diff --git a/libc/stdio/stdin.c b/libc/stdio/stdin.c
index 2407187fe..e90b18012 100644
--- a/libc/stdio/stdin.c
+++ b/libc/stdio/stdin.c
@@ -17,10 +17,12 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/calls.h"
+#include "libc/calls/struct/stat.h"
 #include "libc/stdio/internal.h"
 #include "libc/stdio/stdio.h"
 #include "libc/sysv/consts/fileno.h"
 #include "libc/sysv/consts/o.h"
+#include "libc/sysv/consts/s.h"
 #include "libc/thread/thread.h"
 
 /**
@@ -31,11 +33,14 @@ FILE *stdin;
 static FILE __stdin;
 
 __attribute__((__constructor__(60))) static textstartup void initin(void) {
+  struct stat st;
   stdin = &__stdin;
   stdin->fd = STDIN_FILENO;
   stdin->iomode = O_RDONLY;
   stdin->buf = stdin->mem;
   stdin->size = sizeof(stdin->mem);
   stdin->lock._type = PTHREAD_MUTEX_RECURSIVE;
+  if (fstat(STDIN_FILENO, &st) || !S_ISREG(st.st_mode))
+    stdin->bufmode = _IONBF;
   __fflush_register(stdin);
 }