[TUHS] the sin of buffering [offshoot of excise process from a pipeline]

Larry McVoy lm at mcvoy.com
Tue Jul 15 12:40:42 AEST 2014

On Mon, Jul 14, 2014 at 10:31:27PM -0400, Doug McIlroy wrote:
> > Err, why is buffering data in the process a sin? (Or was this just a
> humourous aside?)
> Process A spawns process B, which reads stdin with buffering. B gets
> all it deserves from stdin and exits. What's left in the buffer,
> intehded for A, is lost. Sinful.

It really depends on what you want.  That buffering is a big win for
some use cases.  Even on today's processors reading a byte at a time via
read(2) is costly.  Like 5000x more costly on the laptop I'm typing on:

calvin:~/tmp lmdd opat=1 move=100m of=XXX
104.8576 MB in 0.1093 secs, 959.5578 MB/sec
calvin:~/tmp time a.out fd < XXX

real    0m14.754s
user    0m1.516s
sys     0m13.201s
calvin:~/tmp time a.out stdio < XXX

real    0m0.003s
user    0m0.000s
sys     0m0.000s
calvin:~/tmp bc

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define unless(x)       if (!(x))
#define streq(a, b)     !strcmp(a, b)

main(int ac, char **av)
        char    c;

        unless (ac == 2) exit(1);
        if (streq(av[1], "stdio")) {
                while ((c = fgetc(stdin)) != EOF)
        } else {
                while (read(0, &c, 1) == 1)

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