diff --git a/changelog.d/9776-pthread-extern-signature-clash.md b/changelog.d/9776-pthread-extern-signature-clash.md
new file mode 100644
index 0000000000..523fe121e2
--- /dev/null
+++ b/changelog.d/9776-pthread-extern-signature-clash.md
@@ -0,0 +1,18 @@
+**`perry-runtime` compiles again under `-D warnings` on linux-x86_64.** The crate
+declared `pthread_getattr_np`, `pthread_attr_getstack` and `pthread_attr_destroy`
+in three separate `extern "C"` blocks using two different spellings of the
+`pthread_attr_t` buffer — `[u64; 8]` in `gc::roots::get_stack_bottom`, `*mut u8`
+in `gc::roots::stack_maps::fp_chain` and in the Error-stack frame walk. Declaring
+one symbol twice in a crate with different signatures is
+`clashing_extern_declarations`, which `-D warnings` denies, and the crate stopped
+building outright (#9486's frame walk supplied the third copy).
+
+The `*mut u8` spelling was not itself new, but its only previous home was
+`fp_chain`, which is gated to `target_arch = "aarch64"`; on an x86_64 host that
+module is compiled out and the two spellings never met.
+
+All three blocks now use `[u64; 8]`, the spelling the collector has always used.
+That also clears the latent aarch64-linux collision between `fp_chain` and
+`gc::roots`, and replaces a `[u8; 128]` buffer — 128 bytes but aligned to 1 —
+with one that is both large enough for `pthread_attr_t` on every supported
+glibc/musl target (56 bytes on both) and correctly aligned for it.
diff --git a/crates/perry-runtime/src/error_stack_frames.rs b/crates/perry-runtime/src/error_stack_frames.rs
index 5f7c03693e..49e8162756 100644
--- a/crates/perry-runtime/src/error_stack_frames.rs
+++ b/crates/perry-runtime/src/error_stack_frames.rs
@@ -195,25 +195,33 @@ mod walk {
 
     #[cfg(all(target_os = "linux", not(target_vendor = "apple")))]
     fn stack_top_uncached() -> usize {
+        // These signatures must stay identical to the ones in
+        // `gc::roots::get_stack_bottom` and `gc::roots::stack_maps::fp_chain`.
+        // Two `extern "C"` blocks in one crate that declare the same symbol
+        // with different argument types are a hard error under `-D warnings`
+        // (`clashing_extern_declarations`), and `[u64; 8]` — 64 bytes, which
+        // covers `pthread_attr_t` on every supported glibc/musl target — is
+        // the spelling the collector has always used. Unlike a `[u8; N]`
+        // buffer it is also correctly aligned for the attr union.
         unsafe extern "C" {
             fn pthread_self() -> usize;
-            fn pthread_getattr_np(thread: usize, attr: *mut u8) -> i32;
+            fn pthread_getattr_np(thread: usize, attr: *mut [u64; 8]) -> i32;
             fn pthread_attr_getstack(
-                attr: *const u8,
-                stackaddr: *mut *mut core::ffi::c_void,
+                attr: *const [u64; 8],
+                stackaddr: *mut *mut u8,
                 stacksize: *mut usize,
             ) -> i32;
-            fn pthread_attr_destroy(attr: *mut u8) -> i32;
+            fn pthread_attr_destroy(attr: *mut [u64; 8]) -> i32;
         }
-        let mut attr = [0u8; 128];
-        let mut addr: *mut core::ffi::c_void = core::ptr::null_mut();
+        let mut attr = [0u64; 8];
+        let mut addr: *mut u8 = core::ptr::null_mut();
         let mut size: usize = 0;
         unsafe {
-            if pthread_getattr_np(pthread_self(), attr.as_mut_ptr()) != 0 {
+            if pthread_getattr_np(pthread_self(), &mut attr) != 0 {
                 return 0;
             }
-            let ok = pthread_attr_getstack(attr.as_ptr(), &mut addr, &mut size) == 0;
-            pthread_attr_destroy(attr.as_mut_ptr());
+            let ok = pthread_attr_getstack(&attr, &mut addr, &mut size) == 0;
+            pthread_attr_destroy(&mut attr);
             if !ok {
                 return 0;
             }
diff --git a/crates/perry-runtime/src/gc/roots/stack_maps.rs b/crates/perry-runtime/src/gc/roots/stack_maps.rs
index 4e52ed3915..b1162161e7 100644
--- a/crates/perry-runtime/src/gc/roots/stack_maps.rs
+++ b/crates/perry-runtime/src/gc/roots/stack_maps.rs
@@ -1786,27 +1786,31 @@ mod fp_chain {
     /// back to the platform unwinder (fail-closed like every other anomaly).
     #[cfg(target_os = "linux")]
     fn stack_top() -> usize {
+        // Identical to `gc::roots::get_stack_bottom` and the Error-stack
+        // capture in `error_stack_frames.rs`: one crate may not declare the
+        // same `extern "C"` symbol with two different signatures
+        // (`clashing_extern_declarations`, denied via `-D warnings`).
         unsafe extern "C" {
             fn pthread_self() -> usize;
-            fn pthread_getattr_np(thread: usize, attr: *mut u8) -> i32;
+            fn pthread_getattr_np(thread: usize, attr: *mut [u64; 8]) -> i32;
             fn pthread_attr_getstack(
-                attr: *const u8,
-                stackaddr: *mut *mut c_void,
+                attr: *const [u64; 8],
+                stackaddr: *mut *mut u8,
                 stacksize: *mut usize,
             ) -> i32;
-            fn pthread_attr_destroy(attr: *mut u8) -> i32;
+            fn pthread_attr_destroy(attr: *mut [u64; 8]) -> i32;
         }
         // pthread_attr_t is at most 64 bytes on glibc/musl for the supported
-        // targets; over-allocate defensively.
-        let mut attr = [0u8; 128];
-        let mut addr: *mut c_void = std::ptr::null_mut();
+        // targets, and `[u64; 8]` is both that size and correctly aligned.
+        let mut attr = [0u64; 8];
+        let mut addr: *mut u8 = std::ptr::null_mut();
         let mut size: usize = 0;
         unsafe {
-            if pthread_getattr_np(pthread_self(), attr.as_mut_ptr()) != 0 {
+            if pthread_getattr_np(pthread_self(), &mut attr) != 0 {
                 return 0;
             }
-            let ok = pthread_attr_getstack(attr.as_ptr(), &mut addr, &mut size) == 0;
-            pthread_attr_destroy(attr.as_mut_ptr());
+            let ok = pthread_attr_getstack(&attr, &mut addr, &mut size) == 0;
+            pthread_attr_destroy(&mut attr);
             if !ok {
                 return 0;
             }