Got "attempted to call an unknown symbol" for an hello world example

[dov]

In order to learn how to use the loadlbrary library with the goal of accessing a commercial win32 library from linux, I started off with a simple hello world example.

Here are my files:

• say-hello.h

  
  #ifndef SAY_HELLO_H
  #define SAY_HELLO_H
  void say_hello();
  #endif /* SAY-HELLO */

* say-hello.c

include

void say_hello() { printf("Say Hello!\n"); }

* hello-main.c - For testing the dll with wine

include "say-hello.h"

int main(int argc, char*argv[]) { say_hello(); }

* The build command of the dll:

i686-w64-mingw32-gcc -o hello-main.obj -c -O2 -mms-bitfields hello-main.c i686-w64-mingw32-gcc -o say-hello.obj -c -fPIC -O2 -mms-bitfields say-hello.c i686-w64-mingw32-gcc -o say-hello.dll -shared say-hello.obj i686-w64-mingw32-gcc -o hello-main.exe hello-main.obj say-hello.dll

* Testing the dll works:

wine hello-main.exe 000b:fixme:winediag:start_process Wine Staging 4.0-rc2 is a testing version containing experimental patches. 000b:fixme:winediag:start_process Please mention your exact version when filing bug reports on winehq.org. Say Hello!

Now I wanted to load this dll into load library and I started at mpclient.c sources and removed as much as I could and then tried to call my say_hello() function. This is what I got:

ifndef _GNU_SOURCE

define _GNU_SOURCE

endif

include

include

include

include

include

include

include "winnt_types.h"

include "pe_linker.h"

include "ntoskernel.h"

int main(int argc, char argv, char envp) { PIMAGE_DOS_HEADER DosHeader; PIMAGE_NT_HEADERS PeHeader; struct pe_image image = { .entry = NULL, .name = "say-hello.dll", };

// Load the mpengine module.
if (pe_load_library(image.name, &image.image, &image.size) == false) {
    printf("You must add the dll and vdm files to the engine directory");
    return 1;
}

// Handle relocations, imports, etc.
link_pe_images(&image, 1);

// Fetch the headers to get base offsets.
DosHeader   = (PIMAGE_DOS_HEADER) image.image;
PeHeader    = (PIMAGE_NT_HEADERS)(image.image + DosHeader->e_lfanew);

// Try calling into the library.
void (*SayHelloPtr)();
if (get_export("say_hello", &SayHelloPtr) != -1) {
    (*SayHelloPtr)();
}
else
    printf("Failed getting pointer to say_hello()!\n");

return 0;

}

It coompiles, the dll loads, the `say_hello` symbol is found, but it then crashes at `(*SayHelloPtr)()`. What did I miss?

Thanks!

[dov]

After I wrote the above, I realized that perhaps the "attempt to call an unknown symbol" error is the call to printf(). I then added a new function to my dll:

int32_t add_hello(int32_t a, int32_t b);

and called it as follows:

    // Try calling into the library.
    int32_t (*AddHelloPtr)(int32_t a, int32_t b);
    if (get_export("add_hello", &AddHelloPtr) != -1) {
        int a=10,b=32;
        printf("%d+%d=%d\n", a,b,AddHelloPtr(a,b));
    }
    else
        printf("Failed getting pointer to say_hello()!\n");

and this worked!

But I still wonder how would I load the windows CRT library so that I can call printf (and other io functions)? Or is there a way to redirect the DLL printf() call to the gcc libc printf() .