PackFontRanges() creates a sigsegv crash

[heysokam]

Describe the bug
On file: stb_truetype.h
Before execution of line #4118, access to spc->pixels works normally
After execution of the loop, the address stops being accesible, and code that needs to access the pixel buffer just segfaults the application.

STBTT_DEF int stbtt_PackFontRanges(stbtt_pack_context *spc, const unsigned char *fontdata, int font_index, stbtt_pack_range *ranges, int num_ranges)
{
   stbtt_fontinfo info;
   int i,j,n, return_value = 1;
   //stbrp_context *context = (stbrp_context *) spc->pack_info;
   stbrp_rect    *rects;

// <---- spc->pixels memory is readable correctly, and zeroed as expected
   // flag all characters as NOT packed
   for (i=0; i < num_ranges; ++i)
      for (j=0; j < ranges[i].num_chars; ++j)
         ranges[i].chardata_for_range[j].x0 =
         ranges[i].chardata_for_range[j].y0 =
         ranges[i].chardata_for_range[j].x1 =
         ranges[i].chardata_for_range[j].y1 = 0;
// <---- spc->pixels memory is not accessible anymore
   n = 0;

The line that crashes with SIGSEGV is at #3192

result->pixels[j*result->stride + i] = (unsigned char) m;

Code in between this line, and the previously mentioned for loop, works correctly without crashing.

To Reproduce

• Follow the Font Packing documentation in stb_truetype
• Try to pack font LiberationMono-Bold.ttf, with charset range 0x20 to 0x7F
• Compile on linux, with gcc -ggdb
• Execution will end with a Segmentation Fault, during the call to stbtt_PackFontRange

Expected behavior
If the spc->pixels buffer is accessible correctly right before the loop to flag all characters as not packed, code that doesn't modify that pixel buffer shouldn't break access to the buffer either.

Screenshots

[nothings]

The loop in question only processes ranges. ranges is a user-supplied array of structs. It sounds like you haven't initialized ranges correctly in your code.

[heysokam]

As mentioned on "steps to reproduce", I'm using stbtt_PackFontRange, not stbtt_PackFontRanges (note the missing S) which generates such ranges struct from the function inputs int first_unicode_codepoint_in_range and int num_chars_in_range in stb_truetype.h from lines #4150 to #4155, right before calling stbtt_PackFontRanges with such self generated data

As said, the value for first is 0x20 and the value for num is 0x7F - 0x20, which resolves to 95. This covers all ascii printable characters, starting from space and ending in tilde. I checked with the debugger, and the integer values supplied to PackRange are indeed those at runtime.

Are these incorrect integer values to pass to the function?

[heysokam]

I gathered a bit more data.
The loop breaks the spc->pixels address right before line #4120, when the value of j becomes 3. So assuming this happens during the value assignment of the previous iteration, when j=2
Stepping through the loop, I noticed that the contents of spc->nodes are also being updated. I don't know if this is expected behavior, but noting it just in case.

[nothings]

I can only guess because you're not actually showing YOUR code which is where the bug lies.

You pass in chardata_for_range to stbtt_PackFontRange. Because you're asking for 95 characters, chardata_for_range needs to be an array of 95 elements. But probably you only have one, and the loop in question is overwriting random memory on the stack. Probably the spc->pxiels pointer is different after the loop than it was before the loop, because it got overwritten.

[heysokam]

Been trying to figure out how to use stb_truetype.h to create a font bitmap atlas.
This is the code I have so far:
https://gist.github.com/heysokam/0270e3bcc36c6c0d537b476d5862dbc2#file-fontatlas-c-L291
As you can see, I haven't generated the chardata_for_range array, I just created an empty stbtt_packedchar struct, since I didn't know what to do with it.

As far as my understanding reaches (first year into programming) this data structure contains information that is stored in the font file itself?
But I don't know how to generate such data.

Is there existing documentation somewhere about this that I missed?
If not, how can one create the required array?

[nothings]

It is an ambiguity of C that a function prototype with a pointer parameter like stbtt_packedchar *chardata_for_range may either expect a single struct or an array of structs, an ambiguity that you must resolve by reading the documentation.

In this case it expects an array of structs, which you could try to declare as:

PackChar fontPack[charNum];

However, in most versions of C this won't actually work because of how const works, so you need to either use a #define instead, or just directly declare the size you need (which isn't future-proof):

PackChar fontPack[95];

You don't need to initialize it beyond that, as it is the job of stbtt_PackFontRange to fill it out.

[heysokam]

That worked, thank you. I always fail at the smallest things possible 😔

Goal is to use this in a game engine, but such engine doesn't render quads, and also asking for render calls from the virtual machine overcomplicates things by a million (its not coded to allow that at all).
So aiming to just write a TGA file (the default format expected by the engine), that I can then cache directly using the pre-existing engine methods for that task.

How do you access the packed bitmap buffer, after the call to PackFontRanges has ended?
The documentation mentions stbtt_GetPackedQuad, but is this function usable to generate a bitmap image file directly from code?

[nothings]

The bitmap buffer is the same one you created, atlasBit. The info needed to indicate where each character is in the pixel buffer is in fontPack. stbtt_GetPackedQuad is irrelevant, it's a convenience function for rendering quads which is irrelevant if you're not rendering quads.