A Little Story About the `yes` Unix Command(matthias-endler.de)

  #ifndef _GNU_SOURCE
  #define _GNU_SOURCE 1
  #endif
  
  #include <errno.h>    /* ERANGE errno */
  #include <inttypes.h> /* strtoumax(3) */
  #include <limits.h>   /* LONG_MAX */
  #include <stdint.h>   /* uintmax_t */
  #include <stdio.h>    /* fflush(3) fprintf(3) fread(3) fwrite(3) tmpfile(3) */
  #include <stdlib.h>   /* EXIT_FAILURE */
  #include <string.h>   /* strlen(3) */
  
  #include <err.h>      /* err(3) errx(3) */
  #include <fcntl.h>    /* loff_t splice(2) */
  #include <sys/time.h> /* struct timeval gettimeofday(2) */
  #include <unistd.h>   /* _SC_PAGESIZE pread(2) sysconf(3) write(2) */
  
  #ifndef HAVE_SPLICE
  #define HAVE_SPLICE __linux__
  #endif
  
  #define MIN(a, b) (((a) < (b))? (a) : (b))
  #define MAX(a, b) (((a) > (b))? (a) : (b))
  
  #ifndef howmany
  #define howmany(x, y)  (((x) + ((y) - 1)) / (y))
  #endif
  
  #define UMAX_PREC (sizeof (uintmax_t) * CHAR_BIT) /* NB: assumes no padding */
  #define UMAX_HALF ((UMAX_PREC + 1) / 2)
  #define UMAX_LO(n) ((n) & ((UINTMAX_C(1) << UMAX_HALF) - 1))
  #define UMAX_HI(n) ((n) >> UMAX_HALF)
  
  static inline _Bool
  add_overflow(uintmax_t *r, const uintmax_t a, const uintmax_t b)
  {
          if (~a < b)
                  return 1;
          *r = a + b;
          return 0;
  }
  
  /*
   * Implement multiplication using a polynomial with four multiplications and
   * three additions, except we can optimize out some operations.
   */
  static inline _Bool
  mul_overflow(uintmax_t *_r, const uintmax_t _a, const uintmax_t _b)
  {
    uintmax_t a[2] = { UMAX_LO(_a), UMAX_HI(_a) };
    uintmax_t b[2] = { UMAX_LO(_b), UMAX_HI(_b) };
    uintmax_t r[2];
  
    /* if both are non-0, we'd always overflow */
    if (a[1] && b[1])
      return 1;
  
    /* either a[1] or b[1] must be 0 here, so no intermediate overflow */
    r[1] = (a[1] * b[0]) + (a[0] * b[1]);
  
    /* if the result has MSW bits set, we'd overflow */
    if (UMAX_HI(r[1]))
      return 1;
  
    r[0] = a[0] * b[0];
  
    return add_overflow(_r, r[0], r[1] << UMAX_HALF);
  }
  
  #if 0
  static uintmax_t
  add(uintmax_t a, uintmax_t b)
  {
    uintmax_t r;
    if (add_overflow(&r, a, b))
      errx(1, "arithmetic overflow (%ju * %ju)", a, b);
    return r;
  }
  #endif
  
  static uintmax_t
  mul(uintmax_t a, uintmax_t b)
  {
    uintmax_t r;
    if (mul_overflow(&r, a, b))
      errx(1, "arithmetic overflow (%ju * %ju)", a, b);
    return r;
  }
  
  static uintmax_t
  toumax(const char *p)
  {
    char *pe;
    uintmax_t n;
  
    errno = 0;
    if (UINTMAX_MAX == (n = strtoumax(p, &pe, 10)) && errno != 0)
      err(1, "%s", p);
    if (*pe != '\0' || p == pe)
      errx(1, "%s: invalid number", p);
  
    return n;
  }
  
  static uintmax_t
  gcd(uintmax_t a, uintmax_t b)
  {
    uintmax_t c;
  
    while (a != 0) {
      c = a;
      a = b % a;
      b = c;
    }
  
    return b;
  }
  
  static uintmax_t
  lcm(uintmax_t a, uintmax_t b)
  {
    return mul(a, b) / gcd(a, b);
  }
  
  static size_t
  xgetpagesize(void)
  {
    long n = sysconf(_SC_PAGESIZE);
    if (n <= 0)
      err(1, "sysconf");
    return n;
  }
  
  static double
  gtod(void)
  {
    struct timeval now;
    gettimeofday(&now, NULL);
    return now.tv_sec + (now.tv_usec / 1000000.0);
  }
  
  int
  main(int argc, char **argv)
  {
    _Bool cflag = 0, nflag = 0;
    uintmax_t count = UINTMAX_MAX, lines = 0, total = 0;
    int optc;
  
    while (-1 != (optc = getopt(argc, argv, "c:n:"))) {
      switch (optc) {
      case 'c':
        if (nflag)
          errx(1, "-c and -n mutually exclusive");
        count = toumax(optarg);
        cflag = 1;
        break;
                  case 'n':
                    if (cflag)
          errx(1, "-c and -n mutually exclusive");
        lines = toumax(optarg);
        nflag = 1;
        break;
      default:
        return EXIT_FAILURE;
      }
    }
    argc -= optind;
    argv += optind;
  
    const char *const line = (argc > 0)? argv[0] : "y";
    const size_t linesize = strlen(line) + 1;
    const size_t pagesize = xgetpagesize();
    const size_t blocksize = MAX(mul(pagesize, 16), lcm(linesize, pagesize));
    const uintmax_t filesize = mul(blocksize, howmany((1UL << 20), blocksize));
  
    if (nflag)
      count = mul(linesize, lines);
  
