I've been exploring dynamic linking, and tried the following:
#include <stdio.h>
int main() {
printf("Address of main: %lx\n", (unsigned long)main);
printf("Address of printf: %lx\n", (unsigned long)printf);
return 0;
}
When I run this, I get
Address of main: 107264f20
Address of printf: 7ff81651af0b
The address of printf loaded from the GOT is in the shared library, but it is odd. I was expecting it to be 8-byte aligned.
I must be missing something obvious, but what?
I presume you’re testing this on Intel. In that case, the result you’re seeing is perfectly reasonable. Intel instructions do not have alignment requirements [1].
I set a breakpoint on the return statement in your code and saw this:
Address of main: 100003f20
Address of printf: 7ff81d13df0b
(lldb) disas -s 7ff81d13df0b
libsystem_c.dylib`printf:
0x7ff81d13df0b <+0>: pushq %rbp
0x7ff81d13df0c <+1>: movq %rsp, %rbp
0x7ff81d13df0f <+4>: subq $0xd0, %rsp
0x7ff81d13df16 <+11>: movq %rdi, %r10
0x7ff81d13df19 <+14>: testb %al, %al
As you can see, printf just happens to start at an odd address [2]. That’s perfectly normal an Intel.
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[1] Well, there may be some obscure ones that do — I’m not enough of an expert on that ISA to say for sure one way or the other — so I’m just talking about the standard instructions you use day-to-day.
[2] The specific address is different due to ASLR.