1 - Program

Object file

Observe sections in Object file

Create file memory_layout.c

#include <stdio.h>
int variable_data = 100;
int variable_bss;
int main(){
    int variable_local_x = 200;
    int variable_local_y;

    return 0;
}

Create object file by gcc memory_layout.c -c memory_layout.o

c@c:~/cos3105$ gcc memory_layout.c -c memory_layout.o
gcc: warning: memory_layout.o: linker input file unused because linking not done

List all of sections in Object file readelf -S memory_layout.o

c@c:~/cos3105$ readelf -S memory_layout.o
 There are 12 section headers, starting at offset 0x228:

 Section Headers:
 [Nr] Name              Type             Address           Offset
      Size              EntSize          Flags  Link  Info  Align
 [ 0]                   NULL             0000000000000000  00000000
      0000000000000000  0000000000000000           0     0     0
 [ 1] .text             PROGBITS         0000000000000000  00000040
      0000000000000016  0000000000000000  AX       0     0     1
 [ 2] .data             PROGBITS         0000000000000000  00000058
      0000000000000004  0000000000000000  WA       0     0     4
 [ 3] .bss              NOBITS           0000000000000000  0000005c
      0000000000000004  0000000000000000  WA       0     0     4
 [ 4] .comment          PROGBITS         0000000000000000  0000005c
      000000000000002c  0000000000000001  MS       0     0     1
 [ 5] .note.GNU-stack   PROGBITS         0000000000000000  00000088
      0000000000000000  0000000000000000           0     0     1
 [ 6] .note.gnu.pr[...] NOTE             0000000000000000  00000088
      0000000000000020  0000000000000000   A       0     0     8
 [ 7] .eh_frame         PROGBITS         0000000000000000  000000a8
      0000000000000038  0000000000000000   A       0     0     8
 [ 8] .rela.eh_frame    RELA             0000000000000000  000001a8
      0000000000000018  0000000000000018   I       9     7     8
 [ 9] .symtab           SYMTAB           0000000000000000  000000e0
      0000000000000090  0000000000000018          10     3     8
 [10] .strtab           STRTAB           0000000000000000  00000170
      0000000000000031  0000000000000000           0     0     1
 [11] .shstrtab         STRTAB           0000000000000000  000001c0
      0000000000000067  0000000000000000           0     0     1
 Key to Flags:
 W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
 L (link order), O (extra OS processing required), G (group), T (TLS),
 C (compressed), x (unknown), o (OS specific), E (exclude),
 D (mbind), l (large), p (processor specific)

Specific to .text by readelf -x .text memory_layout.o

c@c:~/cos3105$ readelf -x .text memory_layout.o

Hex dump of section '.text':
 NOTE: This section has relocations against it, but these have NOT been applied to this dump.
  0x00000000 f30f1efa 554889e5 4883ec10 c745fcc8 ....UH..H....E..
  0x00000010 00000048 8d050000 00004889 c6488d05 ...H......H..H..
  0x00000020 00000000 4889c7b8 00000000 e8000000 ....H...........
  0x00000030 00b80000 0000c9c3                   ........

Specific to .data by readelf -x .data memory_layout.o

c@c:~/cos3105$ readelf -x .data memory_layout.o

Hex dump of section '.data':
  0x00000000 64000000                            d...

Specific to .bss by readelf -x .bss memory_layout.o

c@c:~/cos3105$ readelf -x .bss memory_layout.o
Section '.bss' has no data to dump.

.text (22 bytes = 0x16):

Your compiled main() function code Flags: CODE - contains executable instructions

.data (4 bytes):

Your variable_data = 100 (one int = 4 bytes) Flags: DATA - initialized, writable data

.bss (4 bytes):

Your variable_bss (uninitialized int = 4 bytes) Notice: No CONTENTS flag! BSS doesn’t take space in the file Only ALLOC - memory will be allocated at runtime and zeroed

If add constant variables, then goes to .rodata (read-only data)

String literals:

printf("Hello World");  // "Hello World" goes in .rodata

Const variables:

const int max_users = 100;  // Goes in .rodata
const char* message = "Error";  // Both pointer and string in .rodata

Arrays of constants:

const int primes[] = {2, 3, 5, 7, 11};  // Goes in .rodata

Why separate from DATA segment:

Memory protection: OS can mark .rodata pages as read-only, preventing accidental modification
Security: Helps prevent certain types of buffer overflow attacks
Optimization: Can be shared between multiple instances of the same program

Observe symbol/variable of each sections in Object file

Observe symbol in .text by nm memory_layout.o | grep " T "

c@c:~/cos3105$ nm memory_layout.o | grep " T "
0000000000000000 T main

Observe symbol in .data by nm memory_layout.o | grep " D "

c@c:~/cos3105$ nm memory_layout.o | grep " D "
0000000000000000 D variable_data

Observe symbol in .bss by nm memory_layout.o | grep " B "

c@c:~/cos3105$ nm memory_layout.o | grep " B "
0000000000000000 B variable_bss

Observe symbol/variable size of data section in Object file

Create file memory_layout.c

 #include <stdio.h>
 int variable_data = 100; // Goes to DATA (initialized global) - 4 bytes
 int variable_bss; // Goes to BSS (uninitialized global) - not in DATA
 int main(){
      int variable_local_x = 200; // Goes to STACK (local variable) - not in DATA
      int variable_local_y; // Goes to STACK (local variable) - not in DATA

      return 0;
 }

Create object file by gcc memory_layout.c -c memory_layout.o

c@c:~/cos3105$ gcc memory_layout.c -c memory_layout.o
gcc: warning: memory_layout.o: linker input file unused because linking not done

Run size for observe size of object file

c@c:~/cos3105$ size memory_layout.o
text    data     bss     dec     hex filename
147       4       4     155      9b memory_layout.o