WebAssembly · alexcrichton · Jul 31, 2026
diff --git a/CMakeLists.txt b/CMakeLists.txt
@@ -204,6 +204,17 @@ add_link_options(
   -resource-dir ${tmp_resource_dir}
 )
 
+if (HAVE_WASM_LIBCALL_THREAD_CONTEXT)
+  # TODO: shouldn't have to pass these as it means it's required by all end
+  # users as well. Ideally this would be part of the driver in rustc/clang, but
+  # that's left for a future change.
+  add_link_options(
+    -Wl,--export-if-defined=__wasm_init_tls
+    -Wl,--export-if-defined=__tls_size
+    -Wl,--export-if-defined=__tls_align
+  )
+endif()
+
 # Expose the public headers to the implementation. We use `-isystem` for
 # purpose for two reasons:
 #

diff --git a/cmake/scripts/run-check-symbols.cmake b/cmake/scripts/run-check-symbols.cmake
@@ -98,7 +98,7 @@ foreach(symbol IN LISTS final_undefined_symbols)
      NOT symbol MATCHES "__stack_pointer" AND
      NOT symbol MATCHES "__init_stack_pointer" AND
      NOT symbol MATCHES "__wasm_component_model_builtin.*" AND
-     NOT symbol STREQUAL "__tls_base")
+     NOT symbol MATCHES "^__tls_(base|size|align)$")
     file(APPEND ${out_undefined_symbols} "${symbol}\n")
   endif()
 endforeach()

diff --git a/expected/wasm32-wasip1-threads/include-all.c b/expected/wasm32-wasip1-threads/include-all.c
@@ -1,4 +1,3 @@
-#include <__errno.h>
 #include <__errno_values.h>
 #include <__fd_set.h>
 #include <__function___isatty.h>

diff --git a/expected/wasm32-wasip1-threads/predefined-macros.txt b/expected/wasm32-wasip1-threads/predefined-macros.txt
@@ -3105,7 +3105,6 @@
 #define __wasi_libc_h 
 #define __wasi_libc_nocwd_h 
 #define __wasi_wasip1_h 
-#define __wasilibc___errno_h 
 #define __wasilibc___errno_values_h 
 #define __wasilibc___fd_set_h 
 #define __wasilibc___function___isatty_h 

diff --git a/expected/wasm32-wasip1-threads/undefined-symbols.txt b/expected/wasm32-wasip1-threads/undefined-symbols.txt
@@ -64,8 +64,6 @@ __letf2
 __lttf2
 __netf2
 __subtf3
-__tls_align
-__tls_size
 __trunctfdf2
 __trunctfsf2
 __unordtf2

diff --git a/expected/wasm32-wasip1/include-all.c b/expected/wasm32-wasip1/include-all.c
@@ -1,4 +1,3 @@
-#include <__errno.h>
 #include <__errno_values.h>
 #include <__fd_set.h>
 #include <__function___isatty.h>

diff --git a/expected/wasm32-wasip1/predefined-macros.txt b/expected/wasm32-wasip1/predefined-macros.txt
@@ -3104,7 +3104,6 @@
 #define __wasi_libc_h 
 #define __wasi_libc_nocwd_h 
 #define __wasi_wasip1_h 
-#define __wasilibc___errno_h 
 #define __wasilibc___errno_values_h 
 #define __wasilibc___fd_set_h 
 #define __wasilibc___function___isatty_h 

diff --git a/expected/wasm32-wasip2/include-all.c b/expected/wasm32-wasip2/include-all.c
@@ -1,4 +1,3 @@
-#include <__errno.h>
 #include <__errno_values.h>
 #include <__fd_set.h>
 #include <__function___isatty.h>

diff --git a/expected/wasm32-wasip2/predefined-macros.txt b/expected/wasm32-wasip2/predefined-macros.txt
@@ -3102,7 +3102,6 @@
 #define __wasi_libc_h 
 #define __wasi_libc_nocwd_h 
 #define __wasi_wasip1_h 
-#define __wasilibc___errno_h 
 #define __wasilibc___errno_values_h 
 #define __wasilibc___fd_set_h 
 #define __wasilibc___function___isatty_h 

