Getting The Size of a C++ Function

问题:

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 targeted platform is windows

The method I currently have in my head is the following:
1. Obtain a pointer to the function
2. Increment the Pointer (& counter) until I reach the machine code value for ret
3. The counter will be the size of the function?

Edit1: To clarify what I mean by 'size' I mean the number of bytes (machine code) that make up the function.
Edit2: There have been a few comments asking why or what do I plan to do with this. The honest answer is I have no intention, and I can't really see the benefits of knowing a functions length pre-compile time. (although I'm sure there are some)

This seems like a valid method to me, will this work?

回答1:

No, this will not work:

1. There is no guarantee that your function only contains a single ret instruction.
2. Even if it only does contain a single ret, you can't just look at the individual bytes - because the corresponding value could appear as simply a value, rather than an instruction.

The first problem can possibly be worked around if you restrict your coding style to, say, only have a single point of return in your function, but the other basically requires a disassembler so you can tell the individual instructions apart.

回答2:

It is possible to obtain all blocks of a function, but is an unnatural question to ask what is the 'size' of a function. Optimized code will rearrange code blocks in the order of execution and will move seldom used blocks (exception paths) into outer parts of the module. For more details, see Profile-Guided Optimizations for example how Visual C++ achieves this in link time code generation. So a function can start at address 0x00001000, branch at 0x00001100 into a jump at 0x20001000 and a ret, and have some exception handling code 0x20001000. At 0x00001110 another function starts. What is the 'size' of your function? It does span from 0x00001000 to +0x20001000, but it 'owns' only few blocks in that span. So your question should be unasked.

There are other valid questions in this context, like the total number of instructions a function has (can be determined from the program symbol database and from the image), and more importantly, what is the number of instructions in the frequent executed code path inside the function. All these are questions normally asked in the context of performance measurement and there are tools that instrument code and can give very detailed answers.

Chasing pointers in memory and searching for ret will get you nowhere I'm afraid. Modern code is way way way more complex than that.

回答3:

Wow, I use function size counting all the time and it has lots and lots of uses. Is it reliable? No way. Is it standard c++? No way. But that's why you need to check it in the disassembler to make sure it worked, every time that you release a new version. Compiler flags can mess up the ordering.

static void funcIwantToCount()
{
   // do stuff
}
static void funcToDelimitMyOtherFunc()
{
   __asm _emit 0xCC
   __asm _emit 0xCC
   __asm _emit 0xCC
   __asm _emit 0xCC
}

int getlength( void *funcaddress )
{
   int length = 0;
   for(length = 0; *((UINT32 *)(&((unsigned char *)funcaddress)[length])) != 0xCCCCCCCC; ++length);
   return length;
}

It seems to work better with static functions. Global optimizations can kill it.

P.S. I hate people, asking why you want to do this and it's impossible, etc. Stop asking these questions, please. Makes you sound stupid. Programmers are often asked to do non-standard things, because new products almost always push the limits of what's availble. If they don't, your product is probably a rehash of what's already been done. Boring!!!

回答4:

This won't work... what if there's a jump, a dummy ret, and then the target of the jump? Your code will be fooled.

In general, it's impossible to do this with 100% accuracy because you have to predict all code paths, which is like solving the halting problem. You can get "pretty good" accuracy if you implement your own disassembler, but no solution will be nearly as easy as you imagine.

A "trick" would be to find out which function's code is after the function that you're looking for, which would give pretty good results assuming certain (dangerous) assumptions. But then you'd have to know what function comes after your function, which, after optimizations, is pretty hard to figure out.

---

Edit 1:

What if the function doesn't even end with a ret instruction at all? It could very well just jmp back to its caller (though it's unlikely).

---

Edit 2:

Don't forget that x86, at least, has variable-length instructions...

---

Update:

For those saying that flow analysis isn't the same as solving the halting problem:

Consider what happens when you have code like:

foo:
    ....
    jmp foo

You will have to follow the jump each time to figure out the end of the function, and you cannot ignore it past the first time because you don't know whether or not you're dealing with self-modifying code. (You could have inline assembly in your C++ code that modifies itself, for instance.) It could very well extend to some other place of memory, so your analyzer will (or should) end in an infinite loop, unless you tolerate false negatives.

Isn't that like the halting problem?

回答5:

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.

回答6:

This can work in very limited scenarios. I use it in part of a code injection utility I wrote. I don't remember where I found the information, but I have the following (C++ in VS2005):

#pragma runtime_checks("", off)

static DWORD WINAPI InjectionProc(LPVOID lpvParameter)
{
    // do something
    return 0;
}

static DWORD WINAPI InjectionProcEnd()
{
    return 0;
}

#pragma runtime_checks("", on)

And then in some other function I have:

size_t cbInjectionProc = (size_t)InjectionProcEnd - (size_t)InjectionProc;

You have to turn off some optimizations and declare the functions as static to get this to work; I don't recall the specifics. I don't know if this is an exact byte count, but it is close enough. The size is only that of the immediate function; it doesn't include any other functions that may be called by that function. Aside from extreme edge cases like this, "the size of a function" is meaningless and useless.

