How to get the symbol name for a memory address in GDB?

Question

For instance, I know that 0x46767f0 belongs to an NSString*, is there any way I can find out what NSString it is to help me find some bugs I'm after?

Answer 1

I believe you're looking for:

info symbol <addresss>

Print the name of a symbol which is stored at the address addr. If no symbol is stored exactly at addr, GDB prints the nearest symbol and an offset from it.

Example:

(gdb) info symbol 0x400225
_start + 5 in section .text of /tmp/a.out

(gdb) info symbol 0x2aaaac2811cf
__read_nocancel + 6 in section .text of /usr/lib64/libc.so.6

You can read more about it here.

Answer 2

gdb> list *0xAABBCCDD

That tells you the filename and line number.

Answer 3

If it is a stack variable, there is no way that I am aware to do it. Otherwise, try p/a <pointer symbol or address> and it will print the symbol name (or offset to the nearest symbol name).

Answer 4

addr2line

This Binutils utility can handle any symbol address, including variables and function names.

It is non-interactive by default, which is useful in some cases, when doing post-mortems.

main.c

#include <stdio.h>

int myvar;

int main(void) {
    printf("myvar = %d\n", myvar);
}

Compile and disassemble:

gcc -O 0 -g gdb3 -o main -pedantic-errors -std=c89 -Wextra main.c
readelf -s tmp.out  | grep -E ' (main|myvar)'

Gives:

55: 0000000000201014     4 OBJECT  GLOBAL DEFAULT   24 myvar
65: 000000000000064a    32 FUNC    GLOBAL DEFAULT   14 main

And now we can try:

addr2line -e main 201014 64a

which gives:

/full/path/to/main.c:3
/full/path/to/main.c:5

Boost stack trace library uses it for example to show stack trace lines: print call stack in C or C++

Answer 5

po 0x46767f0

will send a -description message to the object. That will print out the contents of your NSString but I suggest using Brian's answer to check the contents of your address before you send random messages to random addresses.