diff --git a/expected/wasm32-wasip3-coop/defined-symbols.txt b/expected/wasm32-wasip3-coop/defined-symbols.txt
@@ -90,6 +90,7 @@ __getdelim
 __getentropy
 __getopt_msg
 __gmtime_r
+__h_errno_location
 __hashmap_bucket_item
 __hashmap_clear
 __hashmap_count
@@ -352,7 +353,6 @@ __wasilibc_get_service_entry_by_name
 __wasilibc_get_service_entry_by_port
 __wasilibc_getsockopt_timeout
 __wasilibc_iftodt
-__wasilibc_init_async_task
 __wasilibc_init_stdio
 __wasilibc_initialize_environ
 __wasilibc_link
@@ -387,6 +387,7 @@ __wasilibc_rename_newat
 __wasilibc_rename_oldat
 __wasilibc_reset_preopens
 __wasilibc_rmdirat
+__wasilibc_set_task_tls
 __wasilibc_setsockopt_timeout
 __wasilibc_sockaddr_to_wasi
 __wasilibc_sockaddr_validate

diff --git a/expected/wasm32-wasip3-coop/include-all.c b/expected/wasm32-wasip3-coop/include-all.c
@@ -1,4 +1,3 @@
-#include <__errno.h>
 #include <__errno_values.h>
 #include <__fd_set.h>
 #include <__function___isatty.h>

diff --git a/expected/wasm32-wasip3-coop/predefined-macros.txt b/expected/wasm32-wasip3-coop/predefined-macros.txt
@@ -3109,7 +3109,6 @@
 #define __wasi_libc_h 
 #define __wasi_libc_nocwd_h 
 #define __wasi_wasip1_h 
-#define __wasilibc___errno_h 
 #define __wasilibc___errno_values_h 
 #define __wasilibc___fd_set_h 
 #define __wasilibc___function___isatty_h 
@@ -3227,7 +3226,7 @@
 #define dirent64 dirent
 #define erf(x) __tg_real(erf, (x))
 #define erfc(x) __tg_real(erfc, (x))
-#define errno errno
+#define errno (*__errno_location())
 #define exp(x) __tg_real_complex(exp, (x))
 #define exp2(x) __tg_real(exp2, (x))
 #define expm1(x) __tg_real(expm1, (x))
@@ -3259,7 +3258,7 @@
 #define glob64_t glob_t
 #define globfree64 globfree
 #define h_addr h_addr_list[0]
-#define h_errno h_errno
+#define h_errno (*__h_errno_location())
 #define howmany(n,d) (((n)+((d)-1))/(d))
 #define htobe16(x) __bswap16(x)
 #define htobe32(x) __bswap32(x)

diff --git a/expected/wasm32-wasip3-coop/undefined-symbols.txt b/expected/wasm32-wasip3-coop/undefined-symbols.txt
@@ -27,8 +27,6 @@ __subtask_drop
 __subtf3
 __task_cancel
 __thread_yield
-__tls_align
-__tls_size
 __trunctfdf2
 __trunctfsf2
 __unordtf2
@@ -215,6 +213,8 @@ stream_drop_writable_unit
 stream_new_unit
 stream_read_unit
 stream_write_unit
+wasip3_context_get_1
+wasip3_context_set_1
 wasip3_thread_index
 wasip3_thread_new_indirect
 wasip3_thread_resume_later

diff --git a/expected/wasm32-wasip3/defined-symbols.txt b/expected/wasm32-wasip3/defined-symbols.txt
@@ -89,6 +89,7 @@ __getdelim
 __getentropy
 __getopt_msg
 __gmtime_r
+__h_errno_location
 __hwcap
 __inet_aton
 __inhibit_ptc

diff --git a/expected/wasm32-wasip3/include-all.c b/expected/wasm32-wasip3/include-all.c
@@ -1,4 +1,3 @@
-#include <__errno.h>
 #include <__errno_values.h>
 #include <__fd_set.h>
 #include <__function___isatty.h>

diff --git a/expected/wasm32-wasip3/predefined-macros.txt b/expected/wasm32-wasip3/predefined-macros.txt
@@ -3108,7 +3108,6 @@
 #define __wasi_libc_h 
 #define __wasi_libc_nocwd_h 
 #define __wasi_wasip1_h 
-#define __wasilibc___errno_h 
 #define __wasilibc___errno_values_h 
 #define __wasilibc___fd_set_h 
 #define __wasilibc___function___isatty_h 
@@ -3226,7 +3225,7 @@
 #define dirent64 dirent
 #define erf(x) __tg_real(erf, (x))
 #define erfc(x) __tg_real(erfc, (x))
-#define errno errno
+#define errno (*__errno_location())
 #define exp(x) __tg_real_complex(exp, (x))
 #define exp2(x) __tg_real(exp2, (x))
 #define expm1(x) __tg_real(expm1, (x))
@@ -3258,7 +3257,7 @@
 #define glob64_t glob_t
 #define globfree64 globfree
 #define h_addr h_addr_list[0]
-#define h_errno h_errno
+#define h_errno (*__h_errno_location())
 #define howmany(n,d) (((n)+((d)-1))/(d))
 #define htobe16(x) __bswap16(x)
 #define htobe32(x) __bswap32(x)

