Use SIGEV_PAD_SIZE in __c_anonymous_sigev_un on Linux

Author: asomers

src/unix/linux_like/mod.rs

@@ -269,10 +269,7 @@ s_no_extra_traits! {
     // Can't correctly impl Debug for unions
     #[allow(missing_debug_implementations)]
     pub union __c_anonymous_sigev_un {
-        #[cfg(target_pointer_width = "64")]
-        _pad: [::c_int; (64 - 2 * 4 - 8) / 4],
-        #[cfg(target_pointer_width = "32")]
-        _pad: [::c_int; (64 - 2 * 4 - 4) / 4],
+        _pad: [::c_int; SIGEV_PAD_SIZE],
         pub _tid: ::c_int,
         pub _sigev_thread: __c_anonymous_sigev_thread,
     }
@@ -576,6 +573,13 @@ pub const SIGPIPE: ::c_int = 13;
 pub const SIGALRM: ::c_int = 14;
 pub const SIGTERM: ::c_int = 15;
 
+const SIGEV_MAX_SIZE: usize = 64;
+#[cfg(target_pointer_width = "64")]
+const __ARCH_SIGEV_PREAMBLE_SIZE: usize = 4 * 2 + 8;
+#[cfg(target_pointer_width = "32")]
+const __ARCH_SIGEV_PREAMBLE_SIZE: usize = 4 * 2 + 4;
+const SIGEV_PAD_SIZE: usize = (SIGEV_MAX_SIZE - __ARCH_SIGEV_PREAMBLE_SIZE) / 4;
+
 pub const PROT_NONE: ::c_int = 0;
 pub const PROT_READ: ::c_int = 1;
 pub const PROT_WRITE: ::c_int = 2;