  //  fprintf(stderr, "linesize:  %zu\n", linesize);
  //  fprintf(stderr, "pagesize:  %zu\n", xgetpagesize());
  //  fprintf(stderr, "blocksize: %ju\n", blocksize);
  //  fprintf(stderr, "filesize:  %ju\n", filesize);
  
    FILE *fh;
    if (!(fh = tmpfile()))
      err(1, "tmpfile");
    for (size_t i = 0; i < filesize; i += linesize) {
      int n = fprintf(fh, "%s\n", line);
      if (n == -1)
        err(1, "fprintf");
      if ((size_t)n != linesize)
        errx(1, "wrote %d bytes, expected %zu", n, linesize);
    }
    if (0 != fflush(fh))
      err(1, "fflush");
  
    double begin = gtod();
  #if HAVE_SPLICE
    int fd = fileno(fh);
    _Bool eof = 0;
    do {
      loff_t p = 0;
      size_t r;
      ssize_t n;
  
      /* NB: no LOFF_MAX available */
      _Static_assert(sizeof p <= sizeof (long), "unexpected type for loff_t");
      if (filesize > LONG_MAX)
        errx(1, "filesize too large (%ju > %jd)", filesize, (intmax_t)LONG_MAX);
  
      while ((size_t)p < filesize && count >= blocksize && !eof) {
        r = MIN(blocksize, count);
        r -= r % blocksize;
        n = splice(fd, &p, STDOUT_FILENO, NULL, r, SPLICE_F_MOVE);
        if (n == -1)
          err(1, "splice");
        p += n;
        count -= n;
        total += n;
        eof = n == 0;
      }
    } while (!eof && (count || (!cflag && !nflag)));
  #endif
    {
      char buf[BUFSIZ], *p, *pe;
      size_t n;
  
      while (count) {
        rewind(fh);
        if (!(n = fread(buf, 1, MIN(sizeof buf, count), fh))) {
          if (ferror(fh))
            err(1, "fread");
          break;
        }
  
        p = buf;
        pe = &buf[n];
        while (p < pe) {
          if (!(n = fwrite(p, 1, (size_t)(pe - p), stdout)))
            err(1, "fwrite");
          p += n;
          count -= n;
          total += n;
        }
      }
    }
    if (0 != fflush(stdout))
      err(1, "fflush");
  
    double elapsed = gtod() - begin;
    fprintf(stderr, "%.3f GB/s (%zu bytes in %.3f seconds)\n", ((double)total / elapsed) / (1UL<<30), total, elapsed);
  
    return 0;
  }

    #!/usr/bin/python
    import sys

    yes = "yes\n"
    print(sys.argv)
    if len(sys.argv) == 2:
        yes = sys.argv[1] + "\n"

    size = 4096
    buf = bytes(yes * size, 'utf-8')

    while True:
	sys.stdout.buffer.write(buf)

  #include <sys/uio.h>
  
  #define str(x) #x
  #define expand(x) str(x)
  #define y1 y\n
  #define y2 expand(y1) expand(y1)
  #define y4 y2 y2
  #define y8 y4 y4
  #define y16 y8 y8
  #define y32 y16 y16
  #define y64 y32 y32
  #define y128 y64 y64
  #define y256 y128 y128
  #define y512 y256 y256
  #define y1024 y512 y512
  #define y2048 y1024 y1024

  //2048 "y\n" s
  static char __attribute__((aligned(4096))) str[] = y2048;
  static struct iovec v[] = {
  	{str, 4096},			//adjust number of these to taste
  	{str, 4096},
  	{str, 4096},
  	{str, 4096},
  	{str, 4096},
  	{str, 4096},
  	{str, 4096},
  	{str, 4096},
  	{str, 4096},
  	{str, 4096},
  	{str, 4096},
  	{str, 4096},
  	{str, 4096},
  	{str, 4096},
  	{str, 4096},
  	{str, 4096},
  };
  
  
  int main(int argc, char** argv) {
  
  	while(1) {
  		//scatter-gather write (on linux there is basically no way for write to write an odd number of characters, except EOF, so we need not check for partial writes
  		(void)writev(1, v, sizeof(v) / sizeof(*v));
  	}
  	return 0;
  }

  while True:
      print(b'y')

  while True:
      print(u'y')

    import os
    ys = 'y\n' * 2048
    while True:
        os.write(1, ys)

  perl -e 'print "y\n" while 1' | pv -r > /dev/null
  [3.37MB/s]
  perl -e 'print "y\n" x 2048 while 1' | pv -r > /dev/null
  [ 425MB/s]
  yes | pv -r > /dev/null
  [11.1MB/s]

    for p in `seq 1 $(sysctl -n hw.ncpu)`; do yes > /dev/null & done

  parallel -- yes yes yes yes >/dev/null

    (or (cadr (args)) "y")