fs: reject open()/read() on a directory (EISDIR)

open() now checks the target's inode type: opening a directory as a file
returns $FE (EISDIR) instead of a valid fd, so 'cat docs' no longer streams the
raw directory block as bytes. cat reports 'is a directory'; wc/head/save treat
any invalid fd (>3) as an open failure. (ls is unaffected - it uses the
structural opendir/list path, not open/getb.)

1 files changed, 1 insertions, 1 deletions

diff --git a/c/save.c b/c/save.c
index dd2b8cd..1755fea 100644
--- a/c/save.c
+++ b/c/save.c
@@ -7,7 +7,7 @@ void main(void) {
     char c;
     if (argc < 1) { puts("usage: save <name>"); nl(); return; }
     fd = open(argv[0], O_WRITE);
-    if (fd == NOFD) { puts("save: cannot create"); nl(); return; }
+    if (fd > 3) { puts("save: cannot create"); nl(); return; }
     for (;;) {
         c = readc();
         if (c == EOF) break;