C function pointer in using for dlsym()

[van7hu]

Hi all,
I am currently learning a tutorial about library.(Original :http://www.yolinux.com/TUTORIALS/LibraryArchives-StaticAndDynamic.html)
The tutorial is to create a "Dynamically linked shared object libraries",it has 3 sample file as following:
ctest1.c:


void ctest1(int *i)
{
*i=5;
} ctest2.c


void ctest2(int *i)
{
*i=100;
}Using these 2 files to create a library name:libctest.so
Now,the program to use the library is: prog.c


#include <stdio.h> #include <dlfcn.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;
}
This program is compiled and working well.
Now,my question is about the function pointer 'fn' in prog,as I think prog.c calls function ctest1() via function pointer 'fn',here is where I am confused,as ctest1() is defined to return void ,and *fn is defined to return double,so I think if using them together this way,we'll get warning: 'assignment from incompatible pointer type'. But I don't know why the program worked ?

[spjackson]

Now,my question is about the function pointer 'fn' in prog,as I think prog.c calls function ctest1() via function pointer 'fn',here is where I am confused,as ctest1() is defined to return void ,and *fn is defined to return double,so I think if using them together this way,we'll get warning: 'assignment from incompatible pointer type'. But I don't know why the program worked ?
When you compile (and link) prog.c, the compiler knows nothing about ctest1(): there's no function prototype available, and you do not call the function directly.

The magic happens in


fn = dlsym(lib_handle, "ctest1");
If you were assigning the address of a known function, type checking would be possible, but you are not. The address is found at runtime and returned via void * which dlsym returns. When data (in this case the address of a function) is passed via void *, no type checking is possible.

[van7hu]

thank spjackson,it makes me understand more clearly about type checking .