Call a function in a shared library
You are encouraged to solve this task according to the task description, using any language you may know.
Show how to call a function in a shared library (without dynamically linking to it at compile-time). In particular, show how to call the shared library function if the library is available, otherwise use an internal equivalent function.
C
Tested with gcc on a GNU/Linux system (on GNU/Linux dl* functions are available linking to libdl, i.e. with -ldl option)
<lang c>#include <stdio.h>
- include <stdlib.h>
- include <dlfcn.h>
int myopenimage(char *in) {
static int handle=0; fprintf(stderr, "internal openimage opens %s...\n", in); return handle++;
}
int main() {
void *imglib; int (*extopenimage)(char *); int imghandle;
imglib = dlopen("./fakeimglib.so", RTLD_LAZY);
if ( imglib != NULL ) {
/* extopenimage = (int (*)(char *))dlsym(imglib,...)
"man dlopen" says that C99 standard leaves casting from
"void *" to a function pointer undefined. The following is the
POSIX.1-2003 workaround found in man */
*(void **)(&extopenimage) = dlsym(imglib, "openimage");
/* the following works with gcc, gives no warning even with
-Wall -std=c99 -pedantic options... :D */
/* extopenimage = dlsym(imglib, "openimage"); */
imghandle = extopenimage("fake.img");
} else {
imghandle = myopenimage("fake.img");
}
printf("opened with handle %d\n", imghandle);
/* ... */
if (imglib != NULL ) dlclose(imglib);
return EXIT_SUCCESS;
}</lang>
The fake fakeimglib.so code is
<lang c>#include <stdio.h> /* gcc -shared -nostartfiles fakeimglib.c -o fameimglib.so */ int openimage(char *s) {
static int handle = 100; fprintf(stderr, "opening %s\n", s); return handle++;
}</lang>
When the library fakeimglib.c exists in the current directory (this choice is senseful only for testing purposes), the output is:
opening fake.img opened with handle 100
otherwise the output is:
internal openimage opens fake.img... opened with handle 0