Mach-O file LC_FUNCTION_STARTS load command

Question

Does anyone know what the format of the data pointed to by the Mach-O LC_FUNCTION_STARTS command is?

The most information I could find is in the loader.h header file:

#define LC_FUNCTION_STARTS 0x26 /* compressed table of function start addresses */

I see the dyldinfo tool has a -function_starts option which analyzes this data and the tool is open source, but the latest released version of the tool doesn't contain the support:

http://opensource.apple.com/source/ld64/ld64-97.2/src/other/dyldinfo.cpp

Does anyone know where I can get the source for the latest version of dyldinfo, or where I can get more information on this load command?

Thanks!

Answer 1

Since I haven't got any additional answers or comments in a few days I thought I might as well answer this myself. The solution is basically what I put in a comment above:

The newest version of dyldinfo is located here:

http://opensource.apple.com/source/ld64/ld64-127.2/src/other/dyldinfo.cpp

However I still never figured out exactly what the function starts are used for, if anyone has info on that I'd still appreciate it.

Answer 2

It's used by tools that need to symbolicate addresses in crash logs, samples, spindumps, etc. to determine if a given address falls inside a function. It could also be useful to debuggers to help them more quickly find the bounds of the function that a given address is within.

The data within this section is formatted as a zero-terminated sequence of DWARF-style ULEB128 values. The first value is the offset from the start of the __TEXT segment to the start of the first function. The remaining values is the offset to the start of the next function.