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Bug 4451

Summary: [patch] Loading GIFs on multiple threads returns garbage output and crashes
Product: SDL_image Reporter: David Lönnhager <dv.lnh.d>
Component: miscAssignee: Sam Lantinga <slouken>
Status: RESOLVED FIXED QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2 CC: gapkalov
Version: 2.0.4   
Hardware: x86_64   
OS: Windows 10   
Attachments: Load GIFs on multiple threads
fix access violation

Description David Lönnhager 2019-01-10 22:22:56 UTC 
Created attachment 3565 [details]
Load GIFs on multiple threads

It's due to the use of global variables and static local variables. A quick fix was moving all of that into a "state" struct and declaring it in IMG_LoadGIF_RW(), which apparently fixed the issue.

The left image shows what the output might have looked like before the patch: https://i.imgur.com/bs26rG8.png

Loading PNGs, JPGs, and BMPs on multiple threads seems to work.
Comment 1 Sam Lantinga 2019-06-10 23:32:58 UTC 
Fixed, thanks!
https://hg.libsdl.org/SDL_image/rev/07fe7a23da89
Comment 2 Dmitry Gapkalov 2019-06-18 16:28:43 UTC 
Hello, I have a crash with this changes. Crash happens in:

    state->buf[0] = state->buf[state->last_byte - 2];   // access violation becasu in last_byte - trash
    state->buf[1] = state->buf[state->last_byte - 1];

    if ((count = GetDataBlock(src, &state->buf[2], state)) <= 0)
        state->done = TRUE;

    state->last_byte = 2 + count; // last_byte  initialization
Comment 3 Dmitry Gapkalov 2019-06-18 16:35:34 UTC 
Created attachment 3829 [details]
fix access violation

patch to fix access violation
Comment 4 Sam Lantinga 2019-06-19 13:51:54 UTC 
Patch added, thanks!
https://hg.libsdl.org/SDL_image/rev/02e1f7b5746c