dlsym
- C course
- Advanced
- Lesson 50/83
- 5 min
Learn to resolve symbols with dlsym.
What is dlsym?
dlsym retrieves the address of a symbol (function or variable) from a loaded library. You get the address of printf with void *sym = dlsym(h, "printf"). The return is void *; you cast it to the type you want.
On failure it returns NULL; but the real check is done with dlerror (next lesson). You can cast the address obtained via dlsym to a function pointer and call it: typedef void (*Fn)(void); Fn f = (Fn)dlsym(h, "foo"); f();. For testing, dlsym(h, "printf") usually succeeds from the main program.
The symbol resolution pattern
The typical flow: void *h = dlopen(NULL, RTLD_LAZY);, void *sym = dlsym(h, "printf");, printf("symbol found\n");, if (h) dlclose(h);. This prints that the symbol was resolved.
Actually calling the symbol comes in the next lesson; here we just show the address being fetched. The output is deterministic: printf is always found in the main program. If you look for a different symbol and it is not found, sym becomes NULL; then dlerror gives the error message.
Practice Resolve a symbol
Get a handle with void *h = dlopen(NULL, RTLD_LAZY);. Resolve the printf symbol with void *sym = dlsym(h, "printf");. Print with printf("symbol found\n");. Close with if (h) dlclose(h);.
Starter code
#include <stdio.h>
#include <dlfcn.h>
// Load library
// Resolve symbol
// Print and close
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
#include <dlfcn.h>
int main(void) {
void *h = dlopen(NULL, RTLD_LAZY);
void *sym = dlsym(h, "printf");
printf("symbol found\n");
if (h) dlclose(h);
return 0;
}Expected output
symbol found
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match dlsym parts
| Term | Meaning |
|---|---|
dlsym |
Symbol address |
void * |
Return type |
cast |
Type conversion |
NULL |
Error |
2 What type does dlsym return?
- int
- void * Correct answer
- char *
- long
Explanation: dlsym returns void *; you cast to use it.
3 dlsym call
void *sym = ____(h, "printf");void *sym = dlsym(h, "printf");Explanation: dlsym gets the symbol address from the h handle.