Fix issue 4884, preventing a crash in the socket code when python is compiled

with llvm-gcc and run with a glibc <2.10.

Include/pyport.h

@@ -709,6 +709,15 @@ typedef	struct fd_set {
 #define Py_FORMAT_PARSETUPLE(func,p1,p2)
 #endif
 
+/*
+ * Specify alignment on compilers that support it.
+ */
+#if defined(__GNUC__) && __GNUC__ >= 3
+#define Py_ALIGNED(x) __attribute__((aligned(x)))
+#else
+#define Py_ALIGNED(x)
+#endif
+
 /* Eliminate end-of-loop code not reached warnings from SunPro C
  * when using do{...}while(0) macros
  */

Modules/socketmodule.c

@@ -3334,7 +3334,11 @@ socket_gethostbyaddr(PyObject *self, PyObject *args)
 #ifdef HAVE_GETHOSTBYNAME_R_3_ARG
 	struct hostent_data data;
 #else
-	char buf[16384];
+	/* glibcs up to 2.10 assume that the buf argument to
+	   gethostbyaddr_r is 8-byte aligned, which at least llvm-gcc
+	   does not ensure. The attribute below instructs the compiler
+	   to maintain this alignment. */
+	char buf[16384] Py_ALIGNED(8);
 	int buf_len = (sizeof buf) - 1;
 	int errnop;
 #endif