What is _IO_stdin_used - c++

Question

Can someone explain me what _IO_stdin_used is in the following line:
114a: 48 8d 3d b3 0e 00 00 lea rdi,[rip+0xeb3] # 2004 <_IO_stdin_used+0x4>
Sorry for the noob question.

Answer 1

Expanding on https://stackoverflow.com/users/224132/peter-cordes 's comment (What is _IO_stdin_used ), some tools can help make sense of this. Starting with burt.c:
#include <stdio.h>
void main (void) {
puts("hello!");
}
built like so: LDFLAGS=-Wl,-Map=burt.map make burt
We will look at the map file in a moment, first inspect the executable:
$ objdump -xsd -M intel burt
...
0000000000002000 g O .rodata 0000000000000004 _IO_stdin_used
...
Contents of section .rodata:
2000 01000200 68656c6c 6f2100 ....hello!.
...
1148: 48 8d 05 b5 0e 00 00 lea rax,[rip+0xeb5] # 2004 <_IO_stdin_used+0x4>
114f: 48 89 c7 mov rdi,rax
1152: e8 d9 fe ff ff call 1030 <puts#plt>
...
Note, the string's location (2004h) does not have an entry in the symbol table (where the 2000h entry _IO_stdin_used occurs). So, objdump's disassembly comments mention it relative to another symbol it knows about.
But why does it know about _IO_stdin_used, or, why does is that symbol included and why does it have a name? burt.map shows it came from Scrt1.o:
...
.rodata 0x0000000000002000 0xb
*(.rodata .rodata.* .gnu.linkonce.r.*)
.rodata.cst4 0x0000000000002000 0x4 /usr/lib/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../lib/Scrt1.o
.rodata 0x0000000000002004 0x7 /tmp/ccYG1XvK.o
...
On my computer, Scrt1.o belongs to glibc, and the symbol can be traced back here: https://sourceware.org/git/?p=glibc.git;a=blob;f=csu/init.c;h=c2f978f3da565590bcab355fefa3d81cf211cb36;hb=63fb8f9aa9d19f85599afe4b849b567aefd70a36.
To show that this is displayed in the objdump disassembly because there is no name for the address, we can give our string a name to add it to the symbol table, so that objdump will show it instead. To do this let's modify burt.c like so:
#include <stdio.h>
const char greeting[] = "hello!";
void main (void) {
puts(greeting);
}
Build, then inspect:
...
0000000000002004 g O .rodata 0000000000000007 greeting
...
0000000000002000 g O .rodata 0000000000000004 _IO_stdin_used
...
Contents of section .rodata:
2000 01000200 68656c6c 6f2100 ....hello!.
...
113d: 48 8d 05 c0 0e 00 00 lea rax,[rip+0xec0] # 2004 <greeting>
1144: 48 89 c7 mov rdi,rax
1147: e8 e4 fe ff ff call 1030 <puts#plt>
The offset relative to rip has changed a little bit, because the .text and .rodata sections are now a little bit further apart, although the .rodata contents remain the same, because the source modification was chosen to avoid modifying .rodata -- if greetings were not const, it would be located in .data instead. The map file below now shows burt.c's temporary object files's contribution to .rodata defining the greeting symbol, as compared to the previous map file where the same data had no name:
.rodata 0x0000000000002000 0xb
*(.rodata .rodata.* .gnu.linkonce.r.*)
.rodata.cst4 0x0000000000002000 0x4 /usr/lib/gcc/x86_64-pc-linux-gnu/12.2.1/../../../..
/lib/Scrt1.o
0x0000000000002000 _IO_stdin_used
.rodata 0x0000000000002004 0x7 /tmp/ccDnWXjG.o
0x0000000000002004 greeting