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c - SIZE command in UNIX

The following is my C file:

int main()
{
    return 36;
}

It contains only return statement. But if I use the size command, it shows the output like this:

mohanraj@ltsp63:~/Development/chap8$ size a.out 
text       data     bss     dec     hex filename
1056        252       8    1316     524 a.out
mohanraj@ltsp63:~/Development/chap8$

Even though my program does not contain any global variable, or undeclared data. But, the output shows data segment have 252 and the bss have 8 bytes. So, why the output is like this? what is 252 and 8 refers.

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Size Command

First see the definition of each column:

  • text - Actual machine instructions that your CPU going to execute. Linux allows to share this data.
  • data - All initialized variables (declarations) declared in a program (e.g., float salary=123.45;).
  • bss - The BSS consists of uninitialized data such as arrays that you have not set any values to or null pointers.

As Blue Moon said. On Linux, the execution starts by calling _start() function. Which does environment setup. Every C program has hidden "libraries" that depends on compilator you using. There are settings for global parameters, exit calls and after complete configuration it finally calls your main() function. ASFAIK there's no way to see how your code looks encapsulated with configuration and _start() function. But I can show you that even your code contains more information than you thought the closer to hardware we are.

Hint: Type readelf -a a.out to see how much information your exec really carrying.

What is inside?

Do not compare code in your source file to the size of executable file, it depends on the OS, compilator, and used libraries.

In my example, with exactly the same code, SIZE returns:

eryk@eryk-pc:~$ gcc a.c 
eryk@eryk-pc:~$ size a.out 
   text    data     bss     dec     hex filename
   1033     276       4    1313     521 a.out

Let's see what is inside...

eryk@eryk-pc:~$ gcc -S a.c

This will run the preprocessor over a.c, perform the initial compilation and then stop before the assembler is run.

eryk@eryk-pc:~$ cat a.s
    .file   "a.c"
    .text
    .globl  main
    .type   main, @function
main:
.LFB0:
    .cfi_startproc
    pushl   %ebp
    .cfi_def_cfa_offset 8
    .cfi_offset 5, -8
    movl    %esp, %ebp
    .cfi_def_cfa_register 5
    movl    $36, %eax
    popl    %ebp
    .cfi_restore 5
    .cfi_def_cfa 4, 4
    ret
    .cfi_endproc
.LFE0:
    .size   main, .-main
    .ident  "GCC: (Ubuntu 4.8.2-19ubuntu1) 4.8.2"
    .section    .note.GNU-stack,"",@progbits

Then look on the assembly code

eryk@eryk-pc:~$ objdump -d -M intel -S a.out

a.out:     file format elf32-i386


Disassembly of section .init:

08048294 <_init>:
 8048294:   53                      push   ebx
 8048295:   83 ec 08                sub    esp,0x8
 8048298:   e8 83 00 00 00          call   8048320 <__x86.get_pc_thunk.bx>
 804829d:   81 c3 63 1d 00 00       add    ebx,0x1d63
 80482a3:   8b 83 fc ff ff ff       mov    eax,DWORD PTR [ebx-0x4]
 80482a9:   85 c0                   test   eax,eax
 80482ab:   74 05                   je     80482b2 <_init+0x1e>
 80482ad:   e8 1e 00 00 00          call   80482d0 <__gmon_start__@plt>
 80482b2:   83 c4 08                add    esp,0x8
 80482b5:   5b                      pop    ebx
 80482b6:   c3                      ret    

Disassembly of section .plt:

080482c0 <__gmon_start__@plt-0x10>:
 80482c0:   ff 35 04 a0 04 08       push   DWORD PTR ds:0x804a004
 80482c6:   ff 25 08 a0 04 08       jmp    DWORD PTR ds:0x804a008
 80482cc:   00 00                   add    BYTE PTR [eax],al
    ...

