Getting The Size of a C++ Function

Question

I was reading this question because I\'m trying to find the size of a function in a C++ program, It is hinted at that there may be a way that is platform specific. My target

Answer 1

In C++, the there is no notion of function size. In addition to everything else mentioned, preprocessor macros also make for an indeterminate size. If you want to count number of instruction words, you can't do that in C++, because it doesn't exist until it's been compiled.

Answer 2

The real solution to this is to dig into your compiler's documentation. The ARM compiler we use can be made to produce an assembly dump (code.dis), from which it's fairly trivial to subtract the offsets between a given mangled function label and the next mangled function label.

I'm not certain which tools you will need for this with a windows target, however. It looks like the tools listed in the answer to this question might be what you're looking for.

Also note that I (working in the embedded space) assumed you were talking about post-compile-analysis. It still might be possible to examine these intermediate files programmatically as part of a build provided that:

• The target function is in a different object
• The build system has been taught the dependencies
• You know for sure that the compiler will build these object files

Note that I'm not sure entirely WHY you want to know this information. I've needed it in the past to be sure that I can fit a particular chunk of code in a very particular place in memory. I have to admit I'm curious what purpose this would have on a more general desktop-OS target.

Answer 3

The non-portable, but API-based and correctly working approach is to use program database readers - like dbghelp.dll on Windows or readelf on Linux. The usage of those is only possible if debug info is enabled/present along with the program. Here's an example on how it works on Windows:

SYMBOL_INFO symbol = { };

symbol.SizeOfStruct = sizeof(SYMBOL_INFO);

// Implies, that the module is loaded into _dbg_session_handle, see ::SymInitialize & ::SymLoadModule64
::SymFromAddr(_dbg_session_handle, address, 0, &symbol);

You will get the size of the function in symbol.Size, but you may also need additional logic identifying whether the address given is a actually a function, a shim placed there by incremental linker or a DLL call thunk (same thing).

I guess somewhat similar can be done via readelf on Linux, but maybe you'll have to come up with the library on top of its sourcecode...

You must bear in mind that although disassembly-based approach is possible, you'll basically have to analyze a directed graph with endpoints in ret, halt, jmp (PROVIDED you have incremental linking enabled and you're able to read jmp-table to identify whether the jmp you're facing in function is internal to that function (missing in image's jmp-table) or external (present in that table; such jmps frequently occur as part of tail-call optimization on x64, as I know)), any calls that are meant to be nonret (like an exception generating helper), etc.

Answer 4

There is no facilities in Standard C++ to obtain the size or length of a function.
See my answer here: Is it possible to load a function into some allocated memory and run it from there?

In general, knowing the size of a function is used in embedded systems when copying executable code from a read-only source (or a slow memory device, such as a serial Flash) into RAM. Desktop and other operating systems load functions into memory using other techniques, such as dynamic or shared libraries.