How to link a dll, to an exe file in run-time? - c

Question

I am new to programming and I created a dll project in which, i will just print a line. In the application project I have called the function defined in the dll project.
My question is, I am getting the dll file, soon after building the dll project. But when I build the main app, i.e., application project i am getting the following error.
--------------------Configuration: test_bench - Win32 Debug--------------------
Compiling...
main.c
Linking...
main.obj : error LNK2001: unresolved external symbol _print_dll
../../exec/test_bench.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.
test_bench.exe - 2 error(s), 0 warning(s)
If I link the obj before the build, it gets built. But if I change the code of dll project, again i have to re-build the main project, which should not be necessary while running a dll.
Please help me to achieve this

Answer 1

I wrote a little binary template to help with dlopen related calls. You'd need to tweak it for your specific use case (as it is, it only handles strings), but I have found it handy on many occasions.
Usage: dlopener /path/to/library.extension function_name [args...]
//gcc -rdynamic -o dlopener dlopener.c -ldl
#include <dlfcn.h> /*for dlopen,dlsym,dlclose*/
int main(int argc, char **argv){
/* get a "handle" for a shared library*/
void *handle = dlopen(argv[1], RTLD_LAZY);
/* make sure we got a handle before continuing*/
if (! handle) return 1;
/*undefined, but workable solution : POSIX.1-2003 (Technical Corrigendum 1) */
void* (*f)()=dlsym(handle, argv[2]);
/*now call the function f(argv[3],argv[4],...argv[argc]); */
//TODO convert args to unsigned char representations for other types
while (argc > 2) /*ugh, have to use asm to preserve stack*/
asm("push %0"::"r"(argv[argc--])); /*from right to left*/
asm("call *%0"::"r"(f)); //TODO "=a"(ret) where is uchar[XXX]
/*remember that shared library we opened?*/
dlclose(handle);
return 0;
}
Note: for some WIN32 functions (cdecl calling convention IIRC) you need to use a different asm mechanism than push to get the values in registers instead of on the stack

Answer 2

This error indicates that you didn't link the library (DLL) in your executable, that's why it can't find the function print_dll.
You can load libraries dynamically in Windows through LoadLibray()
Here is a sample on how you do it: http://msdn.microsoft.com/en-us/library/windows/desktop/ms686944(v=vs.85).aspx

Answer 3

For linking the dll project in run time, you need to implement the run time linking in your main file. If you implement the the run time linking means, no need to re-build the project every time.
For run time linking you need the following functions
#include <dlfcn.h>
void *dlopen(const char *filename, int flag);
char *dlerror(void);
void *dlsym(void *handle, const char *symbol); //symbol is your function name
int dlclose(void *handle);
Link with -ldl.
See the man page for more details.
but while changing your code, if you change the function name you should use the same name in the place of symbol.