my solution to lab5

lab/user/writemotd.c

@@ -0,0 +1,40 @@
+#include <inc/lib.h>
+
+void
+umain(int argc, char **argv)
+{
+	int rfd, wfd;
+	char buf[512];
+	int n, r;
+
+	if ((rfd = open("/newmotd", O_RDONLY)) < 0)
+		panic("open /newmotd: %e", rfd);
+	if ((wfd = open("/motd", O_RDWR)) < 0)
+		panic("open /motd: %e", wfd);
+	cprintf("file descriptors %d %d\n", rfd, wfd);
+	if (rfd == wfd)
+		panic("open /newmotd and /motd give same file descriptor");
+
+	cprintf("OLD MOTD\n===\n");
+	while ((n = read(wfd, buf, sizeof buf-1)) > 0)
+		sys_cputs(buf, n);
+	cprintf("===\n");
+	seek(wfd, 0);
+
+	if ((r = ftruncate(wfd, 0)) < 0)
+		panic("truncate /motd: %e", r);
+
+	cprintf("NEW MOTD\n===\n");
+	while ((n = read(rfd, buf, sizeof buf-1)) > 0) {
+		sys_cputs(buf, n);
+		if ((r = write(wfd, buf, n)) != n)
+			panic("write /motd: %e", r);
+	}
+	cprintf("===\n");
+
+	if (n < 0)
+		panic("read /newmotd: %e", n);
+
+	close(rfd);
+	close(wfd);
+}