I have written a simple Hello World program.
#include <stdio.h>
int main() {
printf("Hello World");
return 0;
}
I wanted to understand how the relocatable object file and executable file look like. The object file corresponding to the main function is
0000000000000000 <main>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: bf 00 00 00 00 mov $0x0,%edi
9: b8 00 00 00 00 mov $0x0,%eax
e: e8 00 00 00 00 callq 13 <main+0x13>
13: b8 00 00 00 00 mov $0x0,%eax
18: c9 leaveq
19: c3 retq
Here the function call for printf is callq 13. One thing i don't understand is why is it 13. That means call the function at adresss 13, right??. 13 has the next instruction, right?? Please explain me what does this mean??
The executable code corresponding to main is
00000000004004cc <main>:
4004cc: 55 push %rbp
4004cd: 48 89 e5 mov %rsp,%rbp
4004d0: bf dc 05 40 00 mov $0x4005dc,%edi
4004d5: b8 00 00 00 00 mov $0x0,%eax
4004da: e8 e1 fe ff ff callq 4003c0 <printf@plt>
4004df: b8 00 00 00 00 mov $0x0,%eax
4004e4: c9 leaveq
4004e5: c3 retq
Here it is callq 4003c0. But the binary instruction is e8 e1 fe ff ff. There is nothing that corresponds to 4003c0. What is that i am getting wrong?
Thanks. Bala
In the first case, take a look at the instruction encoding - it's all zeroes where the function address would go. That's because the object hasn't been linked yet, so the addresses for external symbols haven't been hooked up yet. When you do the final link into the executable format, the system sticks another placeholder in there, and then the dynamic linker will finally add the correct address for printf() at runtime. Here's a quick example for a "Hello, world" program I wrote.
First, the disassembly of the object file:
00000000 <_main>:
0: 8d 4c 24 04 lea 0x4(%esp),%ecx
4: 83 e4 f0 and $0xfffffff0,%esp
7: ff 71 fc pushl -0x4(%ecx)
a: 55 push %ebp
b: 89 e5 mov %esp,%ebp
d: 51 push %ecx
e: 83 ec 04 sub $0x4,%esp
11: e8 00 00 00 00 call 16 <_main+0x16>
16: c7 04 24 00 00 00 00 movl $0x0,(%esp)
1d: e8 00 00 00 00 call 22 <_main+0x22>
22: b8 00 00 00 00 mov $0x0,%eax
27: 83 c4 04 add $0x4,%esp
2a: 59 pop %ecx
2b: 5d pop %ebp
2c: 8d 61 fc lea -0x4(%ecx),%esp
2f: c3 ret
Then the relocations:
main.o: file format pe-i386
RELOCATION RECORDS FOR [.text]:
OFFSET TYPE VALUE
00000012 DISP32 ___main
00000019 dir32 .rdata
0000001e DISP32 _puts
As you can see there's a relocation there for _puts, which is what the call to printf turned into. That relocation will get noticed at link time and fixed up. In the case of dynamic library linking, the relocations and fixups might not get fully resolved until the program is running, but you'll get the idea from this example, I hope.