Make shared library executable

You may know that it is possible to run the C shared library. For example, on my Linux system, if I run it, it displays its version:

$ /lib/libc.so.6  
GNU C Library (Ubuntu EGLIBC 2.11.1-0ubuntu7.8) stable release version 2.11.1, by Roland McGrath et al.  
Copyright (C) 2009 Free Software Foundation, Inc.  
This is free software; see the source for copying conditions.  
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A  
PARTICULAR PURPOSE.  
Compiled by GNU CC version 4.4.3.  
Compiled on a Linux >>2.6.24-27-server<< system on 2011-01-21.  
Available extensions:  
        crypt add-on version 2.1 by Michael Glad and others  
        GNU Libidn by Simon Josefsson  
        Native POSIX Threads Library by Ulrich Drepper et al  
        BIND-8.2.3-T5B  
For bug reporting instructions, please see:  
<http://www.debian.org/Bugs/>.

Example of shared library

// service.c
#include <stdio.h>
void lib_service(void) {
	printf("This is a service of the shared library\n");
}
 
 
// main.c
#include <stdio.h>
extern void lib_service(void);
int main(int ac, char *av[]) {
	lib_service();
	return 0;
}

To build this program along with its shared library, we do:

$gcc -shared service.c -o libservice.so -Wl,-soname,libservice.so
$gcc main.c -o main -lservice -L.
 
$export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:`pwd`
$./main
This is a service of the shared library

If we run the library we get a segmentation fault:

$chmod +x ./libservice.so
$./libservice.so
exec format error: ./libservice.so

Makeing an executable shared library

#include <stdio.h>
void lib_service(void) {
	printf("This is a service of the shared library\n");
}
 
void lib_entry(void) {
	printf("Entry point of the service library\n");
}

$gcc -shared service.c -o libservice.so -Wl,-soname,libservice.so -Wl,-e,lib_entry

But the execution still fails.

We must make the entry point be a “no return” function thanks to the “_exit()” service:

#include <stdio.h>
#include <unistd.h>
void lib_service(void) {
	printf("This is a service of the shared library\n");
}
 
void lib_entry(void) {
	printf("Entry point of the service library\n");
	_exit(0);
}

But it still fails. We specify the interpreter file to be the dynamic linker. On our Linux system, it is /lib64/ld-linux-x86-64.so.2. Make sure to use the right linker.

#include <stdio.h>
#include <unistd.h>
 
const char service_interp[] __attribute__((section(".interp"))) = "/lib64/ld-linux-x86-64.so.2";
void lib_service(void) {
	printf("This is a service of the shared library\n");
}
 
extern "C" {
void lib_entry(void) {
	printf("Entry point of the service library\n");
	_exit(0);
}
}

Now it works.