Editing Nm (Unix)

{{short description|Unix command}}
{{lowercase|nm (Unix)}}
{{Infobox software
| name                   = nm
| logo                   = 
| screenshot             = 
| screenshot size        = 
| caption                = 
| author                 = [[Dennis Ritchie]],<br />[[Ken Thompson]]<br />([[AT&T Bell Laboratories]])
| developer              = Various [[open-source software|open-source]] and [[commercial software|commercial]] developers
| released               = {{Start date and age|1971|11|3}}
| latest release version = 
| latest release date    = 
| programming language   = [[C (programming language)|C]]
| operating system       = [[Unix]], [[Unix-like]], [[Plan 9 from Bell Labs|Plan 9]]
| platform               = [[Cross-platform]]
| genre                  = [[Command (computing)|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 file|binary]] [[executable]] file (including [[Library (computing)|libraries]], compiled [[Object file|object modules]], shared-object files, and standalone [[executable]]s).

The output from <code>nm</code> distinguishes between various symbol types. For example, it differentiates between a [[subroutine|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 [[Unix-like|similar]] [[operating system]]s including [[Plan 9 from Bell Labs|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}}</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 [[GNU Compiler Collection|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]]

== 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}}

{{Unix commands}}
{{Plan 9 commands}}

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

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