nm (Unix)

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.

/*
 * 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;
}

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

# 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

When the C++ compiler is used, the output differs:

# 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

The differences between the outputs also show an example of solving the name mangling problem by using extern "C" in C++ code.

• objdump

The Wikibook Guide to Unix has a page on the topic of: Commands

• nm: write the name list of an object file – Shell and Utilities Reference, The Single UNIX Specification, Version 4 from The Open Group
• nm(1) – Plan 9 Programmer's Manual, Volume 1

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