diff --git a/extern/p.c b/extern/p.c
new file mode 100644
index 0000000..9a3a353
--- /dev/null
+++ b/extern/p.c
@@ -0,0 +1,12 @@
+// gcc p.c q.c -lm でビルド
+// ./a.out で実行
+
+#include "q.h"
+#include <stdio.h>
+
+int main(){
+  float d = 1.23;
+  int id = q(d);
+  printf("floor(d) = %d\n", id);
+  return 0;
+}
diff --git a/extern/q.c b/extern/q.c
new file mode 100644
index 0000000..706d771
--- /dev/null
+++ b/extern/q.c
@@ -0,0 +1,8 @@
+#include <stdio.h>
+#include <math.h>
+
+int q(float x)
+{
+  printf("d = %f\n", x*2);
+  return floor(x*2); 
+}
diff --git a/extern/q.h b/extern/q.h
new file mode 100644
index 0000000..771b0f1
--- /dev/null
+++ b/extern/q.h
@@ -0,0 +1,5 @@
+// q.h
+
+extern int q(float);
+
+