On Tue, 2021-04-20 at 08:21 -0500, Peng Yu via Gcc-help wrote:
> 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"?
They are in relocation records. Try objdump -r.
> 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?
"q" means "qword", i. e. it's a 64-bit operation. On my system objdump
(binutils-2.36.1) just print "call" instead of "callq", because there is
no "calll" ("32-bit call") operation in a 64-bit x86 program.
> $ 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(%rip) means "the value of %rip (the program counter) plus 0xec4",
i. e. (the location of <main>) + (the offset of the next instruction in
<main>) + 0xec4. It's somewhere in .rodata, and the nearest symbol
before it is _IO_stdin_used. Then just a substraction can get 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?
None that I'm conscious of.
Anyway this thread is very off-topic now. gcc-help is not for
discussion about general questions about linking and relocation.
stackoverflow.com may be a better place.
--
Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University
