDL subsystems and may not be disabled")
+  message(FATAL_ERROR "Timers are needed by many SDL subsystems and may not be disabled")
 endif()
 
 # Most platforms use this.

src/dynapi/SDL_dynapi.h

@@ -65,8 +65,8 @@
 #define SDL_DYNAMIC_API 0 // devkitARM doesn't support dynamic linking
 #elif defined(SDL_PLATFORM_NGAGE)
 #define SDL_DYNAMIC_API 0
-#elif defined(SDL_PLATFORM_DOS)  // !!! FIXME: maybe with DXE3 support...
-#define SDL_DYNAMIC_API 0
+#elif defined(SDL_PLATFORM_DOS)
+#define SDL_DYNAMIC_API 0 // DJGPP doesn't support dynamic linking
 #elif defined(DYNAPI_NEEDS_DLOPEN) && !defined(HAVE_DLOPEN)
 #define SDL_DYNAMIC_API 0 // we need dlopen(), but don't have it....
 #endif

src/io/SDL_iostream.c

@@ -751,7 +751,7 @@ static Sint64 SDLCALL stdio_seek(void *userdata, Sint64 offset, SDL_IOWhence whe
     const bool is_noop = (whence == SDL_IO_SEEK_CUR) && (offset == 0);
 
     if (is_noop || fseek(iodata->fp, (fseek_off_t)offset, stdiowhence) == 0) {
-        #ifdef SDL_PLATFORM_DOS  // !!! FIXME: I don't know know why, but seeking seems to be broken in djgpp if buffering is enabled. Dump the buffer and recreate it for now.
+        #ifdef SDL_PLATFORM_DOS  // DJGPP libc bug: fseek doesn't invalidate the read buffer, so subsequent reads return stale data. Flush and recreate the buffer as a workaround.
         setvbuf(iodata->fp, NULL, _IONBF, 0);
         setvbuf(iodata->fp, NULL, _IOFBF, 16 * 1024);
         #endif

src/timer/dos/SDL_systimer.c

@@ -63,6 +63,7 @@ void SDL_SYS_DelayNS(Uint64 ns)
             }
             DOS_Yield();
             delay(chunk);
+            DOS_Yield();
             ms -= chunk;
         }
         ns -= (Uint64)((Uint32)(ns / SDL_NS_PER_MS)) * SDL_NS_PER_MS;

test/CMakeLists.txt

@@ -151,7 +151,6 @@ function(add_sdl_test_executable TARGET)
     SDL_AddCommonCompilerFlags(${TARGET})
     target_include_directories(${TARGET} PRIVATE "${SDL3_SOURCE_DIR}/src/video/khronos")
     target_link_libraries(${TARGET} PRIVATE SDL3::SDL3_test SDL3::${sdl_name_component})
-
     if(AST_DEPENDS)
         add_dependencies(${TARGET} ${AST_DEPENDS})
     endif()