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.)

5 files changed, 8 insertions, 6 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 {
diff --git a/c/gbos.h b/c/gbos.h
index c93414f..2a99c0e 100644
--- a/c/gbos.h
+++ b/c/gbos.h
@@ -34,6 +34,7 @@ void          poweroff(void);   /* clean emulator shutdown (saves) */
 #define O_READ  0
 #define O_WRITE 1
 #define NOFD    0xFF
+#define EISDIR  0xFE     /* open() failed: path is a directory */
 
 /* process control (c/libc.s) */
 unsigned char fork(void);
diff --git a/c/head.c b/c/head.c
index e8bf846..3298e54 100644
--- a/c/head.c
+++ b/c/head.c
@@ -16,10 +16,10 @@ void main(void) {
     }
     if (fname) {
         fd = open(fname, O_READ);
-        if (fd == NOFD) { puts("head: no such file"); nl(); return; }
+        if (fd > 3) { puts("head: no such file"); nl(); return; }
     }
     for (;;) {
-        if (fd == NOFD) { char c = readc(); if (c == EOF) break; ch = c; }
+        if (fd > 3) { char c = readc(); if (c == EOF) break; ch = c; }
         else { ch = fgetc(fd); if (ch < 0) break; }
         if (lines < n) putc((char)ch);
         if (ch == '\n') lines++;
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;
diff --git a/c/wc.c b/c/wc.c
index 424876d..4fa48ee 100644
--- a/c/wc.c
+++ b/c/wc.c
@@ -14,10 +14,10 @@ void main(void) {
     for (i = 0; i < argc; i++) if (argv[i][0] != '-') { fname = argv[i]; break; }
     if (fname) {
         fd = open(fname, O_READ);
-        if (fd == NOFD) { puts("wc: no such file"); nl(); return; }
+        if (fd > 3) { puts("wc: no such file"); nl(); return; }
     }
     for (;;) {
-        if (fd == NOFD) { char c = readc(); if (c == EOF) break; ch = c; }
+        if (fd > 3) { char c = readc(); if (c == EOF) break; ch = c; }
         else { ch = fgetc(fd); if (ch < 0) break; }
         chars++;
         if (ch == '\n') lines++;