Add mesg(1)

No manpage yet.

Makefile

@@ -53,6 +53,7 @@ SRC = \
 	login.c             \
 	lsmod.c             \
 	lsusb.c             \
+	mesg.c              \
 	mknod.c             \
 	mkswap.c            \
 	mount.c             \

mesg.c

@@ -0,0 +1,53 @@
+/* See LICENSE file for copyright and license details. */
+#include <sys/stat.h>
+#include <sys/types.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include "util.h"
+
+static void
+usage(void)
+{
+	eprintf("usage: %s [n|y]\n", argv0);
+}
+
+int
+main(int argc, char *argv[])
+{
+	struct stat sb;
+	mode_t mode;
+
+	ARGBEGIN {
+	default:
+		usage();
+	} ARGEND;
+
+	if (argc > 1)
+		usage();
+
+	if (isatty(STDIN_FILENO) == 0)
+		eprintf("stdin: not a tty\n");
+
+	if (fstat(STDIN_FILENO, &sb) < 0)
+		eprintf("fstat stdin:");
+
+	if (argc == 0) {
+		puts(sb.st_mode & (S_IWGRP | S_IWOTH) ? "is y" : "is n");
+		return EXIT_SUCCESS;
+	}
+
+	if (argv[0][0] == 'y')
+		mode = sb.st_mode | S_IWGRP | S_IWOTH;
+	else if (argv[0][0] == 'n')
+		mode = sb.st_mode & ~(S_IWGRP | S_IWOTH);
+	else
+		usage();
+
+	if (fchmod(STDIN_FILENO, mode) < 0)
+		eprintf("fchmod stdin:");
+
+	return EXIT_SUCCESS;
+}