WIP: Use a more type safe alternative for sigaction types

libc-test/build.rs

@@ -349,6 +349,13 @@ fn test_apple(target: &str) {
         }
     });
 
+    cfg.skip_union(move |u| {
+        match u.ident() {
+            "sighandler_t" => true,
+            _ => false,
+        }
+    });
+
     cfg.skip_const(move |constant| {
         match constant.ident() {
             // They're declared via `deprecated_mach` and we don't support it anymore.
@@ -368,6 +375,8 @@ fn test_apple(target: &str) {
             // FIXME(macos): bumped up on macOS 26, it's sizeof `vm_statistics64_data_t`
             "HOST_VM_INFO64_COUNT" => true,
 
+            "SIG_DFL" | "SIG_IGN" | "SIG_ERR" => true,
+
             _ => false,
         }
     });
@@ -426,9 +435,6 @@ fn test_apple(target: &str) {
             .then_some(ty.to_string())
     });
 
-    // OSX calls this something else
-    cfg.rename_type(|ty| (ty == "sighandler_t").then_some("sig_t".to_string()));
-
     cfg.rename_struct_ty(|ty| ty.ends_with("_t").then_some(ty.to_string()));
     cfg.rename_union_ty(|ty| ty.ends_with("_t").then_some(ty.to_string()));
 

src/unix/bsd/apple/mod.rs

@@ -347,8 +347,8 @@ s! {
         _pad: Padding<[usize; 9]>,
     }
 
+    #[allow(unpredictable_function_pointer_comparisons)]
     pub struct sigaction {
-        // FIXME(union): this field is actually a union
         pub sa_sigaction: crate::sighandler_t,
         pub sa_mask: sigset_t,
         pub sa_flags: c_int,

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

@@ -129,6 +129,13 @@ impl siginfo_t {
     }
 }
 
+s_no_extra_traits! {
+    pub union sa_union_t {
+        pub sa_handler: crate::sighandler_t,
+        pub sa_sigaction: extern "C" fn(c_int, *mut crate::siginfo_t, *mut c_void),
+    }
+}
+
 s! {
     pub struct aiocb {
         pub aio_fildes: c_int,
@@ -161,7 +168,7 @@ s! {
     // FIXME(1.0): This should not implement `PartialEq`
     #[allow(unpredictable_function_pointer_comparisons)]
     pub struct sigaction {
-        pub sa_sigaction: crate::sighandler_t,
+        pub sa_sigaction: sa_union_t,
         pub sa_mask: crate::sigset_t,
         pub sa_flags: c_int,
         pub sa_restorer: Option<extern "C" fn()>,

src/unix/mod.rs

@@ -17,9 +17,19 @@ pub type ssize_t = isize;
 pub type pid_t = i32;
 pub type in_addr_t = u32;
 pub type in_port_t = u16;
-pub type sighandler_t = size_t;
 pub type cc_t = c_uchar;
 
+// All derivatives use sighandler_t to mean something which is either a
+// function pointer or a size_t. And this has implications for its size
+// (especially on CHERI architectures) compared to just blindly using size_t to
+// represent it.
+s_no_extra_traits! {
+    pub union sighandler_t {
+        pub sighandler_id: size_t,
+        pub sighandler_fn: extern "C" fn(c_int),
+    }
+}
+
 cfg_if! {
     if #[cfg(any(
         target_os = "espidf",
@@ -229,9 +239,9 @@ s! {
 pub const INT_MIN: c_int = -2147483648;
 pub const INT_MAX: c_int = 2147483647;
 
-pub const SIG_DFL: sighandler_t = 0 as sighandler_t;
-pub const SIG_IGN: sighandler_t = 1 as sighandler_t;
-pub const SIG_ERR: sighandler_t = !0 as sighandler_t;
+pub const SIG_DFL: size_t = 0 as size_t;
+pub const SIG_IGN: size_t = 1 as size_t;
+pub const SIG_ERR: size_t = !0 as size_t;
 
 cfg_if! {
     if #[cfg(all(