nm Command in Linux

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The nm command in Linux is a useful utility for examining binary files and libraries. It is primarily used to list the symbols from object files, which can include functions, variables, and other entities. This command is useful for understanding the structure and contents of compiled binaries and libraries.

The nm command lists the symbols from object files (objfile). If no object files are specified, nm assumes the file a.out. For each symbol, nm shows −

Attributes of Each Symbol −

• Value − The symbol value, in the radix selected by options (hexadecimal by default).
• Type − The symbol type. Lowercase indicates a local symbol, and uppercase indicates a global (external) symbol.
• Name − The symbol name.

Symbol Types −

Table of Contents

Here is a comprehensive guide to the options available with the nm command −

• Syntax of nm Command in Linux
• nm Command Options
• Examples of nm Command in Linux

Syntax of nm Command in Linux

The following is the general syntax for the nm command −

nm [OPTIONS] [FILE]...

Where −

• OPTIONS − Specifies the options to modify the behavior of the nm command.
• FILE − One or more object files, archives of object files, or shared libraries to examine. If no files are provided, nm assumes the file out.

nm Command Options

The following are different options available for the nm command, including both long and short forms where applicable −

Examples of nm Command in Linux

The following examples will give you a good grasp of how to use the nm command with various options to analyze object files −

Display All Symbols Including Debugger-Only Symbols

To display all symbols, including those meant for debugging, you can use the following command −

sudo nm -a example.o

This lists all symbols in the object file example.o, even those that are typically hidden.

Print Name of Input File before Each Symbol

To precede each symbol with the name of the input file, use the following command −

sudo nm -A example.o

This is useful when dealing with multiple object files, as it clearly identifies the source file for each symbol.

Decode Mangled C++ Symbols

To make C++ function names readable, simply use the following command −

sudo nm -C example.o

This demangles the C++ symbols, making the output more understandable by converting low-level symbols to user-friendly names.

Display Dynamic Symbols Instead of Normal Symbols

To show only dynamic symbols, you can run the following command −

sudo nm -D libexample.so

This option is useful for examining symbols in shared libraries, as it focuses on dynamic symbols.

Use BSD Format for Output

To format the output in BSD style, you can use the following command −

sudo nm -f bsd example.o

The bsd format is the default and this command explicitly sets it for clarity.

Display Only External Symbols

To list only external (global) symbols, you can simply use the nm command with the "-g" flag −

sudo nm -g example.o

This helps in identifying symbols that are accessible outside the given object file.

List Symbol Names Only

To display just the names of the symbols, you can use the following command −

sudo nm -j example.o

This option filters out additional details and shows only the symbol names.

Display Filename and Line Number for Each Symbol

To use debugging information for showing filename and line number, you can run the following command −

sudo nm -l example.o

This is particularly useful for debugging, as it associates symbols with their source code locations.

Sort Symbols Numerically by Address

To sort symbols based on their addresses, you can simply use the nm command with the "-n" flag −

sudo nm -n example.o

This command sorts the symbols numerically, providing a clear view of their memory layout.

Reverse the Sort Order of Symbols

To reverse the order of symbols in the output, you can use the nm command with the "-r" flag −

sudo nm -r example.o

This command sorts the symbols in descending order by their addresses.

Print the Size of Defined Symbols

To include the size of each defined symbol in the output, you can run the nm command with the "-S" flag −

sudo nm -S example.o

This is useful for understanding the memory footprint of each symbol.

Include Index for Symbols from Archive Members

To include the index for symbols from archive members, you can use the nm command with the "-s" flag −

sudo nm -s libexample.a

This option is helpful when working with archive files, as it maps symbols to their respective modules.

Display Only Undefined Symbols

To show only the undefined symbols in the object file, you can use the nm command with the "-u" flag −

sudo nm -u example.o

This highlights symbols that need to be resolved by linking with other object files or libraries.

Display Only Defined Symbols

To show only the defined symbols in the object file, you can use the nm command with the "-U" flag −

sudo nm -U example.o

This filters out undefined symbols, focusing solely on those that are defined within the file.

Conclusion

The nm command is an indispensable tool for developers and system administrators working with binary files and libraries. It provides detailed insights into the symbols within object files, enabling better understanding and debugging of compiled code.

With its wide array of options, including the ability to display symbol types, sort output, filter specific symbols, and decode complex symbol names, the nm command offers flexibility and precision for analyzing binary structures.

Mastering this command can significantly enhance your proficiency in managing and debugging software at the binary level, making it an essential skill in any Unix or Linux toolkit.