buffer - Should I set stdout and stdin to be unbuffered in C? -

because of stdin , stdout buffering printf, scanf , getchar not executed. flush output buffer using fflush(stdout) code can become unreadable because of that. if set stdin , stdout unbuffered using setbuf(stdin, null) , setbuf(stdout, null) make program perform better or worse?

making stdin or stdout unbuffered can make program perform worse if handles large quantities of input / output , files. i/o requests broken down system calls on byte byte basis.

note buffering not cause printf, scanf , getchar not executed: printf output final destination can delayed, input operation via scanf or getchar may occur without prompt.

note setting intput unbuffered may not effective terminal because terminal performs own buffering controlled via stty or ioctl.

most c libraries have hack causes stdout flushed when reading stdin requires getting data system, behavior not specified in c standard, libraries not implement it. safe add calls fflush(stdout); before input operations, , after transitory messages such progress meters. purposes, best let c startup determine appropriate buffering strategy depending upon type of system handle associated stdin , stdout streams. common default line buffered devices , buffered size of bufsiz files.

to idea of potential performance hit, compile naive ccat program:

#include <stdio.h> #include <stdlib.h> #include <string.h>  int main(int argc, char *argv[]) {     int c, size;      if (argc > 1) {         if (!strcmp(argv[1], "bufsiz"))             size = bufsiz;         else             size = strtol(argv[1], null, 0);          if (size == 0) {             /* make stdin , stdout unbuffered */             setvbuf(stdin, null, _ionbf, 0);             setvbuf(stdout, null, _ionbf, 0);         } else         if (size > 0) {             /* make stdin , stdout buffered */             setvbuf(stdin, null, _iofbf, size);             setvbuf(stdout, null, _iofbf, size);         } else {             /* make stdin , stdout line buffered */             setvbuf(stdin, null, _iolbf, -size);             setvbuf(stdout, null, _iolbf, -size);         }     }     while ((c = getchar()) != eof) {         putchar(c);     }     return 0; } 

time program execution copying large file several times minimize file caching side effects.

on debian linux box these timings 3.8mb text file:

chqrlie@linux:~/dev/stackoverflow$ time wc w  396684  396684 3755392 w real    0m0.072s user    0m0.068s sys     0m0.000s  chqrlie@linux:~/dev/stackoverflow$ time cat < w > ww real    0m0.008s user    0m0.000s sys     0m0.004s  chqrlie@linux:~/dev/stackoverflow$ time ./ccat < w > ww real    0m0.060s user    0m0.056s sys     0m0.000s  chqrlie@linux:~/dev/stackoverflow$ time ./ccat 0x100000 < w > ww real    0m0.060s user    0m0.058s sys     0m0.000s  chqrlie@linux:~/dev/stackoverflow$ time ./ccat 0 < w > ww real    0m5.326s user    0m0.632s sys     0m4.684s  chqrlie@linux:~/dev/stackoverflow$ time ./ccat -0x1000 < w > ww real    0m0.533s user    0m0.104s sys     0m0.428s 

as can see:

• setting stdin , stdout unbuffered causes program slow down factor of 100,
• using line buffering slows down factor of 10 (because lines short, 9 10 bytes on average)
• using larger buffer not show improvement, timing difference not significant,
• the naive implementation quite fast, real cat utility uses faster apis 6 times faster execution times.

as conclusion: not set stdin , stdout unbuffered, impact performance moderately large files.