c - Global variable vs macro expansion for string literal -


i'm trying understand of intricacies preprocessor , of c compiler (specifically, gnu gcc) , string literals. more efficient assign global variable string literal occupies 1 place in memory vs using #define preprocessor directive?

as in example, string literal on place in memory , accessed several times:

#include <stdio.h> #include <string.h> char output[20] = "hello, world!!!"; int main (){     printf("%s %d characters long.\n", output, strlen(output));     return 0; }

vs doing preprocessor:

#include <stdio.h> #include <string.h> #define output "hello, world!!!"  int main (){     printf("%s %d characters long.\n", output, (int) strlen(output));     return 0; }

which translates as:

#include <stdio.h> #include <string.h> #define output "hello, world!!!"  int main (){     printf("%s %d characters long.\n", "hello, world!!!", (int) strlen("hello, world!!!"));     return 0; }

what i'm asking in last 2 examples example using preprocessor, compiler have 2 separate instances of "hello, world!!!" in 2 separate memory locations or compiler smart enough make 1 memory location?

if 2 separate memory locations, isn't more resource-friendly use global variable rather macro expansion program constants?

your compiler should smart enough store 1 instance of string. can verify checking assembly outputs programs.

for example, using gcc:

assume first example called "global.c". 

gcc -wall -s global.c  .file   "global.c" .globl  output .data .align 16 .type   output, @object .size   output, 20 output: .string "hello, world!!!" .zero   4 .section    .rodata .lc0: .string "%s %d characters long.\n" .text .globl  main .type   main, @function main:  // more code...

assume preprocessor example called "preproc.c".

gcc -wall -s preproc.c .file   "preproc.c" .section    .rodata .lc0: .string "%s %d characters long.\n" .lc1: .string "hello, world!!!" .text .globl  main .type   main, @function main: // more code...

in both cases, 1 copy of "hello, world!!!" , "%s %d characters long.\n" exist. in first example, have save space 20 characters because code has modifiable array. if changed this

char output[20] = "hello, world!!!";

to

const char * const output = "hello, world!!!";

you get:

.file   "global.c" .globl  output .section    .rodata .lc0: .string "hello, world!!!" .align 8 .type   output, @object .size   output, 8 output: .quad   .lc0 .lc1: .string "%s %d characters long.\n" .text .globl  main .type   main, @function main: // more code...

now saving space pointer , string.

which way better negligible in situation, though recommend using preprocessor scope of strings stays within main function.

both emit identical code optimizations.

global.c (const char * const output):

gcc -wall -o3 -s global.c  .file   "global.c" .section    .rodata.str1.1,"ams",@progbits,1 .lc0: .string "hello, world!!!" .lc1: .string "%s %d characters long.\n" .section    .text.startup,"ax",@progbits .p2align 4,,15 .globl  main .type   main, @function main: .lfb44: .cfi_startproc subq    $8, %rsp .cfi_def_cfa_offset 16 movl    $15, %ecx movl    $.lc0, %edx movl    $.lc1, %esi movl    $1, %edi xorl    %eax, %eax call    __printf_chk xorl    %eax, %eax addq    $8, %rsp .cfi_def_cfa_offset 8 ret .cfi_endproc .lfe44: .size   main, .-main .globl  output .section    .rodata .align 8 .type   output, @object .size   output, 8 output: .quad   .lc0 .ident  "gcc: (ubuntu/linaro 4.6.3-1ubuntu5) 4.6.3" .section    .note.gnu-stack,"",@progbits

preproc.c

gcc -wall -o3 -s preproc.c      .file   "preproc.c" .section    .rodata.str1.1,"ams",@progbits,1 .lc0: .string "hello, world!!!" .lc1: .string "%s %d characters long.\n" .section    .text.startup,"ax",@progbits .p2align 4,,15 .globl  main .type   main, @function main: .lfb44: .cfi_startproc subq    $8, %rsp .cfi_def_cfa_offset 16 movl    $15, %ecx movl    $.lc0, %edx movl    $.lc1, %esi movl    $1, %edi xorl    %eax, %eax call    __printf_chk xorl    %eax, %eax addq    $8, %rsp .cfi_def_cfa_offset 8 ret .cfi_endproc .lfe44: .size   main, .-main .ident  "gcc: (ubuntu/linaro 4.6.3-1ubuntu5) 4.6.3" .section    .note.gnu-stack,"",@progbits

looking @ both main functions, can see instructions identical.


Comments

Post a Comment

Popular posts from this blog

SPSS keyboard combination alters encoding -

my question not have programming practical issue on spss. accident keyword combination cannot remember because did not intend press changed encoding in spss , text in data in different language english shown small boxes. nevertheless, when kind of analysis performed different language text shown in output. does know reason? tried change encoding did not produce result. first, @ edit > options , see whether setting unicode or locale. might fix it. if have text in multiple languages, want unicode. otherwise, may have specific locale set wrong. can change set locale, need know locale want use.
Read more

Add new record to the table by click on the button in Microsoft Access -

in ms access form want implement separate button, adds new record table. in order added button , attached button event: private sub btnaddrec_click() refresh codecontextobject on error resume next docmd.gotorecord , , acnewrec if err.number <> 0 btnaddrec.enabled = false end if end end sub everything ok when open window , click btnaddrec button, problem when first of perform navigation through existed records , after click on button. got runtime error: 2105: «you can't go specified record. may @ end of recordset» . how solve issue, need have ability add new record on click on specific button, no matter, have walked or not walked through records before. thanks. i created simple form field call description (and autonumber) , created button code follows click event. filled number of records , navigated through them, clicked addnewrec button. form navigated new record without issues. able click addnewrec button directly after
Read more

CSS3 Transition to highlight new elements created in JQuery -

i want use css3 color transition apply highlight-fading color effect (yellow transparent) new elements appended markup using jquery. css #content div { background-color:transparent; -moz-transition:background-color 2s; -webkit-transition:background-color 2s; -o-transition:background-color 2s; transition:background-color 2s; } #content div.new { background-color:yellow; } html <div id="content"></div> <a id="add-element" href="#">add new element</a> js $('#add-element').click(function() { var newelement = $('<div class="new">new element</div>'); $('#content').append(newelement); newelement.removeclass('new'); }); when click link, new element created. class "new" (background color yellow) , it's appended html markup. should able remove "new" class trigger background color transition transparent (or whate
Read more