feat(unistd): cwd/link/readlink/path ops

src/unistd/isatty.c

@@ -0,0 +1,30 @@
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include <termios.h>
+
+#include "../internal/syscall.h"
+
+/*
+ * isatty: report whether fildes is associated with a terminal.
+ *
+ * The probe is the kernel's own: issue the TCGETS ioctl, which a real
+ * terminal answers with 0 and everything else rejects with -ENOTTY (a
+ * regular file, directory, pipe, socket, or a character device that is
+ * not a terminal such as /dev/null). A char-device fstat check is NOT
+ * sufficient — /dev/null is a character device but not a tty — so the
+ * ioctl is the correct test.
+ *
+ * The result is derived from the raw syscall return (negative -errno on
+ * failure), so isatty never writes errno: POSIX permits an errno set on
+ * a 0 return but does not require one, and skipping the write keeps the
+ * probe free of thread-local side effects.
+ */
+int
+isatty(int fildes)
+{
+    struct termios t;
+
+    return __syscall3(SYS_ioctl, fildes, 0x5401 /* TCGETS */, (long)&t) == 0 ? 1 : 0;
+}