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Bug 5509

Summary: SDL_RWsize() returns wrong value if start pos != 0 ?
Product: SDL Reporter: Ozkan Sezer <sezeroz>
Component: fileAssignee: Sam Lantinga <slouken>
Status: RESOLVED WONTFIX QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2    
Version: HG 2.0   
Hardware: All   
OS: All   

Description Ozkan Sezer 2021-01-28 03:07:32 UTC 
Suppose that I do:

    FILE *f = fopen("test","rb"); /* 100 bytes file */
    fseek(f, 20, SEEK_SET);
    SDL_RWops *rw = SDL_RWFromFP(f);

SDL_RWsize() would give me 100 instead of 80.  Only if I apply the
following then it gives me 80.  Is this an unsupported scenario or
is it an oversight?

diff --git a/src/file/SDL_rwops.c b/src/file/SDL_rwops.c
--- a/src/file/SDL_rwops.c
+++ b/src/file/SDL_rwops.c
@@ -357,7 +357,7 @@ stdio_size(SDL_RWops * context)
     size = SDL_RWseek(context, 0, RW_SEEK_END);
 
     SDL_RWseek(context, pos, RW_SEEK_SET);
-    return size;
+    return size - pos;
 }
 
 static Sint64 SDLCALL
Comment 1 Ozkan Sezer 2021-01-28 03:30:02 UTC 
Another scenario is the test case from bug #4410:

    SDL_RWops *file = SDL_RWFromFile("test.dat", "rb");
    SDL_RWseek(file, 69531, RW_SEEK_SET); /* MP3 is 69531 bytes from start */
    Mix_Music *music = Mix_LoadMUS_RW(file, 0);
    Mix_PlayMusic(music, -1);

And see current hg for this:
https://hg.libsdl.org/SDL_mixer/file/a341f68abf34/src/codecs/mp3utils.c#l29

I can easily fix bug #4410 by doing the following:
diff --git a/src/codecs/music_mpg123.c b/src/codecs/music_mpg123.c
--- a/src/codecs/music_mpg123.c
+++ b/src/codecs/music_mpg123.c
@@ -231,6 +231,7 @@ static void *MPG123_CreateFromRW(SDL_RWo
     music->mp3file.src = src;
     music->volume = MIX_MAX_VOLUME;
 
+    music->mp3file.start  = SDL_RWtell(src);
     music->mp3file.length = SDL_RWsize(src);
     if (mp3_skiptags(&music->mp3file, SDL_TRUE) < 0) {
         SDL_free(music);

... _but only if_ I apply the patch stdio_size() patch to SDL2.

SDL_rwops.c::windows_file_size() has the same issue.
Comment 2 Ozkan Sezer 2021-01-28 14:07:51 UTC 
I fixed the bug #4410 issue by not using SDL_RWsize() and fixing
other mistakes in the code there.

However the SDL_RWsize() concerns I noted here still remain. Not
sure if they are to be fixed, though.
Comment 3 Ozkan Sezer 2021-02-05 12:26:27 UTC 
Closing this as WONTFIX -- any changes to RWsize() would be too
intrusive and result in other possible bugs.