Is there any way to compile additional code at runtime in C or C++?

Question

Here is what I want to do:

1. Run a program and initialize some data structures.
2. Then compile additional code that can access/modify the existing data structures.
3. Repeat step 2 as needed.

I want to be able to do this with both C and C++ using gcc (and eventually Java) on Unix-like systems (especially Linux and Mac OS X). The idea is to basically implement a read-eval-print loop for these languages that compiles expressions and statements as they are entered and uses them to modify existing data structures (something that is done all the time in scripting languages). I am writing this tool in python, which generates the C/C++ files, but this should not be relevant.

I have explored doing this with shared libraries but learned that modifying shared libraries does not affect programs that are already running. I have also tried using shared memory but could not find a way to load a function onto the heap. I have also considered using assembly code but have not yet attempted to do so.

I would prefer not to use any compilers other than gcc unless there is absolutely no way to do it in gcc.

If anyone has any ideas or knows how to do this, any help will be appreciated.

Answer 1

There is one simple solution:

1. create own library having special functions
2. load created library
3. execute functions from that library, pass structures as function variables

To use your structures you have to include same header files like in host application.

structs.h:

struct S {
    int a,b;
};

main.cpp:

#include <iostream>
#include <fstream>
#include <dlfcn.h>
#include <stdlib.h>

#include "structs.h"

using namespace std;

int main ( int argc, char **argv ) {

    // create own program
    ofstream f ( "tmp.cpp" );
    f << "#include<stdlib.h>\n#include \"structs.h\"\n extern \"C\" void F(S &s) { s.a += s.a; s.b *= s.b; }\n";
    f.close();

    // create library
    system ( "/usr/bin/gcc -shared tmp.cpp -o libtmp.so" );

    // load library        
    void * fLib = dlopen ( "./libtmp.so", RTLD_LAZY );
    if ( !fLib ) {
        cerr << "Cannot open library: " << dlerror() << '\n';
    }

    if ( fLib ) {
        int ( *fn ) ( S & ) = dlsym ( fLib, "F" );

        if ( fn ) {
            for(int i=0;i<11;i++) {
                S s;
                s.a = i;
                s.b = i;

                // use function
                fn(s);
                cout << s.a << " " << s.b << endl;
            }
        }
        dlclose ( fLib );
    }

    return 0;
}

output:

0 0
2 1
4 4
6 9
8 16
10 25
12 36
14 49
16 64
18 81
20 100

You can also create mutable program that will be changing itself (source code), recompiling yourself and then replace it's actual execution with execv and save resources with shared memory.