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Asked: 2026-05-10T22:05:21+00:00

As I stated in this question , I am using SDL for a small

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As I stated in this question, I am using SDL for a small game I’m developing. Now I am having problems with SDL_DisplayFormatAlpha. I am trying to create a surface with an alpha channel from a PNG image. It was working before, but now that I’ve done some slight refactoring something got broken. I’ve narrowed it down to this constructor:

 Surface::Surface( tfilename file ) {     // initialize the surface data member to the image indicated by filename     SDL_Surface *tempSurface;     tempSurface = IMG_Load( file.c_str() );     if ( !tempSurface ) {         surface = NULL;         exit(1);     }     else {         surface = SDL_DisplayFormatAlpha( tempSurface );         //surface = tempSurface;     }     SDL_FreeSurface( tempSurface ); }

This compiles just fine, but when I run it I get a Segmentation fault. The error reported by gdb:

 Program received signal SIGSEGV, Segmentation fault. [Switching to Thread 0xb79c16c0 (LWP 8089)] 0xb7e8b9a3 in SDL_DisplayFormatAlpha () from /usr/lib/libSDL-1.2.so.0

The stack trace is as follows:

 #0  0xb7e8b9a3 in SDL_DisplayFormatAlpha () from /usr/lib/libSDL-1.2.so.0 #1  0x0804987e in Surface (this=0x804d060, file=@0xbfb20760) at Surface.cpp:16 #2  0x0804a159 in Image (this=0x804d038, x=0, y=0, file=@0xbfb207a0)     at Image.cpp:16 #3  0x0804a3de in Object (this=0x804d028, imageFile=@0xbfb207dc)     at Object.cpp:4 #4  0x080491cb in Application (this=0xbfb20810) at Application.cpp:8 #5  0x08048e0d in main () at main.cpp:5

If I comment out surface = SDL_DisplayFormatAlpha( tempSurface ); and SDL_FreeSurface( tempSurface ); and uncomment surface = tempSurface; like so:

  Surface::Surface( tfilename file ) {     // initialize the surface data member to the image indicated by filename     SDL_Surface *tempSurface;     tempSurface = IMG_Load( file.c_str() );     if ( !tempSurface ) {         surface = NULL;         exit(1);     }     else {         //surface = SDL_DisplayFormatAlpha( tempSurface );         surface = tempSurface;     }     //SDL_FreeSurface( tempSurface ); }

Then it seems to work just fine. Can anyone tell me what’s going on? Actually, the transparency seems to work, too when I comment out SDL_DisplayFormatAlpha. Is that function only meant to be used with images that do not already have an alpha channel?

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1 Answer

  1. IMG_Load should handle transparent PNG’s automatically, as the end of your post notes. What is the actual exception/error being thrown? Your stack trace doesn’t show that.

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