Thread: Creating a long, specific, c string

Originally Posted by Dino

In the #define solution above, will the, for instance, B500 fields, since not directly used, be part of the executable, affecting the final size of the executable?

But they are directly used! And, yes, using a literal can increase the executable size. but not always. As an example, these two functions are the same in my machine, using GCC 10.2):

Code:

#include <stdio.h>
#include <string.h>

#define B10 "          "
#define B50 B10 B10 B10 B10 B10
#define B100 B50 B50

#define STRSIZE 100

void f( void )
{
  char s[STRSIZE+1] = B100;

  puts( s );
}

void g( void )
{
  char s[STRSIZE+1];

  memset( s, ' ', sizeof s - 1 );
  s[sizeof s - 1] = '\0';

  puts( s );
}

Both create the same asm code with maximum optimization:

Code:

$ cc -O2 -fomit-frame-pointer -fcf-protection=none -fno-stack-protector -c -o test.o test.c

Code:

  section .text

f:    ; and g:
  ; Reserve space for s.
  sub     rsp, 120

  ; Fill the array s.
  movdqa  xmm0, [spaces]
  mov     rdi, rsp
  mov     DWORD [rsp+96], 0x20202020
  mov     BYTE [rsp+100], 0
  movups  [rsp], xmm0
  movups  [rsp+16], xmm0
  movups  [rsp+32], xmm0
  movups  [rsp+48], xmm0
  movups  [rsp+64], xmm0
  movups  [rsp+80], xmm0

  call    puts wrt ..plt

  ; Dispose of s.
  add     rsp, 120
  ret

  section .rodata
  align 16
spaces:
  times 2 dq 0x2020202020202020

In other cases, like this:

Code:

#include <stdio.h>
#include <string.h>

#define STRSIZE 79

#ifdef STATIC
void f( void )
{
  char s[STRSIZE+1] = "abcdefghijklmnopqrstuvwxyz"
                      "abcdefghijklmnopqrstuvwxyz"
                      "abcdefghijklmnopqrstuvwxyz";

  puts( s );
}
#else
void f( void )
{
  char s[STRSIZE+1];
  char *p;

  s[sizeof s - 1] = '\0';
  p = s;
  for ( int j = 0; j < 3; j++ )
    for ( int i = 'a'; i <= 'z'; i++ )
      *p++ = i;

  puts( s );
}
#endif

We have:

Code:

$ cc -DSTATIC -O2 -fomit-frame-pointer -fcf-protection=none -fno-stack-protector -c -o test1.o test.c
$ cc -O2 -fomit-frame-pointer -fcf-protection=none -fno-stack-protector -c -o test2.o test.c
$ objdump -x test1.o | sed -n '/^Sec/,/^SYM/p'
Sections:
Idx Name          Siz.      VMA               LMA               File off  Algn
  0 .text         00000051  0000000000000000  0000000000000000  00000040  2**4
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 .data         00000000  0000000000000000  0000000000000000  00000091  2**0
                  CONTENTS, ALLOC, LOAD, DATA
  2 .bss          00000000  0000000000000000  0000000000000000  00000091  2**0
                  ALLOC
  3 .rodata.cst16 00000050  0000000000000000  0000000000000000  000000a0  2**4
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  4 .comment      00000028  0000000000000000  0000000000000000  000000f0  2**0
                  CONTENTS, READONLY
  5 .note.GNU-stack 00000000  0000000000000000  0000000000000000  00000118  2**0
                  CONTENTS, READONLY
  6 .eh_frame     00000030  0000000000000000  0000000000000000  00000118  2**3
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
SYMBOL TABLE:

$ objdump -x test2.o | sed -n '/^Sec/,/^SYM/p'
Sections:
Idx Name          Siz.      VMA               LMA               File off  Algn
  0 .text         00000049  0000000000000000  0000000000000000  00000040  2**4
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 .data         00000000  0000000000000000  0000000000000000  00000089  2**0
                  CONTENTS, ALLOC, LOAD, DATA
  2 .bss          00000000  0000000000000000  0000000000000000  00000089  2**0
                  ALLOC
  3 .comment      00000028  0000000000000000  0000000000000000  00000089  2**0
                  CONTENTS, READONLY
  4 .note.GNU-stack 00000000  0000000000000000  0000000000000000  000000b1  2**0
                  CONTENTS, READONLY
  5 .eh_frame     00000030  0000000000000000  0000000000000000  000000b8  2**3
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
SYMBOL TABLE:

Notice the extra .rodata section with 80 bytes (values in hexadecimal) on test1.o.

[]s
Fred