Understanding storage class using nm

Prepared by
Mohammed Sikander
Technical Lead
Cranes Software International Limited

int g_var = 5;
int main( )
{
int l_var = 8;
}
mohammed.sikander@cranessoftware.com 2
00000000 D g_var
00000000 T main
• Symbol table does not contain the
entries of local/automatic variables.
• gl is stored in Initialized Data Segment
• main is stored inText / Code Segment

int global = 5;
static int stglobal = 9;
int main( )
{
int local = 8;
static int stlocal = 3;
}
00000000 D global
00000000 T main
00000004 d stglobal
00000008 d stlocal.0
• d stands for Internal Linkage
• External variables will not have any
name mangling.
• Internal Linkage variables might have
name mangling.

1. Can we have two static
variables with same name
but different functions.
2. What is the memory
segment of x.
3. How does the compiler
avoid name clashes.
void print( )
{
static int x = 5;
}
void display( )
{
static int x = 8;
}
int main( )
{
}
00000005 T display
0000000a T main
00000000 T print
00000000 d x.0
00000004 d x.1
Compiler uses different names for x.

int x = 5;
int main( )
{
int x = 10;
printf(“x = %d n “ , x);
}
static int x = 5;
int main( )
{
static int x = 10;
{
static int x = 12;
printf(“x = %d n “ , x);
}
}

FILE1.C
int x = 5;
void func( );
int main( )
{
func( );
}
FILE2.C
int x = 8;
void func( )
{
printf(" x = %d n" , x);
}
• Compile file1.c
• Compile file2.c
• Link file1.o and file2.o
• Observe the output

• [sikander@localhost nm]$ gcc file1.c -c
• [sikander@localhost nm]$ gcc file2.c –c
• [sikander@localhost nm]$ gcc file1.o file2.o
• file2.o(.data+0x0): multiple definition of `x'
• file1.o(.data+0x0): first defined here
• collect2: ld returned 1 exit status
[sikander@localhost nm]$ nm file1.o
U func
00000000 T main
00000000 D x
[sikander@localhost nm]$ nm file2.o
00000000 T func
U printf
00000000 D x

FILE1.C
static int x = 5;
void func( );
int main( )
{
printf(__func__ );
printf(" x = %d n",x);
func( );
}
FILE2.C
static int x = 8;
void func( )
{
printf("%s x = %d n" ,__func__ , x);
}
• Compile file1.c
• Compile file2.c

• $ gcc -c file1.c
• $ gcc file1.o file2.o
• $ ./a.out
• main x = 5
• func x = 8
$ nm file1.o
U func
00000000r __func__.0
00000000T main
U printf
00000000 d x
$ nm file2.o
00000000T func
00000000r __func__.0
U printf
00000000 d x
080495dc d x
080495e0 d x

FILE1.C
int x = 5;
void func( );
int main( )
{
printf(__func__ );
func( );
}
FILE2.C
extern int x;
void func( )
{
}
• Compile file1.c
• Compile file2.c

• $ ./a.out
 main x = 5
 &x = 0x8049618
 func x = 5
 &x = 0x8049618
]$ nm file1.o
U func
00000000 r __func__.0
00000000 T main
U printf
00000000 D x
]$ nm file2.o
00000000 T func
00000000 r __func__.0
U printf
U x
$nm a.out
08049618 D x

FILE1.C
static int x = 5;
void func( );
int main( )
{
printf(__func__ );
func( );
}
FILE2.C
extern int x;
void func( )
{
}
• Compile file1.c
• Compile file2.c

• file2.o(.text+0xb): In function `func':
• : undefined reference to `x‘
• collect2: ld returned 1 exit status
]$ nm file1.o
U func
00000000 r __func__.0
00000000 T main
U printf
00000000 d x
]$ nm file2.o
00000000 T func
00000000 r __func__.0
U printf
U x

static int x = 5;
int main( )
{
int x;
printf(“x = %d n”,x);
}

static int x = 5;
int main( )
{
extern int x;
printf(“x = %d n”,x);
}

int a = 5;
int b;
static int c = 7;
static int d;
int main( )
{
}
• Mention the memory segments of variables after
compiling (.o file) and after linking (a.out)
• What is the difference between a and c.
• Use nm utility to view the memory segments
$gcc –c file.c
$nm file.o
$gcc file.c
$nm ./a.out

 $ gcc file.c -c
 $ nm file.o
 00000000 D a
 00000004 C b
 00000004 d c
 00000000 b d
 00000000 T main
•“C” The symbol is common.
• Common symbols are uninitialized
data.
• When linking, multiple common
symbols may appear with the same
name.
• If the symbol is defined anywhere, the
common symbols are treated as
undefined references.$ nm a.out
08049534 D a
08049544 B b
08049538 d c
08049540 b d

int x ;
int main( )
{
printf(“Main &x= %p” , &x);
printf(“x = %d n”, x);
func( );
}
int x ;
void func( )
{
printf(“Func &x= %p” , &x);
}
$ nm file1.o
U func
00000000 T main
U printf
00000004 C x
]$ nm file2.o
00000000 T func
U printf
00000004 C x
$nm a.out
08049600 B x

int x ;
int main( )
{
func( );
}
int x = 5 ;
void func( )
{
}
$ nm file2.o
00000000T func
U printf
00000000 D x
$ nm file1.o
U func
00000000 T main
U printf
00000004 C x
$nm a.out
08049600 D x

C C
B
C D
D
D D
Multiple Definition
d d
NoConflict, two different variables
b b
d b
b d
b D

void func( );
int x = 5; //D
int main( )
{
func( );
}
int x = 10; //D
void func( )
{
}
void func( );
static int x = 5; //d
int main( )
{
func( );
}
int x = 10; //D
void func( )
{
}

const int gc = 10;
static const int gsc = 12;
int main( )
{
const int lc = 5;
static const int lsc = 8;
}
00000000 R gc
00000004 r gsc
00000008 r lsc.0
00000000 T main

const int gc = 15; //ReadOnly
int main( )
{
const int lc = 5; //Stack
printf(“Enter the value for local const variable : “);
scanf(“ %d”,&lc);
printf(“LocalConst = %d n” , lc);
printf(“Enter the value for global const variable : “);
scanf(“ %d”,&gc);
printf(“GlobalConst = %d n”,gc);
}
[sikander@localhost ~]$ ./a.out
Enter the value for local const variable : 89
LocalConst = 89
Enter the value for global const variable : 6
Segmentation fault

 [sikander@localhost ~]$ nm f1.o
 00000000 t display
 0000000a T main
 00000005 T print
static void display()
{
}
void print()
{
}
int main( )
{
display( );
print( );
}

 For any queries and feedback
 Mail to
 Mohammed.sikander@cranessoftware.com
 sikander1248@gmail.com

Understanding storage class using nm

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