14.5.3.c

#include <ctype.h>
#include <fcntl.h>
#include "apue.h"

#define BSZ 4096

unsigned char buf[BSZ];

unsigned char translate(unsigned char c) {
  if (isalpha(c)) {
    if (c >= 'n') {
      c -= 13;
    } else if (c >= 'a') {
      c += 13;
    } else if (c >= 'N') {
      c -= 13;
    } else {
      c += 13;
    }
  }
  return (c);
}

int main(int argc, char* argv[]) {
  int ifd, ofd, i, n, nw;
  if (argc != 3) {
    err_quit("usage: rot13 infile outfile");
  }
  if ((ifd = open(argv[1], O_RDONLY)) < 0) {
    err_sys("can't open %s", argv[1]);
  }
  if ((ofd = open(argv[2], O_RDWR | O_CREAT | O_TRUNC, FILE_MODE)) < 0) {
    err_sys("can't create %s", argv[2]);
  }

  while ((n = read(ifd, buf, BSZ)) > 0) {
    for (i = 0; i < n; i++) {
      buf[i] = translate(buf[i]);
    }
    if ((nw = write(ofd, buf, n)) != n) {
      if (nw < 0) {
        err_sys("write failed");
      } else {
        err_quit("short write (%d/%d)", nw, n);
      }
    }
  }

  fsync(ofd);
  exit(0);
}