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, 2 insertions, 1 deletions

diff --git a/c/cat.c b/c/cat.c
index dde58cd..790cb88 100644
--- a/c/cat.c
+++ b/c/cat.c
@@ -6,7 +6,8 @@ void main(void) {
     if (argc > 0) {
         unsigned char fd = open(argv[0], O_READ);
         int ch;
-        if (fd == NOFD) { puts("cat: no such file"); nl(); return; }
+        if (fd == EISDIR) { puts("cat: is a directory"); nl(); return; }
+        if (fd == NOFD)   { puts("cat: no such file"); nl(); return; }
         while ((ch = fgetc(fd)) >= 0) putc((char)ch);
         close(fd);
     } else {