<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.
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