SDL_DisplayFormatAlpha problem (C ++)

As I stated in this question , I am using SDL for a small game that I am developing. Now I am having problems with SDL_DisplayFormatAlpha. I am trying to create a surface with alpha channel from a PNG image. This used to work, but now that I've reworked a bit, something is broken. I 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 error. 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 looks like this:

# 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 surface = SDL_DisplayFormatAlpha( tempSurface );

and SDL_FreeSurface( tempSurface );

and uncomment surface = tempSurface;

like this:

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 everything seems to work fine. Can anyone tell me what's going on? In fact, transparency seems to work too when I comment out SDL_DisplayFormatAlpha. Is this feature only meant to be used with images that don't already have an alpha channel?