setsockopt MCAST_JOIN_GROUP calls (et cetera) fail with EINVAL when my 32-bit 
binary talks to a 64-bit kernel. this is because struct group_req isn't packed.

/tmp$ cat mc.cpp
#include <errno.h>
#include <inttypes.h>
#include <signal.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>

int main() {
 struct sockaddr_in addr;
 memset (&addr, 0, sizeof(addr));
 addr.sin_family = AF_INET;
 addr.sin_addr.s_addr = inet_addr ("239.192.0.1");

 int recv_sock = socket (AF_INET, SOCK_DGRAM, IPPROTO_UDP);
 if (-1 == recv_sock) { perror("socket failed"); return 1; }
 struct sockaddr_in recv_addr;
 memcpy (&recv_addr, &addr, sizeof(addr));
 recv_addr.sin_port = 7500;
 socklen_t l = sizeof(recv_addr);
 if (0 != bind (recv_sock, (struct sockaddr*)&recv_addr, l)) {
perror("bind failed"); return 1; }

 struct group_req gr;
 memset (&gr, 0, sizeof(gr));
 printf("sizeof(group_req)=%i\n", sizeof(group_req));
 printf("offsetof(group_req, gr_interface)=%i\n", offsetof(group_req,
gr_interface));
 printf("offsetof(group_req, gr_group)=%i\n", offsetof(group_req, gr_group));
 sockaddr_storage ss;
 printf("sizeof(ss.ss_family)=%i\n", sizeof(ss.ss_family));

 ((struct sockaddr*)&gr.gr_group)->sa_family = addr.sin_family;
 ((struct sockaddr_in*)&gr.gr_group)->sin_addr.s_addr = addr.sin_addr.s_addr;
 if (0 != setsockopt (recv_sock, SOL_IP, MCAST_JOIN_GROUP, (const
char*)&gr, sizeof(gr))) { perror("setsockopt failed"); return 1; }

 return 0;
}
/tmp$ g++ -m64    mc.cpp   -o mc && ./mc
sizeof(group_req)=136
offsetof(group_req, gr_interface)=0
offsetof(group_req, gr_group)=8
sizeof(ss.ss_family)=2
/tmp$ g++ -m32    mc.cpp   -o mc && ./mc
sizeof(group_req)=132
offsetof(group_req, gr_interface)=0
offsetof(group_req, gr_group)=4
sizeof(ss.ss_family)=2
setsockopt failed: Invalid argument
/tmp$

i've worked around this by changing my code to recognize EINVAL and try again 
with its own fake struct instead

 struct group_req64 {
   uint32_t gr_interface;
   uint32_t my_padding;
   sockaddr_storage gr_group;
 };

that works, but i wonder if glibc should be doing this for me? i know it does 
stuff with statfs/statfs64 and suchlike.