080482d0 <__gmon_start__@plt>:
 80482d0:   ff 25 0c a0 04 08       jmp    DWORD PTR ds:0x804a00c
 80482d6:   68 00 00 00 00          push   0x0
 80482db:   e9 e0 ff ff ff          jmp    80482c0 <_init+0x2c>

080482e0 <__libc_start_main@plt>:
 80482e0:   ff 25 10 a0 04 08       jmp    DWORD PTR ds:0x804a010
 80482e6:   68 08 00 00 00          push   0x8
 80482eb:   e9 d0 ff ff ff          jmp    80482c0 <_init+0x2c>

Disassembly of section .text:

080482f0 <_start>:
 80482f0:   31 ed                   xor    ebp,ebp
 80482f2:   5e                      pop    esi
 80482f3:   89 e1                   mov    ecx,esp
 80482f5:   83 e4 f0                and    esp,0xfffffff0
 80482f8:   50                      push   eax
 80482f9:   54                      push   esp
 80482fa:   52                      push   edx
 80482fb:   68 70 84 04 08          push   0x8048470
 8048300:   68 00 84 04 08          push   0x8048400
 8048305:   51                      push   ecx
 8048306:   56                      push   esi
 8048307:   68 ed 83 04 08          push   0x80483ed
 804830c:   e8 cf ff ff ff          call   80482e0 <__libc_start_main@plt>
 8048311:   f4                      hlt    
 8048312:   66 90                   xchg   ax,ax
 8048314:   66 90                   xchg   ax,ax
 8048316:   66 90                   xchg   ax,ax
 8048318:   66 90                   xchg   ax,ax
 804831a:   66 90                   xchg   ax,ax
 804831c:   66 90                   xchg   ax,ax
 804831e:   66 90                   xchg   ax,ax

08048320 <__x86.get_pc_thunk.bx>:
 8048320:   8b 1c 24                mov    ebx,DWORD PTR [esp]
 8048323:   c3                      ret    
 8048324:   66 90                   xchg   ax,ax
 8048326:   66 90                   xchg   ax,ax
 8048328:   66 90                   xchg   ax,ax
 804832a:   66 90                   xchg   ax,ax
 804832c:   66 90                   xchg   ax,ax
 804832e:   66 90                   xchg   ax,ax

08048330 <deregister_tm_clones>:
 8048330:   b8 1f a0 04 08          mov    eax,0x804a01f
 8048335:   2d 1c a0 04 08          sub    eax,0x804a01c
 804833a:   83 f8 06                cmp    eax,0x6
 804833d:   77 01                   ja     8048340 <deregister_tm_clones+0x10>
 804833f:   c3                      ret    
 8048340:   b8 00 00 00 00          mov    eax,0x0
 8048345:   85 c0                   test   eax,eax
 8048347:   74 f6                   je     804833f <deregister_tm_clones+0xf>
 8048349:   55                      push   ebp
 804834a:   89 e5                   mov    ebp,esp
 804834c:   83 ec 18                sub    esp,0x18
 804834f:   c7 04 24 1c a0 04 08    mov    DWORD PTR [esp],0x804a01c
 8048356:   ff d0                   call   eax
 8048358:   c9                      leave  
 8048359:   c3                      ret    
 804835a:   8d b6 00 00 00 00       lea    esi,[esi+0x0]

08048360 <register_tm_clones>:
 8048360:   b8 1c a0 04 08          mov    eax,0x804a01c
 8048365:   2d 1c a0 04 08          sub    eax,0x804a01c
 804836a:   c1 f8 02                sar    eax,0x2
 804836d:   89 c2                   mov    edx,eax
 804836f:   c1 ea 1f                shr    edx,0x1f
 8048372:   01 d0                   add    eax,edx
 8048374:   d1 f8                   sar    eax,1
 8048376:   75 01                   jne    8048379 <register_tm_clones+0x19>
 8048378:   c3                      ret    
 8048379:   ba 00 00 00 00          mov    edx,0x0
 804837e:   85 d2                   test   edx,edx
 8048380:   74 f6                   je     8048378 <register_tm_clones+0x18>
 8048382:   55                      push   ebp
 8048383:   89 e5                   mov    ebp,esp
 8048385:   83 ec 18                sub    esp,0x18
 8048388:   89 44 24 04             mov    DWORD PTR [esp+0x4],eax
 804838c:   c7 04 24 1c a0 04 08    mov    DWORD PTR [esp],0x804a01c
 8048393:   ff d2                   call   edx
 8048395:   c9                      leave  
 8048396:   c3                      ret    
 8048397:   89 f6                   mov    esi,esi
 8048399:   8d bc 27 00 00 00 00    lea    edi,[edi+eiz*1+0x0]

