Nm (Unix)

{{short description|Unix command}} {{Lowercase title}} {{Infobox software | name = nm | logo = | screenshot = | screenshot size = | caption = | author = Dennis Ritchie,<br />Ken Thompson<br />(AT&T Bell Laboratories) | developer = Various open-source and commercial developers | released = {{Start date and age|1971|11|3}} | latest release version = | latest release date = | programming language = C | operating system = Unix, Unix-like, Plan 9 | platform = Cross-platform | genre = Command | license = Plan 9: MIT License | website = }} <code>'''nm'''</code> is a Unix command used to dump the symbol table and their attributes from a binary executable file (including libraries, compiled object modules, shared-object files, and standalone executables).

The output from <code>nm</code> distinguishes between various symbol types. For example, it differentiates between a function that is supplied by an object module and a function that is required by it. <code>nm</code> is used as an aid for debugging, to help resolve problems arising from name conflicts and C++ name mangling, and to validate other parts of the toolchain.

This command is shipped with a number of later versions of Unix and similar operating systems including Plan 9. The GNU Project ships an implementation of <code>nm</code> as part of the GNU Binutils package.

The etymology is that in the old Version 7 Unix, <code>nm</code>'s manpage used the term ''name list'' instead of ''symbol table''.<ref>{{Cite web |title=NM(1) |url=https://man.freebsd.org/cgi/man.cgi?query=nm&manpath=Unix%20Seventh%20Edition&format=ascii |access-date=2025-03-14 |website=Unix Seventh Edition General Commands Manual |archive-date=2026-01-05 |archive-url=https://web.archive.org/web/20260105041927/https://man.freebsd.org/cgi/man.cgi?query=nm&manpath=Unix%20Seventh%20Edition&format=ascii |url-status=live }}</ref>

==nm output sample==

<syntaxhighlight lang="c"> /* * File name: test.c * For C code compile with: * gcc -c test.c * * For C++ code compile with: * g++ -c test.cpp */

int global_var; int global_var_init = 26;

static int static_var; static int static_var_init = 25;

static int static_function() { return 0; }

int global_function(int p) { static int local_static_var; static int local_static_var_init=5;

local_static_var = p;

return local_static_var_init + local_static_var; }

int global_function2() { int x; int y; return x+y; }

#ifdef __cplusplus extern "C" #endif void non_mangled_function() { // I do nothing }

int main(void) { global_var = 1; static_var = 2;

return 0; }

</syntaxhighlight>

If the previous code is compiled with the gcc C compiler, the output of the <code>nm</code> command is the following:

<syntaxhighlight lang="console"> # nm test.o 0000000a T global_function 00000025 T global_function2 00000004 C global_var 00000000 D global_var_init 00000004 b local_static_var.1255 00000008 d local_static_var_init.1256 0000003b T main 00000036 T non_mangled_function 00000000 t static_function 00000000 b static_var 00000004 d static_var_init </syntaxhighlight>

When the C++ compiler is used, the output differs: <syntaxhighlight lang="console"> # nm test.o 0000000a T _Z15global_functioni 00000025 T _Z16global_function2v 00000004 b _ZL10static_var 00000000 t _ZL15static_functionv 00000004 d _ZL15static_var_init 00000008 b _ZZ15global_functioniE16local_static_var 00000008 d _ZZ15global_functioniE21local_static_var_init U __gxx_personality_v0 00000000 B global_var 00000000 D global_var_init 0000003b T main 00000036 T non_mangled_function </syntaxhighlight>

The differences between the outputs also show an example of solving the name mangling problem by using extern "C" in C++ code. {| class="wikitable" |+ Symbol types |- ! Symbol type ! Description |- | A |Global absolute symbol |- | a | Local absolute symbol |- | B | Global bss symbol |- | b | Local bss symbol |- | D | Global data symbol |- | d | Local data symbol |- | f | Source file name symbol |- | R | Global read-only symbol |- | r | Local read-only symbol |- | T | Global text symbol |- | t | Local text symbol |- | U | Undefined symbol |}

==See also== *objdump *strings

== References == {{Reflist}}

==External links== {{Wikibooks|Guide to Unix|Commands}} *{{man|cu|nm|SUS|write the name list of an object file}} *{{man|1|nm|Plan 9}}

Category:Unix programming tools Category:Unix SUS2008 utilities Category:Articles with example C code Category:Plan 9 commands