Auto merge of #3183 - ChrisDenton:align-stack-buffer, r=<try>

Use aligned `cmsghdr` structs `test_cmsg_nxthdr`

Fixes #3181.

I could find no reason for using unaligned structs in this test.

Author: bors

libc-test/test/cmsg.rs

@@ -11,8 +11,6 @@ mod t {
 
     extern "C" {
         pub fn cmsg_firsthdr(msgh: *const msghdr) -> *mut cmsghdr;
-        // see below
-        #[cfg(not(target_arch = "sparc64"))]
         pub fn cmsg_nxthdr(mhdr: *const msghdr, cmsg: *const cmsghdr) -> *mut cmsghdr;
         pub fn cmsg_space(length: c_uint) -> usize;
         pub fn cmsg_len(length: c_uint) -> usize;
@@ -51,28 +49,24 @@ mod t {
         }
     }
 
-    // Skip on sparc64
-    // https://github.com/rust-lang/libc/issues/1239
-    #[cfg(not(target_arch = "sparc64"))]
     #[test]
-    // FIXME: This triggers alignment checks for pointer dereferences:
-    // https://github.com/rust-lang/libc/issues/3181
-    #[ignore]
     fn test_cmsg_nxthdr() {
         use std::ptr;
+        // Stack alignment helper.
+        // Note: on Sparc32, cmsghdr is 8 byte aligned.
+        #[repr(align(8))]
+        struct Align8<T>(T);
 
-        let mut buffer = [0u8; 256];
+        let mut buffer = Align8([0_u8; 260]);
         let mut mhdr: msghdr = unsafe { mem::zeroed() };
         let pmhdr = &mhdr as *const msghdr;
         for start_ofs in 0..64 {
-            let pcmsghdr = &mut buffer[start_ofs] as *mut u8 as *mut cmsghdr;
+            let pcmsghdr = &mut buffer.0 as *mut u8 as *mut cmsghdr;
             mhdr.msg_control = pcmsghdr as *mut c_void;
             mhdr.msg_controllen = (160 - start_ofs) as _;
             for cmsg_len in 0..64 {
                 for next_cmsg_len in 0..32 {
-                    for i in buffer[start_ofs..].iter_mut() {
-                        *i = 0;
-                    }
+                    buffer.0.fill(0);
                     unsafe {
                         (*pcmsghdr).cmsg_len = cmsg_len;
                         let libc_next = libc::CMSG_NXTHDR(pmhdr, pcmsghdr);