Can a running C program access its own symbol table?

Question

I have a linux C program that handles request sent to a TCP socket (bound to a particular port). I want to be able to query the internal state of the C program via a request to

Answer 1

This is actually fairly easy. You use dlopen / dlsym to access symbols. In order for this to work, the symbols have to be present in the dynamic symbol table. There are multiple symbol tables!

#include 
#include 

__attribute__((visibility("default")))
const char A[] = "Value of A";

__attribute__((visibility("hidden")))
const char B[] = "Value of B";

const char C[] = "Value of C";

int main(int argc, char *argv[])
{
    void *hdl;
    const char *ptr;
    int i;

    hdl = dlopen(NULL, 0);
    for (i = 1; i < argc; ++i) {
        ptr = dlsym(hdl, argv[i]);
        printf("%s = %s\n", argv[i], ptr);
    }
    return 0;
}

In order to add all symbols to the dynamic symbol table, use -Wl,--export-dynamic. If you want to remove most symbols from the symbol table (recommended), set -fvisibility=hidden and then explicitly add the symbols you want with __attribute__((visibility("default"))) or one of the other methods.

~ $ gcc dlopentest.c -Wall -Wextra -ldl
~ $ ./a.out A B C
A = (null)
B = (null)
C = (null)
~ $ gcc dlopentest.c -Wall -Wextra -ldl -Wl,--export-dynamic
~ $ ./a.out A B C
A = Value of A
B = (null)
C = Value of C
~ $ gcc dlopentest.c -Wall -Wextra -ldl -Wl,--export-dynamic -fvisibility=hidden
~ $ ./a.out A B C
A = Value of A
B = (null)
C = (null)

Safety

Notice that there is a lot of room for bad behavior.

$ ./a.out printf
printf = ▯▯▯▯ (garbage)

If you want this to be safe, you should create a whitelist of permissible symbols.