Getting SO's to compile and load properly

Getting SO's to compile and load properly

DanielH

Member #934

January 2001

In another thread I mentioned my Sudoku game. I'm trying to get it to work in Linux. After some lengthy effort I'm getting some errors. In the game. generator.so is loaded, but I can't extract the functions.

After some work, I've made a simple test program and can't get the so to load.

// yeah.cpp
#include <stdio.h>

int p = 0;

int yeah()
{
    printf( "%d\n", ++p ); 
    
    return 0;
}

Compiled with:
g++ -o yeah.so -shared -fPIC yeah.cpp

1// main.cpp
2#include <stdio.h>
3#include <stdlib.h>
4#include <dlfcn.h>
5 
6int main( int argc, char **argv )
7{
8    int rt = 0;
9    const char modName[ 40 ] = "yeah.so";
10    void *library = NULL;
11    int (*yeah)() = NULL;
12 
13    
14    if ( !( library = dlopen( modName, RTLD_LAZY) ) )
15    {
16        printf( "Could not load: %s\n", modName );
17        return -1;
18    }
19 
20    yeah = (int (*)())dlsym( library, "yeah");
21    
22    if ( !yeah )
23    {
24        printf( "Could not extract function\n" );
25        return -2;
26    }
27 
28    for ( int i = 0; i < 5; i++ )
29    {
30        yeah();
31    }
32    
33    dlclose(library);
34    
35    return 0;
36}

Compiled with:
g++ -o main main.cpp -ldl

yeah.so and main are created with permissions 777. But when main is ran as
./main, I get the 'Could not load: yeah.so'

So, what am I doing wrong? Any suggestions to help me.

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ReyBrujo

Moderator

January 2001

Set your LDPATH (or was it LD_PATH?) to point to the directory where the .so file is. Otherwise, add it to /etc/ld.so.conf (or something similar) and run ldconfig. Or run ldconfig dir/where/so/is to temporarily add it to your linker path.

--
RB
光子「あたしただ…奪う側に回ろうと思っただけよ」
Mitsuko's last words, Battle Royale

miran

Member #2,407

June 2002

No, use absolute paths! For example:

char soName[] = "yeah.so"
char fullName[MAX_PATH];
replace_filename(fullName, argv[0], soName, MAX_PATH);
...

ReyBrujo: That's for dynamically loading plugins!

EDIT: Hmm, still doesn't work. Interesting, because I use very similar code and it works... :-/

--
sig used to be here

ReyBrujo

Moderator

January 2001

Oh, you are right. With the full path it loads the library but can't get the handle to the function.

(Edited: Got it working. The problem is that the function name is mangled in C++, thus you can't just load yeah, but instead _Z4yeahv (in my case). Some magic with extern "C" should make it work.

--
RB
光子「あたしただ…奪う側に回ろうと思っただけよ」
Mitsuko's last words, Battle Royale

Evert

Member #794

November 2000

Quote:

Set your LDPATH (or was it LD_PATH?)

LD_LIBRARY_PATH

miran

Member #2,407

June 2002

Oh I forgot he didn't have extern "C".

So, final solution:

1. extern "C" in front of the functions you export
2. load .so with full path

--
sig used to be here

DanielH

Member #934

January 2001

Ok I did that and the program runs fine. It wasn't loading the text correctly, but I fixed that also. I just need to compile it statically and add it to the download page.

Thanks Guys

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