080483a0 <__do_global_dtors_aux>:
 80483a0:   80 3d 1c a0 04 08 00    cmp    BYTE PTR ds:0x804a01c,0x0
 80483a7:   75 13                   jne    80483bc <__do_global_dtors_aux+0x1c>
 80483a9:   55                      push   ebp
 80483aa:   89 e5                   mov    ebp,esp
 80483ac:   83 ec 08                sub    esp,0x8
 80483af:   e8 7c ff ff ff          call   8048330 <deregister_tm_clones>
 80483b4:   c6 05 1c a0 04 08 01    mov    BYTE PTR ds:0x804a01c,0x1
 80483bb:   c9                      leave  
 80483bc:   f3 c3                   repz ret 
 80483be:   66 90                   xchg   ax,ax

080483c0 <frame_dummy>:
 80483c0:   a1 10 9f 04 08          mov    eax,ds:0x8049f10
 80483c5:   85 c0                   test   eax,eax
 80483c7:   74 1f                   je     80483e8 <frame_dummy+0x28>
 80483c9:   b8 00 00 00 00          mov    eax,0x0
 80483ce:   85 c0                   test   eax,eax
 80483d0:   74 16                   je     80483e8 <frame_dummy+0x28>
 80483d2:   55                      push   ebp
 80483d3:   89 e5                   mov    ebp,esp
 80483d5:   83 ec 18                sub    esp,0x18
 80483d8:   c7 04 24 10 9f 04 08    mov    DWORD PTR [esp],0x8049f10
 80483df:   ff d0                   call   eax
 80483e1:   c9                      leave  
 80483e2:   e9 79 ff ff ff          jmp    8048360 <register_tm_clones>
 80483e7:   90                      nop
 80483e8:   e9 73 ff ff ff          jmp    8048360 <register_tm_clones>

080483ed <main>:
 80483ed:   55                      push   ebp
 80483ee:   89 e5                   mov    ebp,esp
 80483f0:   b8 24 00 00 00          mov    eax,0x24
 80483f5:   5d                      pop    ebp
 80483f6:   c3                      ret    
 80483f7:   66 90                   xchg   ax,ax
 80483f9:   66 90                   xchg   ax,ax
 80483fb:   66 90                   xchg   ax,ax
 80483fd:   66 90                   xchg   ax,ax
 80483ff:   90                      nop

08048400 <__libc_csu_init>:
 8048400:   55                      push   ebp
 8048401:   57                      push   edi
 8048402:   31 ff                   xor    edi,edi
 8048404:   56                      push   esi
 8048405:   53                      push   ebx
 8048406:   e8 15 ff ff ff          call   8048320 <__x86.get_pc_thunk.bx>
 804840b:   81 c3 f5 1b 00 00       add    ebx,0x1bf5
 8048411:   83 ec 1c                sub    esp,0x1c
 8048414:   8b 6c 24 30             mov    ebp,DWORD PTR [esp+0x30]
 8048418:   8d b3 0c ff ff ff       lea    esi,[ebx-0xf4]
 804841e:   e8 71 fe ff ff          call   8048294 <_init>
 8048423:   8d 83 08 ff ff ff       lea    eax,[ebx-0xf8]
 8048429:   29 c6                   sub    esi,eax
 804842b:   c1 fe 02                sar    esi,0x2
 804842e:   85 f6                   test   esi,esi
 8048430:   74 27                   je     8048459 <__libc_csu_init+0x59>
 8048432:   8d b6 00 00 00 00       lea    esi,[esi+0x0]
 8048438:   8b 44 24 38             mov    eax,DWORD PTR [esp+0x38]
 804843c:   89 2c 24                mov    DWORD PTR [esp],ebp
 804843f:   89 44 24 08             mov    DWORD PTR [esp+0x8],eax
 8048443:   8b 44 24 34             mov    eax,DWORD PTR [esp+0x34]
 8048447:   89 44 24 04             mov    DWORD PTR [esp+0x4],eax
 804844b:   ff 94 bb 08 ff ff ff    call   DWORD PTR [ebx+edi*4-0xf8]
 8048452:   83 c7 01                add    edi,0x1
 8048455:   39 f7                   cmp    edi,esi
 8048457:   75 df                   jne    8048438 <__libc_csu_init+0x38>
 8048459:   83 c4 1c                add    esp,0x1c
 804845c:   5b                      pop    ebx
 804845d:   5e                      pop    esi
 804845e:   5f                      pop    edi
 804845f:   5d                      pop    ebp
 8048460:   c3                      ret    
 8048461:   eb 0d                   jmp    8048470 <__libc_csu_fini>
 8048463:   90                      nop
 8048464:   90                      nop
 8048465:   90                      nop
 8048466:   90                      nop
 8048467:   90                      nop
 8048468:   90 

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