Alignment issue with zmq_msg_t on SPARC CPU

[baryonix]

One of the fields of zmq::msg_t is a pointer, which in some environments (in our case, this is a SPARC CPU in 64-bit mode) requires that an instance of the class be aligned to an 8-byte boundary in memory. SPARC is very picky about alignment - you will get a SIGBUS if this is not observed. Many other RISC CPUs behave similarly.

However the zmq_msg_t type which is used as a placeholder to contain a zmq::msg_t instance in the C API, only contains a char array, which will make the compiler apply a less strict alignment requirement. This results in several test cases failing with SIGBUS.

This could be fixed by turning zmq_msg_t into a union, having a pointer as one of its fields, which will force the compiler to apply correct alignment when allocating a zmq_msg_t.

--- include/zmq.h.orig  Di. Jan 27 14:43:30 2015
+++ include/zmq.h       Di. Jan 27 14:44:29 2015
@@ -199,7 +199,7 @@
 /*  0MQ message definition.                                                   */
 /******************************************************************************/

-typedef struct zmq_msg_t {unsigned char _ [32];} zmq_msg_t;
+typedef union zmq_msg_t {unsigned char _ [32]; void *p;} zmq_msg_t;

 typedef void (zmq_free_fn) (void *data, void *hint);

[rodgert]

Scratch my previous comment, there should be no chance of false sharing for a zmq_msg_t if used correctly.