回答7:

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.

回答8:

What do you mean "size of a function"?

If you mean a function pointer than it is always just 4 bytes for 32bits systems.

If you mean the size of the code than you should just disassemble generated code and find the entry point and closest ret call. One way to do it is to read the instruction pointer register at the beginning and at the end of your function.

If you want to figure out the number of instructions called in the average case for your function you can use profilers and divide the number of retired instructions on the number of calls.

回答9:

I think it will work on windows programs created with msvc, as for branches the 'ret' seems to always come at the end (even if there are branches that return early it does a jne to go the end). However you will need some kind of disassembler library to figure the current opcode length as they are variable length for x86. If you don't do this you'll run into false positives.

I would not be surprised if there are cases this doesn't catch.

回答10:

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.

回答11:

I'm posting this to say two things:

1) Most of the answers given here are really bad and will break easily. If you use the C function pointer (using the function name), in a debug build of your executable, and possibly in other circumstances, it may point to a JMP shim that will not have the function body itself. Here's an example. If I do the following for the function I defined below:

FARPROC pfn = (FARPROC)some_function_with_possibility_to_get_its_size_at_runtime;

the pfn I get (for example: 0x7FF724241893) will point to this, which is just a JMP instruction:

Additionally, a compiler can nest several of those shims, or branch your function code so that it will have multiple epilogs, or ret instructions. Heck, it may not even use a ret instruction. Then, there's no guarantee that functions themselves will be compiled and linked in the order you define them in the source code.

You can do all that stuff in assembly language, but not in C or C++.

2) So that above was the bad news. The good news is that the answer to the original question is, yes, there's a way (or a hack) to get the exact function size, but it comes with the following limitations:

• It works in 64-bit executables on Windows only.

• It is obviously Microsoft specific and is not portable.

• You have to do this at run-time.

The concept is simple -- utilize the way SEH is implemented in x64 Windows binaries. Compiler adds details of each function into the PE32+ header (into the IMAGE_DIRECTORY_ENTRY_EXCEPTION directory of the optional header) that you can use to obtain the exact function size. (In case you're wondering, this information is used for catching, handling and unwinding of exceptions in the __try/__except/__finally blocks.)

Here's a quick example:

//You will have to call this when your app initializes and then
//cache the size somewhere in the global variable because it will not
//change after the executable image is built.

size_t fn_size; //Will receive function size in bytes, or 0 if error
some_function_with_possibility_to_get_its_size_at_runtime(&fn_size);

and then:

#include <Windows.h>

//The function itself has to be defined for two types of a call:
// 1) when you call it just to get its size, and
// 2) for its normal operation
bool some_function_with_possibility_to_get_its_size_at_runtime(size_t* p_getSizeOnly = NULL)
{
    //This input parameter will define what we want to do:
    if(!p_getSizeOnly)
    {
        //Do this function's normal work
        //...

        return true;
    }
    else
    {
        //Get this function size
        //INFO: Works only in 64-bit builds on Windows!
        size_t nFnSz = 0;

        //One of the reasons why we have to do this at run-time is
        //so that we can get the address of a byte inside 
        //the function body... we'll get it as this thread context:
        CONTEXT context = {0};
        RtlCaptureContext(&context);

        DWORD64 ImgBase = 0;
        RUNTIME_FUNCTION* pRTFn = RtlLookupFunctionEntry(context.Rip, &ImgBase, NULL);
        if(pRTFn)
        {
            nFnSz = pRTFn->EndAddress - pRTFn->BeginAddress;
        }

        *p_getSizeOnly = nFnSz;
        return false;
    }
}

回答12:

Just set PAGE_EXECUTE_READWRITE at the address where you got your function. Then read every byte. When you got byte "0xCC" it means that the end of function is actual_reading_address - 1.

回答13:

Using GCC, not so hard at all.

void do_something(void) { 
   printf("%s!", "Hello your name is Cemetech"); 
   do_something_END: 
} 

... 

   printf("size of function do_something: %i", (int)(&&do_something_END - (int)do_something));

回答14:

below code the get the accurate function block size, it works fine with my test runtime_checks disable _RTC_CheckEsp in debug mode

    #pragma runtime_checks("", off)
DWORD __stdcall loadDll(char* pDllFullPath)
{  
    OutputDebugStringA(pDllFullPath);
    //OutputDebugStringA("loadDll...................\r\n");
    return 0;
    //return test(pDllFullPath);
}
#pragma runtime_checks("", restore)

DWORD __stdcall getFuncSize_loadDll()
{
    DWORD maxSize=(PBYTE)getFuncSize_loadDll-(PBYTE)loadDll;
    PBYTE pTail=(PBYTE)getFuncSize_loadDll-1;
    while(*pTail != 0xC2 && *pTail != 0xC3) --pTail;
    if (*pTail==0xC2)
    {   //0xC3          : ret
        //0xC2 04 00    : ret 4
        pTail +=3;
    }

    return pTail-(PBYTE)loadDll;
};

回答15:

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.