Use anonymous shared memory on FreeBSD

It works in Capsicum capability mode (sandbox)

src/platform/unix/mod.rs

@@ -947,11 +947,9 @@ fn recv(fd: c_int, blocking_mode: BlockingMode)
     Ok((main_data_buffer, channels, shared_memory_regions))
 }
 
-#[cfg(not(all(target_os="linux", feature="memfd")))]
+#[cfg(not(any(target_os="freebsd", all(target_os="linux", feature="memfd"))))]
 fn create_shmem(name: CString, length: usize) -> c_int {
     unsafe {
-        // NB: the FreeBSD man page for shm_unlink states that it requires
-        // write permissions, but testing shows that read-write is required.
         let fd = libc::shm_open(name.as_ptr(),
                                 libc::O_CREAT | libc::O_RDWR | libc::O_EXCL,
                                 0o600);
@@ -962,6 +960,20 @@ fn create_shmem(name: CString, length: usize) -> c_int {
     }
 }
 
+#[cfg(target_os="freebsd")]
+fn create_shmem(_name: CString, length: usize) -> c_int {
+    unsafe {
+        // NB: the FreeBSD man page for shm_unlink states that it requires
+        // write permissions, but testing shows that read-write is required.
+        let fd = libc::shm_open(1 as *mut i8, // SHM_ANON
+                                libc::O_CREAT | libc::O_RDWR | libc::O_EXCL,
+                                0o600);
+        assert!(fd >= 0);
+        assert!(libc::ftruncate(fd, length as off_t) == 0);
+        fd
+    }
+}
+
 #[cfg(all(feature="memfd", target_os="linux"))]
 fn create_shmem(name: CString, length: usize) -> c_int {
     unsafe {