diff --git a/libc-bottom-half/CMakeLists.txt b/libc-bottom-half/CMakeLists.txt
@@ -185,7 +185,7 @@ endif()
 
 if (HAVE_WASM_LIBCALL_THREAD_CONTEXT)
   list(APPEND bottom_half_sources
-    sources/wasip3_non_coop_tls_base.S
+    sources/wasip3_tls.c
     sources/__wasm_init_task.S
   )
 endif()

diff --git a/libc-bottom-half/cloudlibc/src/libc/errno/errno.c b/libc-bottom-half/cloudlibc/src/libc/errno/errno.c
@@ -86,4 +86,7 @@ static_assert(ETXTBSY == __WASI_ERRNO_TXTBSY, "Value mismatch");
 static_assert(EXDEV == __WASI_ERRNO_XDEV, "Value mismatch");
 #endif
 
+// `<errno.h>` may define `errno` as a call to `__errno_location`, so undo that
+// here where the thread-local it ultimately refers to is defined.
+#undef errno
 thread_local int errno = 0;
diff --git a/libc-bottom-half/crt/wasip3_symbol_references.h b/libc-bottom-half/crt/wasip3_symbol_references.h
@@ -16,10 +16,9 @@ __attribute__((used)) static void *__wasm_init_async_task_ref =
 __attribute__((used)) static void *cabi_realloc_ref = cabi_realloc;
 
 // Force `__wasm_{g,s}et_{stack_pointer,tls_base}` to exist as defined symbols.
-// These end up as imported functions which `wit-component` recognizes. Note
-// that for coop threads the tls_base symbols are defined but without coop
-// threads they're defined within libc itself to access a locally-defined
-// global (see `wasip3_non_coop_tls_base.S`).
+// These end up as imported functions which `wit-component` recognizes, and what
+// exactly they're hooked up to will depend on `wit-component` when this is
+// turned into a component.
 __asm__(
 ".globl      __wasm_get_stack_pointer\n"
 ".type       __wasm_get_stack_pointer,@function\n"
@@ -33,7 +32,6 @@ __asm__(
 ".import_module __wasm_set_stack_pointer, \"env\"\n"
 ".import_name __wasm_set_stack_pointer, \"__wasm_set_stack_pointer\"\n"
 
-#ifdef __wasi_cooperative_threads__
 ".globl      __wasm_get_tls_base\n"
 ".type       __wasm_get_tls_base,@function\n"
 ".functype __wasm_get_tls_base () -> (i32)\n"
@@ -45,8 +43,7 @@ __asm__(
 ".functype __wasm_set_tls_base (i32) -> ()\n"
 ".import_module __wasm_set_tls_base, \"env\"\n"
 ".import_name __wasm_set_tls_base, \"__wasm_set_tls_base\"\n"
-#endif
-);
 
+);
 
 #endif
diff --git a/libc-bottom-half/headers/public/__errno.h b/libc-bottom-half/headers/public/__errno.h
diff --git a/libc-bottom-half/sources/__cabi_realloc_wrapper.S b/libc-bottom-half/sources/__cabi_realloc_wrapper.S
@@ -27,11 +27,23 @@
 
