Expose si_pid, si_uid, and si_status from siginfo_t as functions

On Linux, siginfo_t cannot expose these fields directly due to
https://github.com/rust-lang/libc/issues/716 , so expose them as
functions, just like si_addr and si_value.

In order to get alignment correct on both 32-bit and 64-bit
architectures, define an sifields union that includes a pointer field,
to ensure that it has the same alignment as a pointer.

src/unix/bsd/apple/mod.rs

@@ -680,6 +680,14 @@ impl siginfo_t {
 
         (*(self as *const siginfo_t as *const siginfo_timer)).si_value
     }
+
+    pub unsafe fn si_pid(&self) -> ::pid_t {
+        self.si_pid
+    }
+
+    pub unsafe fn si_uid(&self) -> ::uid_t {
+        self.si_uid
+    }
 }
 
 cfg_if! {

src/unix/bsd/freebsdlike/mod.rs

@@ -31,6 +31,14 @@ impl siginfo_t {
     pub unsafe fn si_value(&self) -> ::sigval {
         self.si_value
     }
+
+    pub unsafe fn si_pid(&self) -> ::pid_t {
+        self.si_pid
+    }
+
+    pub unsafe fn si_uid(&self) -> ::uid_t {
+        self.si_uid
+    }
 }
 
 s! {

src/unix/haiku/mod.rs

@@ -38,6 +38,20 @@ impl ::Clone for timezone {
     }
 }
 
+impl siginfo_t {
+    pub unsafe fn si_addr(&self) -> *mut ::c_void {
+        self.si_addr
+    }
+
+    pub unsafe fn si_pid(&self) -> ::pid_t {
+        self.si_pid
+    }
+
+    pub unsafe fn si_uid(&self) -> ::uid_t {
+        self.si_uid
+    }
+}
+
 s! {
     pub struct in_addr {
         pub s_addr: ::in_addr_t,

src/unix/linux_like/linux/gnu/mod.rs

@@ -313,6 +313,55 @@ impl siginfo_t {
     }
 }
 
+cfg_if! {
+    if #[cfg(libc_union)] {
+        // Internal, for casts to access union fields
+        #[repr(C)]
+        #[derive(Copy,Clone)]
+        struct sifields_sigchld {
+            si_pid: ::pid_t,
+            si_uid: ::uid_t,
+            si_status: ::c_int,
+        }
+
+        // Internal, for casts to access union fields
+        #[repr(C)]
+        union sifields {
+            _align_pointer: *mut ::c_void,
+            sigchld: sifields_sigchld,
+        }
+
+        // Internal, for casts to access union fields. Note that some variants
+        // of sifields start with a pointer, which makes the alignment of
+        // sifields vary on 32-bit and 64-bit architectures.
+        #[repr(C)]
+        struct siginfo_f {
+            _si_signo: ::c_int,
+            _si_errno: ::c_int,
+            _si_code: ::c_int,
+            sifields: sifields,
+        }
+
+        impl siginfo_t {
+            unsafe fn sifields(&self) -> &sifields {
+                &(*(self as *const siginfo_t as *const siginfo_f)).sifields
+            }
+
+            pub unsafe fn si_pid(&self) -> ::pid_t {
+                self.sifields().sigchld.si_pid
+            }
+
+            pub unsafe fn si_uid(&self) -> ::uid_t {
+                self.sifields().sigchld.si_uid
+            }
+
+            pub unsafe fn si_status(&self) -> ::c_int {
+                self.sifields().sigchld.si_status
+            }
+        }
+    }
+}
+
 s_no_extra_traits! {
     pub struct utmpx {
         pub ut_type: ::c_short,

src/vxworks/mod.rs

@@ -112,6 +112,24 @@ impl ::Clone for _Vx_semaphore {
     }
 }
 
+impl siginfo_t {
+    pub unsafe fn si_addr(&self) -> *mut ::c_void {
+        self.si_addr
+    }
+
+    pub unsafe fn si_value(&self) -> ::sigval {
+        self.si_value
+    }
+
+    pub unsafe fn si_pid(&self) -> ::pid_t {
+        self.si_pid
+    }
+
+    pub unsafe fn si_uid(&self) -> ::uid_t {
+        self.si_uid
+    }
+}
+
 s! {
     // b_pthread_condattr_t.h
     pub struct pthread_condattr_t {