nm command

 

nm (Name List)

Purpose

The nm command lists symbols (functions, variables, etc.) from object files, static libraries, and executables.

It’s very useful to:

• Inspect which symbols (functions/variables) are defined or undefined in a file.

• Understand linker errors like “undefined reference”.

• Study how the compiler organizes symbols in different sections (.text, .data, .bss).

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Basic Syntax

nm [options] filename

Example:

nm a.out

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What are Symbols?

Symbols represent identifiers (names) in your program — for example:

• Functions (like main, printf)

• Global/static variables

• External references (things defined in another file)

Each symbol has:

1. Address (where it’s stored in memory)

2. Type (function, variable, etc.)

3. Name

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Example 1: Simple Program

sum.c

#include <stdio.h>

int a = 5;        // global variable
int b;            // uninitialized global variable

int sum(int x, int y) {
    return x + y;
}

int main() {
    int c = sum(a, 10);
    printf("%d\n", c);
    return 0;
}

Compile it:

gcc -c sum.c     # creates object file sum.o

Then run:

nm sum.o

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Sample Output:

0000000000000000 T sum
0000000000000010 T main
0000000000000004 D a
0000000000000008 B b
                 U printf

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Understanding the Columns

Column	Meaning
Address	Memory address (if known)
Type	Symbol type (a single letter)
Name	Symbol name

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Symbol Type Codes

Code	Section	Meaning
T	.text	Function defined in this file
t	.text	Local (static) function
D	.data	Initialized global variable
d	.data	Initialized static variable
B	.bss	Uninitialized global variable
b	.bss	Uninitialized static variable
U	–	Undefined (used but not defined here — will be resolved by linker)
C	–	Common (uninitialized data to be allocated by the linker)
R	.rodata	Read-only data (constants, strings)
W	–	Weak symbol
?	–	Unknown type

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From the Example Above

• T sum → sum() function is defined in this file (in text section).

• T main → main() function defined here.

• D a → Global variable a (initialized) in data section.

• B b → Global variable b (uninitialized) in BSS section.

• U printf → Undefined symbol — it’s defined in another file (libc).

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Display Symbols in an Executable

After linking:

gcc sum.c -o sum
nm sum

You’ll now see resolved symbols (like printf) with addresses.

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Sort Output by Symbol Name

nm -n sum

Sorts by symbol address (numeric order).

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Show Only External Symbols

nm -g sum.o

Lists only global (external) symbols.

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Show Undefined Symbols

nm -u sum.o

Displays only undefined symbols — useful for debugging linker errors.

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Show Demangled C++ Names

For C++ programs, you can make nm print readable names (instead of mangled ones):

nm -C hello.o

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Simple Lab Exercise

Aim:

To study the use of nm command for listing symbol information in object and executable files.

Procedure:

1. Write and compile a simple program:

   #include <stdio.h>
   
   int x = 10;
   int y;
   
   int square(int n) {
       return n * n;
   }
   
   int main() {
       printf("%d\n", square(x));
       return 0;
   }
   

2. Compile it to object form:

   gcc -c test.c
   

3. Run:

   nm test.o
   

4. Observe the symbol table output and identify:

   • Defined functions

   • Initialized/uninitialized variables

   • Undefined external symbols

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Learning Outcomes

Students will:

• Understand how functions and variables are stored in different segments.

• Identify undefined and external symbols.

• Gain insight into how linking works in C programs.

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Quick Reference Summary

Command	Purpose
nm file.o	List all symbols
nm -n file.o	Sort symbols by address
nm -g file.o	Show only external (global) symbols
nm -u file.o	Show undefined symbols
nm -C file.o	Demangle C++ names