i386/MIT/course/src/git.lab/user/writemotd.c

   1 #include <inc/lib.h>
   2
   3 void
   4 umain(int argc, char **argv)
   5 {
   6         int rfd, wfd;
   7         char buf[512];
   8         int n, r;
   9
  10         if ((rfd = open("/newmotd", O_RDONLY)) < 0)
  11                 panic("open /newmotd: %e", rfd);
  12         if ((wfd = open("/motd", O_RDWR)) < 0)
  13                 panic("open /motd: %e", wfd);
  14         cprintf("file descriptors %d %d\n", rfd, wfd);
  15         if (rfd == wfd)
  16                 panic("open /newmotd and /motd give same file descriptor");
  17
  18         cprintf("OLD MOTD\n===\n");
  19         while ((n = read(wfd, buf, sizeof buf-1)) > 0)
  20                 sys_cputs(buf, n);
  21         cprintf("===\n");
  22         seek(wfd, 0);
  23
  24         if ((r = ftruncate(wfd, 0)) < 0)
  25                 panic("truncate /motd: %e", r);
  26
  27         cprintf("NEW MOTD\n===\n");
  28         while ((n = read(rfd, buf, sizeof buf-1)) > 0) {
  29                 sys_cputs(buf, n);
  30                 if ((r = write(wfd, buf, n)) != n)
  31                         panic("write /motd: %e", r);
  32         }
  33         cprintf("===\n");
  34
  35         if (n < 0)
  36                 panic("read /newmotd: %e", n);
  37
  38         close(rfd);
  39         close(wfd);
  40 }