How to compile ELF binary so that it can be loaded as dynamic library?

Question

This is theoretical question. I am aware that perhaps best practice would be the use of shared libraries. But I ran into this question and cannot seem to find an answer anywhere.

How to construct the code and compile in ELF format a program in C/C++ so that it can be loaded with dlopen()?

For example if one executable contains implementation of some function int test() and I would like to call this function from my program (and preferably get the result of function), if that is even possible, how would I go about doing that?

In pseudocode I could describe it as follows:

ELF executable source:

void main() {
    int i = test();
    printf("Returned: %d", i);//Prints "Returned: 5"
}

int test() {
    return 5;
}

External program:

// ... Somehow load executable from above
void main() {
    int i = test();
    printf("Returned: %d", i);//Must print "Returned: 5"
}

Answer 1

Based on links provided in comments and other answers here is how it can be done without linking these programs compile time:

test1.c:

#include <stdio.h>

int a(int b)
{
  return b+1;
}

int c(int d)
{
  return a(d)+1;
}

int main()
{
  int b = a(3);
  printf("Calling a(3) gave %d \n", b);
  int d = c(3);
  printf("Calling c(3) gave %d \n", d);
}

test2.c:

#include <dlfcn.h>
#include <stdio.h>


int (*a_ptr)(int b);
int (*c_ptr)(int d);

int main()
{
  void* lib=dlopen("./test1",RTLD_LAZY);
  a_ptr=dlsym(lib,"a");
  c_ptr=dlsym(lib,"c");
  int d = c_ptr(6);
  int b = a_ptr(5);
  printf("b is %d d is %d\n",b,d);
  return 0;
}

Compilation:

$ gcc -fPIC  -pie -o test1 test1.c -Wl,-E
$ gcc -o test2 test2.c -ldl

Execution results:

$ ./test1
Calling a(3) gave 4 
Calling c(3) gave 5
$ ./test2 
b is 6 d is 8

References:

• building a .so that is also an executable
• Compile C program using dlopen and dlsym with -fPIC

PS: In order to avoid symbol clashes imported symbols and pointers they assigned to better have different names. See comments here.