What is the nm command used for?

The nm command displays the symbol table of object files, libraries, and executables. It shows function names, variables, and their memory addresses, which is useful for debugging, reverse engineering, and understanding binary files.

How do I see all symbols in an executable?

Use 'nm executable_name' to display all symbols, or 'nm -a executable_name' to show all symbols including debug symbols. The output will show symbol names, types, and addresses.

What do the symbol types mean in nm output?

Common symbol types include: T/t (text section), D/d (data section), B/b (BSS section), U (undefined), A (absolute), and W/w (weak symbols). Uppercase letters indicate global symbols, lowercase indicate local symbols.

nm

Display symbol tables from object files, libraries, and executables

Syntax

nm [options] [object-file...]

Basic Usage

Display symbols from executable

nm program

Shows the symbol table of the specified executable.

Display symbols from object file

nm file.o

Display symbols from library

nm libexample.so

Common Options

-a, --debug-syms: Display all symbols including debug symbols
-A, --print-file-name: Print filename before each symbol
-B: Use BSD format output
-C, --demangle: Decode mangled C++ names
-D, --dynamic: Display dynamic symbols only
-f format: Output format (bsd, sysv, posix)
-g, --extern-only: Display external symbols only
-l, --line-numbers: Display source file and line number
-n, --numeric-sort: Sort symbols numerically by address
-p, --no-sort: Don't sort symbols
-r, --reverse-sort: Reverse sort order
-S, --print-size: Show size of symbols
-t format: Output format (radix)
-u, --undefined-only: Display undefined symbols only
-v, --version: Display version information

Practical Examples

Show all symbols with demangling

nm -C program

Show only undefined symbols

nm -u program

Show symbols with sizes

nm -S program

Show external symbols only

nm -g program

Show symbols with line numbers

nm -l program

Show symbols sorted by address

nm -n program

Show dynamic symbols from shared library

nm -D libexample.so

Show symbols in POSIX format

nm -f posix program

Understanding Output

Symbol Types

A: Absolute symbol
B: Uninitialized data (BSS)
C: Common symbol
D: Initialized data
G: Global symbol
I: Indirect symbol
N: Debugging symbol
R: Read-only data
T: Text section symbol
U: Undefined symbol
V: Weak symbol
W: Weak symbol
-: Local symbol

Output Format

Default output shows: address type symbol_name

address: Memory address (or size for undefined symbols)
type: Symbol type (see above)
symbol_name: Name of the symbol

Best Practices

Use Cases

Debugging binary files and executables
Reverse engineering and analysis
Checking library dependencies
Verifying symbol exports in shared libraries
Understanding program structure

Important Notes

Symbol information may be stripped from release builds
Use -C flag for readable C++ function names
Dynamic symbols are different from static symbols
Addresses shown are virtual addresses, not physical
Some symbols may be optimized away by the compiler

nm