On 4/20/21, Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote:
> On Tue, 20 Apr 2021 at 06:42, Peng Yu wrote:
>>
>> Hi,
>>
>> I am trying to understand where are unnamed string constants are
>> defined in symbol tables.
>>
>> In the following example, the unnamed string constant is "Hello World!".
>>
>> $ cat a.c
>> #include <stdio.h>
>> int main() { puts("Hello World!"); }
>> $ gcc -c a.c
>> $ gcc -o a.out a.o
>>
>> But I don't find it in the symbol table. Could anybody point to me
>> where it is? Thanks.
>
> The ELF format puts string literals in the .rodata section, not the
> .symtab.
>
>
> $ readelf -p .rodata a.out
>
> String dump of section '.rodata':
> [ 10] Hello World!
How does the linker know that it should look for the string literal in
.rodata just using the object file?
$ objdump --disassemble=main a.o
...
0000000000000000 <main>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 48 8d 3d 00 00 00 00 lea 0x0(%rip),%rdi # b <main+0xb>
b: e8 00 00 00 00 callq 10 <main+0x10>
10: b8 00 00 00 00 mov $0x0,%eax
15: 5d pop %rbp
16: c3 retq
Where is the number "10" in the callq line from? "00 00 00 00" is just
address 0. So the disassembler knows "00 00 00 00" is not a legal
address, so it just put the address of the next instruction which is
"10"?
Is callq just call in the disassembled code? I see people says they
are the same. But I don't understand if they are the same why not just
use call instead?
$ objdump --disassemble=main a.out
...
0000000000001135 <main>:
1135: 55 push %rbp
1136: 48 89 e5 mov %rsp,%rbp
1139: 48 8d 3d c4 0e 00 00 lea 0xec4(%rip),%rdi # 2004
<_IO_stdin_used+0x4>
1140: e8 eb fe ff ff callq 1030 <puts@plt>
1145: b8 00 00 00 00 mov $0x0,%eax
114a: 5d pop %rbp
114b: c3 retq
...
How does objdump figure out 0xec4(%rip) is the address _IO_stdin_used+0x4?
0xec4 is related to the relative layout of .rodata and .text. Is there
a way to show both sections in the same layout as they appear in
memory (data should be shown as hexdump, and code should be shown as
disassembled code) for human readability?
--
Regards,
Peng