 // For coop-threads builds we need to configure TLS to the initial TLS
 // value for all tasks/etc, so import those symbols here.
+//
+// Note that this saves and restores context slot 1 itself rather than this
+// module's TLS base. When several modules are linked together the slot holds an
+// array of per-module base pointers, so clobbering just libc's entry would both
+// leave the other modules pointing at the wrong TLS and scribble through
+// whatever stale pointer the slot happened to contain.
 #ifdef __wasi_cooperative_threads__
-.functype   __wasm_set_tls_base (i32) -> ()
-.functype   __wasm_get_tls_base () -> (i32)
-.type       __wasm_set_tls_base,@function
-.type       __wasm_get_tls_base,@function
+.functype   wasip3_context_set_1 (i32) -> ()
+.functype   wasip3_context_get_1 () -> (i32)
+.functype   __wasilibc_set_task_tls (i32) -> ()
+.type       wasip3_context_set_1,@function
+.type       wasip3_context_get_1,@function
+.type       __wasilibc_set_task_tls,@function
+.import_module wasip3_context_get_1, "$root"
+.import_name wasip3_context_get_1, "[context-get-1]"
+.import_module wasip3_context_set_1, "$root"
+.import_name wasip3_context_set_1, "[context-set-1]"
 .type       __init_tls_base,@global
 #ifdef __PIC__
 .globaltype __init_tls_base, i32
@@ -68,7 +80,7 @@ cabi_realloc:
   call __wasm_get_stack_pointer
   local.set 4
 #ifdef __wasi_cooperative_threads__
-  call __wasm_get_tls_base
+  call wasip3_context_get_1
   local.set 5
 #endif
 
@@ -85,7 +97,7 @@ cabi_realloc:
   // Next up configure TLS. This is only required in coop-threads builds.
 #ifdef __wasi_cooperative_threads__
   global.get   __init_tls_base
-  call __wasm_set_tls_base
+  call __wasilibc_set_task_tls
 #endif
 
   // And finally forward to the actual implementation of `cabi_realloc`
@@ -100,7 +112,7 @@ cabi_realloc:
   call __wasm_set_stack_pointer
 #ifdef __wasi_cooperative_threads__
   local.get 5
-  call __wasm_set_tls_base
+  call wasip3_context_set_1
 #endif
 
   end_function

diff --git a/libc-bottom-half/sources/__errno_location.c b/libc-bottom-half/sources/__errno_location.c
@@ -1,3 +1,8 @@
 #include <errno.h>
 
+// `<errno.h>` may define `errno` as a call to this function, so reach the
+// underlying thread-local directly here to avoid recursing into ourselves.
+#undef errno
+extern _Thread_local int errno;
+
 int *__errno_location(void) { return &errno; }
diff --git a/libc-bottom-half/sources/__wasm_init_task.S b/libc-bottom-half/sources/__wasm_init_task.S
@@ -12,7 +12,7 @@
 #else
 .globaltype __init_tls_base, i32, immutable
 #endif
-.functype   __wasm_set_tls_base (i32) -> ()
+.functype   __wasilibc_set_task_tls (i32) -> ()
 #endif
 
 
@@ -30,10 +30,10 @@
 // the same TLS values are used across all tasks, which seem reasonable-enough
 // for now.
 //
-// Note that configuring TLS is only required in coop threads builds since when
-// coop-threads are disabled TLS is routed through a global in wasi-libc which
-// needs no extra handling (unlike coop threads where TLS routes through
-// `context.{get,set}` which needs setting up).
+// Note that configuring TLS is only required in coop threads builds. When
+// threads are disabled TLS is handled by `wit-component`'s synthesis of
+// functions. With coop threads though delegation goes into C in wasi-libc
+// itself to do a bit of detection logic.
 .globl  __wasm_init_task
 .export_name  __wasm_init_task, __wasm_init_task
 .type  __wasm_init_task,@function
@@ -45,7 +45,7 @@ __wasm_init_task:
 
 #ifdef __wasi_cooperative_threads__
   global.get   __init_tls_base
-  call __wasm_set_tls_base
+  call __wasilibc_set_task_tls
 #endif
   end_function
 

diff --git a/libc-bottom-half/sources/isatty.c b/libc-bottom-half/sources/isatty.c
@@ -1,4 +1,3 @@
-#include <__errno.h>
 #include <__function___isatty.h>
 #include <errno.h>
 #include <features.h>

diff --git a/libc-bottom-half/sources/netdb.c b/libc-bottom-half/sources/netdb.c
@@ -21,8 +21,15 @@ static_assert(SOCK_STREAM == __WASI_FILETYPE_SOCKET_STREAM, "value mismatch");
   NETWORK_ERROR_CODE_PERMANENT_RESOLVER_FAILURE
 #endif
 
+// `<netdb.h>` may define `h_errno` as a call to `__h_errno_location`, so undo
+// that here where the thread-local it ultimately refers to is defined.
+#undef h_errno
 _Thread_local int h_errno = 0;
 
+#if !defined(__wasip1__) && !defined(__wasip2__)
+int *__h_errno_location(void) { return &h_errno; }
+#endif
+
 static struct servent global_serv = {0};
 
 static int map_error(ip_name_lookup_error_code_t *error) {