Listing Symbol Table Information: nm

Listing Symbol Table Information: nm

The nm tool lists symbol table information for object files and archive files.

nm Syntax

The syntax for nm is:

nm options filename1 [filename2...filenamen]

options: One or more of the options listed in Table 2-8.
filename: Specifies the object files or archive files from which symbol table information is to be extracted. If you do not specify a filename, nm assumes the file is named a.out.

nm Symbol Table Options

Table 2-8 lists nm symbol table options. For more information, see the nm(1) reference page.

Symbol Table nm Options
Option Purpose
-a Prints debugging information.
-A Prints the listing in System V format (default).
-b Prints the value field in octal.
-B Prints the listing in BSD format.
-C Prints decoded C++ names.
-d Prints the value field in decimal (the default for System V output).
-g Prints globally visible names.
-h Suppresses printing of headers.
-l Produces a long listing.
-n Sorts external symbols by name for System V format. Sorts all symbols by value for BSD format (by name is the BSD default output).
-o Prints value field in octal (System V output). Prints the filename immediately before each symbol name (BSD output).
-p Produces easily parsible, terse output similar to the BSD format.
-r Prepends the name of the object file or archive to each output line.
-T Truncates characters in exceedingly long symbol names; inserts an asterisk as the last character of the truncated name. This option may make the listing easier to read.
-u Prints only undefined symbols.
-v Sorts external symbols by value (default for BSD format).
-V Prints the version number of nm.
-x Prints the value field in hexadecimal.

Symbol Table nm Options
Option	Purpose
-a	Prints debugging information.
-A	Prints the listing in System V format (default).
-b	Prints the value field in octal.
-B	Prints the listing in BSD format.
-C	Prints decoded C++ names.
-d	Prints the value field in decimal (the default for System V output).
-g	Prints globally visible names.
-h	Suppresses printing of headers.
-l	Produces a long listing.
-n	Sorts external symbols by name for System V format. Sorts all symbols by value for BSD format (by name is the BSD default output).
-o	Prints value field in octal (System V output). Prints the filename immediately before each symbol name (BSD output).
-p	Produces easily parsible, terse output similar to the BSD format.
-r	Prepends the name of the object file or archive to each output line.
-T	Truncates characters in exceedingly long symbol names; inserts an asterisk as the last character of the truncated name. This option may make the listing easier to read.
-u	Prints only undefined symbols.
-v	Sorts external symbols by value (default for BSD format).
-V	Prints the version number of nm.
-x	Prints the value field in hexadecimal.

Table 2-9 defines the one-character codes shown in an nm listing. Refer to the example that follows the table for a sample listing.

Character Code Meanings
Key Description
a Local absolute data
A External absolute data
b Local zeroed data
B External zeroed data
C Common data
d Local initialized data
D External initialized data
E Small common data
G External small initialized data
N Nil storage class (unused external reference)
r Local read-only data
R External read-only data
s Local small zeroed data
S External small zeroed data
t Local text
T External text
U External undefined data
V External small undefined data

Character Code Meanings
Key	Description
a	Local absolute data
A	External absolute data
b	Local zeroed data
B	External zeroed data
C	Common data
d	Local initialized data
D	External initialized data
E	Small common data
G	External small initialized data
N	Nil storage class (unused external reference)
r	Local read-only data
R	External read-only data
s	Local small zeroed data
S	External small zeroed data
t	Local text
T	External text
U	External undefined data
V	External small undefined data

nm Example of Obtaining a Symbol Table Listing

This example demonstrates how to obtain a symbol table listing. Consider the following program, tnm.c:

#include <stdio.h>
#include <math.h>
#define LIMIT 12
int unused_item = 14;
double mydata[LIMIT];

main()
{
    int i;
    for(i = 0; i < LIMIT; i++) {
        mydata[i] = sqrt((double)i);
    }
    return 0;
}

Compile the program into an object file by entering:

cc -c tnm.c

To obtain symbol table information for the object file tnm.o in BSD format, use the nm -B command:

0000000000 T main
0000000000 B mydata
0000000000 U sqrt
0000000000 D unused_item
00000000 N _bufendtab

To obtain symbol table information for the object file tnm.o in SVR4 format, use the nm command without any options:

Symbols from tnm.o:

[Index]   Value     Size    Class    Type         Section    Name

[0]     |        0|        |File    |ref=4       |Text     | tnm.c
[1]     |        0|        |Proc    |end=3 int   |Text     | main
[2]     |      116|        |End     |ref=1       |Text     | main
[3]     |        0|        |End     |ref=0       |Text     | tnm.c
[4]     |        0|        |File    |ref=6       |Text     | /usr/include/math.h
[5]     |        0|        |End     |ref=4       |Text     | /usr/include/math.h
[6]     |        0|        |Global  |            |Data     | unused_item
[7]     |        0|        |Global  |            |Bss      | mydata
[8]     |        0|        |Proc    |ref=1       |Text     | main
[9]     |        0|        |Proc    |            |Undefined| sqrt
[10]    |        0|        |Global  |            |Undefined| _gp_disp