Re: [png-mng-implement] png_read_png() hanging...

[Sujeet A. <SA...@ra...>]

After setting up my custom read function, I discovered that it was 
longjmp'ing away at the same point every time. I suppose this could be a 
problem with the file, however, this is simply the pngtest.png provided 
with libpng. Here is my custom read fn:

void my_read_data(png_structp png_ptr, png_bytep data, png_size_t length)
{
        PNGTK* pngtk = (PNGTK*) png_get_io_ptr(png_ptr); //simple struct 
with a pointer to the buffer with my png,
                                                                        // 
offset from the original start of the buffer, and its size

        NKDbgPrintfW(L"[Sujeet, buf=%p; dwPos=%u; 
dwSize=%u!]\r\n",pngtk->buf, pngtk->dwPos, pngtk->dwSize); //some serial 
output calls
        NKDbgPrintfW(L"[Sujeet, png_ptr=%p; data=%p; 
length=%i!]\r\n",png_ptr, data, length);
        memcpy(data, pngtk->buf, length);
        pngtk->buf += 8;
        pngtk->dwPos += 8;
        NKDbgPrintfW(L"[Sujeet, I copied %i bytes from pngtk->buf to 
data!]\r\n",length);
        return;
}

>From the output, I see that it longjmp to the code where I say "we got 
hosed", and it always does so after 16bytes.

[Sujeet, buf=100010; dwPos=0; dwSize=8574!]
[Sujeet, png_ptr=4DDF0; data=5EDE0; length=8!]
[Sujeet, I copied 8 bytes from pngtk->buf to data!]
[Sujeet, buf=100018; dwPos=8; dwSize=8574!]
[Sujeet, png_ptr=4DDF0; data=160AA1B0; length=4!]
[Sujeet, I copied 4 bytes from pngtk->buf to data!]
[Sujeet, buf=100020; dwPos=16; dwSize=8574!]
[Sujeet, png_ptr=4DDF0; data=4DF0C; length=4!]
[Sujeet, I copied 4 bytes from pngtk->buf to data!]
[Sujeet, we got hosed!]

Any ideas on what's going on here? Would it be worthwhile to use the chunk 
callbacks?

Sujeet


png...@li... wrote on 11/28/2006 
10:10:33 AM:

> 
> It turns out libpng was having problems reading the image: I guess 
> the longjmp/setjmp stuff gets messed up while debugging. Currently, 
> I am trying to write my own read fn's for libpng to use, but I would
> much rather just use the FILE* but do not know how to debug this, as
> it does happen consistently. Any ideas? 
> 
> Sujeet 
> 
> png...@li... wrote on 11/21/2006 
> 01:28:36 PM:
> 
> > 
> > In my call to png_read_png, the function simply hangs. I thought 
> > that if hit a bad chunk or something like that it would longjmp back
> > and return; out of my function. Here is my setup: 
> > 
> > 
> > // the contents of the file named szSaveFileName have been verified 
> > as a PNG already by png_sig_cmp, while it was sitting in a buffer, 
> > before being written out. 
> > 
> > if ((fpPng = fopen(szSaveFileName, "rb")) == NULL ) 
> >         return; 
> > 
> > png_structp png_ptr = png_create_read_struct (PNG_LIBPNG_VER_STRING,
> > NULL, NULL, NULL);  // I thought using NULL for the error fn's could
> > have been why, but the sample 
> > 
> > //  provided by greg readpng.c creates it this way, as well. 
> > 
> > if (!png_ptr) 
> >         return; 
> > 
> > png_infop info_ptr = png_create_info_struct(png_ptr); 
> > if (!info_ptr) 
> > { 
> >         png_destroy_read_struct(&png_ptr, (png_infopp)NULL, 
> (png_infopp)NULL);
> >         return; 
> > } 
> > 
> > if (setjmp(png_jmpbuf(png_ptr))) 
> > { 
> >   png_destroy_read_struct(&png_ptr, &info_ptr, png_infopp_NULL); 
> >   fclose(fpPng); 
> >   return; 
> > } 
> > 
> > // all of these statements complete successfully. 
> > png_init_io(png_ptr, fpPng); 
> > 
> > // while debugging, upon execution of this next statement, the 
> > debugger remains in [run] with nothing happening. no statements 
> > after this are executed 
> > png_read_png(png_ptr, info_ptr, PNG_TRANSFORM_IDENTITY, 
png_voidp_NULL); 
> > 
> > Any help is appreciated. 
> > 
> > Sujeet
> > 
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