Nm (Unix)

Template:Short description Template:Lowercase {{#invoke:Infobox|infobox}}Template:Template other{{#invoke:Check for unknown parameters | check | showblankpositional=1 | unknown = Template:Main other | preview = Page using Template:Infobox software with unknown parameter "_VALUE_"|ignoreblank=y | AsOf | author | background | bodystyle | caption | collapsetext | collapsible | developer | discontinued | engine | engines | genre | included with | language | language count | language footnote | latest preview date | latest preview version | latest release date | latest release version | latest_preview_date | latest_preview_version | latest_release_date | latest_release_version | licence | license | logo | logo alt | logo caption | logo upright | logo size | logo title | logo_alt | logo_caption | logo_upright | logo_size | logo_title | middleware | module | name | operating system | operating_system | other_names | platform | programming language | programming_language | released | replaced_by | replaces | repo | screenshot | screenshot alt | screenshot upright | screenshot size | screenshot title | screenshot_alt | screenshot_upright | screenshot_size | screenshot_title | service_name | size | standard | title | ver layout | website | qid }}Template:Main other nm 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 nm 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. nm 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 nm as part of the GNU Binutils package.

The etymology is that in the old Version 7 Unix, nm's manpage used the term name list instead of symbol table.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</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; }

1. ifdef __cplusplus

extern "C"

1. 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 nm command is the following:

<syntaxhighlight lang="console">

1. 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">

1. 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.

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

External links

• Template:Man
• Template:Man