-t should only return TRUE for file handles connected to a TTY

The Microsoft C version of isatty() returns TRUE for all
character mode devices, including the /dev/null style "nul"
device and printers like "lpt1".

The included test has only been tested on Windows and Linux;
the device names for OS/2 and VMS are just best guesses...

1 files changed, 16 insertions, 1 deletions

diff --git a/win32/perlhost.h b/win32/perlhost.h
index 7464c7a99f..36a716a846 100644
--- a/win32/perlhost.h
+++ b/win32/perlhost.h
@@ -1004,7 +1004,22 @@ PerlLIOIOCtl(struct IPerlLIO* piPerl, int i, unsigned int u, char *data)
 int
 PerlLIOIsatty(struct IPerlLIO* piPerl, int fd)
 {
-    return isatty(fd);
+    /* The Microsoft isatty() function returns true for *all*
+     * character mode devices, including "nul".  Our implementation
+     * should only return true if the handle has a console buffer.
+     */
+    DWORD mode;
+    HANDLE fh = (HANDLE)_get_osfhandle(fd);
+    if (fh == (HANDLE)-1) {
+        /* errno is already set to EBADF */
+        return 0;
+    }
+
+    if (GetConsoleMode(fh, &mode))
+        return 1;
+
+    errno = ENOTTY;
+    return 0;
 }
 
 int