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ایجاد یک کتابخانه پویا و فراخوانی آن در برنامه در لینوکس

Prototype include file for the library: ctest.h

#ifndef CTEST_H
#define CTEST_H

#ifdef __cplusplus
extern "C" {
#endif

void ctest1(int *);
void ctest2(int *);

#ifdef __cplusplus
}
#endif

#endif
Use the notation extern "C" so the libraries can be used with C and C++. This statement prevents the C++ from name mangling and thus creating "unresolved symbols" when linking.

Load and unload the library libctest.so (created above), dynamically:

#include <stdio.h>
#include <dlfcn.h>
#include "ctest.h"

int main(int argc, char **argv) 
{
   void *lib_handle;
   double (*fn)(int *);
   int x;
   char *error;

   lib_handle = dlopen("/opt/lib/libctest.so", RTLD_LAZY);
   if (!lib_handle) 
   {
      fprintf(stderr, "%s\n", dlerror());
      exit(1);
   }

   fn = dlsym(lib_handle, "ctest1");
   if ((error = dlerror()) != NULL)  
   {
      fprintf(stderr, "%s\n", error);
      exit(1);
   }

   (*fn)(&x);
   printf("Valx=%d\n",x);

   dlclose(lib_handle);
   return 0;
}

gcc -rdynamic -o progdl progdl.c -ldl

Explanation:

  • dlopen("/opt/lib/libctest.so", RTLD_LAZY); 
    Open shared library named "libctest.so". 
    The second argument indicates the binding. See include file dlfcn.h. 
    Returns NULL if it fails. 
    Options:
    • RTLD_LAZY: If specified, Linux is not concerned about unresolved symbols until they are referenced.
    • RTLD_NOW: All unresolved symbols resolved when dlopen() is called.
    • RTLD_GLOBAL: Make symbol libraries visible.
  • dlsym(lib_handle, "ctest1"); 
    Returns address to the function which has been loaded with the shared library.. 
    Returns NULL if it fails. 
    Note: When using C++ functions, first use nm to find the "mangled" symbol name or use the extern "C" construct to avoid name mangling. 
    i.e. extern "C" void function-name();

Object code location: Object code archive libraries can be located with either the executable or the loadable library. Object code routines used by both should not be duplicated in each. This is especially true for code which use static variables such as singleton classes. A static variable is global and thus can only be represented once. Including it twice will provide unexpected results. The programmer can specify that specific object code be linked with the executable by using linker commands which are passed on by the compiler.

Use the "-Wl" gcc/g++ compiler flag to pass command line arguments on to the GNU "ld" linker.

Example makefile statement: g++ -rdynamic -o appexe $(OBJ) $(LINKFLAGS) -Wl,--whole-archive -L{AA_libs} -laa -Wl,--no-whole-archive $(LIBS)

  • --whole-archive: This linker directive specifies that the libraries listed following this directive (in this case AA_libs) shall be included in the resulting output even though there may not be any calls requiring its presence. This option is used to specify libraries which the loadable libraries will require at run time.
  • -no-whole-archive: This needs to be specified whether you list additional object files or not. The gcc/g++ compiler will add its own list of archive libraries and you would not want all the object code in the archive library linked in if not needed. It toggles the behavior back to normal for the rest of the archive libraries.


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