Call function from shared library in GCC with inline assembler

Question

I've created shared library (will be used like plugin). There are a lot of functions like a

extern "C" long __attribute__ ((__cdecl__)) SumAgrs(long X, long Y, long Z, long *Out)
{
    *Out = X + Y + Z;
    return 0;
}

I would like to call functions from this library in C++(GCC, Linux), but not in compile time. When I use inline assembler, the "push" instruction corrupt local variables, and i have no idea how to fix it.

typedef int (*FARPROC)();

void *dl_handle = dlopen("plugin.so", RTLD_LAZY);
FARPROC proc = (FARPROC)dlsym(dl_handle, "SumAgrs");

long result;

asm("leal %0, %%eax\n\r" \
    "pushl %%eax" : : "m" (result));
asm("pushl $10");
asm("pushl $15");
asm("pushl $20");
asm("call *%0" : : "m" (proc));

Result binary file contains something like call *24(%esp). So my pushl change %esp and call cause segmentation fault. But how to avoid this behavior?

thx

ams · Accepted Answer · 2012-02-23 15:49:49Z

1

Look at libffi: "A Portable Foreign Function Interface Library"

"The libffi library provides a portable, high level programming interface to various calling conventions. This allows a programmer to call any function specified by a call interface description at run-time."

http://sourceware.org/libffi/

answered Feb 23, 2012 at 15:49

ams

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Comments

Lol4t0 · Accepted Answer · 2012-02-22 20:10:48Z

0

You needn't assembler to call functions, actually:

extern "C" 
{
    typedef long __attribute__ ((__cdecl__)) (*Proc)(long X, long Y, long Z, long *Out);
}

void *dl_handle = dlopen("plugin.so", RTLD_LAZY);
Proc proc = (Proc)dlsym(dl_handle, "SumAgrs");

proc(...); // call

Note, that either you call functions with c-code, or with ASM inline, your code for call generated in compile time. You cannot pass string variable to asm, because code for your asm inline should be generated during program compilation. That means you cannot do

std::string asm_code;
std:: cin >> asm_code;
asm(asm_code);

edited Feb 22, 2012 at 20:10

answered Feb 22, 2012 at 19:48

Lol4t0

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3 Comments

zdenek Over a year ago

Functions will be called from script and parameters are stored in array. So the compile time "call" isn't solution.

zdenek Over a year ago

no no, with the ASM inline, the code will be generated in run-time. For example (pseudocode): foreach (params) { push parm; } call func;

Lol4t0 Over a year ago

@user1173308, You cannot act like that! You never sure, that foreach(params) does not use stack pointer inside it, and of course, it does not assume you will change it by hands! Do not write such code

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