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Dealing with external libraries
In the part 2 of our series we learned how to process relocations in object files in order to properly wire up internal dependencies in the code. In this post we will look into what happens if the code has external dependencies — that is, it tries to call functions from external libraries. As before, we will be building upon the code from part 2. Let's add another function to our toy object file:
obj.c:
#include <stdio.h>
...
void say_hello(void)
{
puts("Hello, world!");
}
In the above scenario our say_hello function now depends on the puts function from the C standard library. To try it out we also need to modify our loader to import the new function and execute it:
loader.c:
...
static void execute_funcs(void)
{
/* pointers to imported functions */
int (*add5)(int);
int (*add10)(int);
const char *(*get_hello)(void);
int (*get_var)(void);
void (*set_var)(int num);
void (*say_hello)(void);
...
say_hello = lookup_function("say_hello");
if (!say_hello) {
fputs("Failed to find say_hello function\n", stderr);
exit(ENOENT);
}
puts("Executing say_hello...");
say_hello();
}
...
Let's run it:
$ gcc -c obj.c
$ gcc -o loader loader.c
$ ./loader
No runtime base address for section
Seems something went Continue reading
