Fix Android aarch64 build: expose posix_spawn via bionic weak symbols

The previous API-28 fix (c6bb70d) was a no-op on the current
dockcross-android-arm64 image and the Android build still failed compiling
mtmd-helper.cpp (vendor/sheredom/subprocess.h -> posix_spawn*).

Root cause: that image is NOT the Google NDK CMake toolchain but a
Debian-style cross-clang (/usr/aarch64-linux-android/bin/clang). It never sets
ANDROID/ANDROID_ABI, so the if(ANDROID_ABI)-guarded
add_compile_definitions(__ANDROID_API__=28) never ran; and it ignores
-DANDROID_PLATFORM=android-28 (CMake flags it as an unused variable). bionic
gates posix_spawn behind __BIONIC_AVAILABILITY_GUARD(28), which tests
__ANDROID_MIN_SDK_VERSION__ (predefined by clang from the target triple) — not
__ANDROID_API__ — so even applying that define would not have helped.

Fix: define __ANDROID_UNAVAILABLE_SYMBOLS_ARE_WEAK__ for the Android build.
That forces __BIONIC_AVAILABILITY_GUARD(api) to 1 for every level (declarations
always visible) and weak-links symbols newer than the baked-in min-SDK, which
resolve at load time on the API-28+ devices the artifact targets. It is never
compiler-predefined, so defining it is clean (no -Wmacro-redefined). It also
subsumes the getifaddrs (API 24) case. Detection no longer relies on
ANDROID_ABI: it uses OS_NAME MATCHES "Android" (CI passes -DOS_NAME=Linux-Android)
and the compiler path, plus ANDROID/ANDROID_ABI for a future NDK switch.

Also drop the inert -DANDROID_PLATFORM=android-28 from both Android jobs (it
only produced an unused-variable warning) and rewrite the CLAUDE.md Android
section to document the real mechanism.

https://claude.ai/code/session_013jKzHGzz97hiY6t7mp3MGr

Author: claude

.github/workflows/publish.yml

@@ -164,7 +164,7 @@ jobs:
       - name: Build libraries
         shell: bash
         run: |
-          .github/dockcross/dockcross-android-arm64 .github/build.sh "-DANDROID_PLATFORM=android-28 -DOS_NAME=Linux-Android -DOS_ARCH=aarch64"
+          .github/dockcross/dockcross-android-arm64 .github/build.sh "-DOS_NAME=Linux-Android -DOS_ARCH=aarch64"
       - name: Upload artifacts
         uses: actions/upload-artifact@v7
         with:
@@ -180,7 +180,7 @@ jobs:
       - name: Build libraries
         shell: bash
         run: |
-          .github/dockcross/dockcross-android-arm64 .github/build_opencl_android.sh "-DANDROID_PLATFORM=android-28 -DOS_NAME=Linux-Android -DOS_ARCH=aarch64 -DGGML_OPENCL=ON -DGGML_OPENCL_EMBED_KERNELS=ON -DGGML_OPENCL_USE_ADRENO_KERNELS=ON"
+          .github/dockcross/dockcross-android-arm64 .github/build_opencl_android.sh "-DOS_NAME=Linux-Android -DOS_ARCH=aarch64 -DGGML_OPENCL=ON -DGGML_OPENCL_EMBED_KERNELS=ON -DGGML_OPENCL_USE_ADRENO_KERNELS=ON"
       - name: Upload artifacts
         uses: actions/upload-artifact@v7
         with:

CLAUDE.md

@@ -42,15 +42,46 @@ git commit -m "Upgrade CUDA from 13.2 to 13.3"
 
 Current Android minimum API level: **28** (Android 9.0 Pie)
 
-To change the minimum API level, update the following **three** places:
-
-1. **`CMakeLists.txt`** — the `add_compile_definitions(__ANDROID_API__=28)` line (controls which NDK header symbols are exposed).
-2. **`.github/workflows/publish.yml`** — `-DANDROID_PLATFORM=android-28` in both the `crosscompile-android-aarch64` and `crosscompile-android-aarch64-opencl` job steps.
-3. **`CLAUDE.md`** (this file) — the "Current Android minimum API level" line above and the `-DANDROID_PLATFORM` values in the local sanity-build examples.
-
-Also update the minimum-API note in **`README.md`** (the `[!NOTE]` block near the Android classifier entries and the "Importing in Android" section).
-
-**Why API 28?** `mtmd-helper.cpp` (part of the upstream llama.cpp `mtmd` multimodal library) includes `vendor/sheredom/subprocess.h`, which calls `posix_spawn`, `posix_spawnp`, and `posix_spawn_file_actions_*`. The Android NDK headers only expose those declarations when `__ANDROID_API__ >= 28`. The symbols exist in `libc.so` at all API levels; the define only gates their header visibility.
+This is enforced through bionic's **weak-symbol** mechanism, *not* by bumping
+`__ANDROID_API__` or passing `-DANDROID_PLATFORM`. See "How the API gate is
+satisfied" below for why. To change anything here, update:
+
+1. **`CMakeLists.txt`** — the `add_compile_definitions(__ANDROID_UNAVAILABLE_SYMBOLS_ARE_WEAK__)`
+   block and its Android-detection guard (`OS_NAME MATCHES "Android"` etc.).
+2. **`CLAUDE.md`** (this file) — the "Current Android minimum API level" line above.
+3. **`README.md`** — the minimum-API note (the `[!NOTE]` block near the Android
+   classifier entries and the "Importing in Android" section).
+
+**Why API 28?** `mtmd-helper.cpp` (part of the upstream llama.cpp `mtmd`
+multimodal library) includes `vendor/sheredom/subprocess.h`, which calls
+`posix_spawn`, `posix_spawnp`, and `posix_spawn_file_actions_*`. Bionic only
+exposes those `<spawn.h>` declarations once the minimum SDK is ≥ 28 (and
+`getifaddrs`/`freeifaddrs` in `<ifaddrs.h>`, used by cpp-httplib, at ≥ 24). The
+symbols exist in `libc.so` at all API levels; bionic only hides the
+*declarations* below the introducing API.
+
+**How the API gate is satisfied (important — the obvious fixes do not work).**
+The CI cross-compiler is the `dockcross-android-arm64` image, which is **not**
+the Google NDK CMake toolchain — it is a Debian-style cross-clang at
+`/usr/aarch64-linux-android/bin/clang`. Consequently:
+
+- It never sets the `ANDROID` / `ANDROID_ABI` CMake variables, so any
+  `if(ANDROID_ABI)`-guarded logic silently does nothing.
+- It **ignores** `-DANDROID_PLATFORM=android-28` (CMake prints it as a
+  "Manually-specified variables were not used by the project" warning).
+- `clang` predefines `__ANDROID_API__` from its baked-in target triple, so
+  `-D__ANDROID_API__=28` would only clash with the builtin (`-Wmacro-redefined`)
+  and would *not* move `__ANDROID_MIN_SDK_VERSION__`, which is what bionic's
+  `__BIONIC_AVAILABILITY_GUARD(api)` actually tests.
+
+The working fix is `add_compile_definitions(__ANDROID_UNAVAILABLE_SYMBOLS_ARE_WEAK__)`
+for the Android build. That macro forces `__BIONIC_AVAILABILITY_GUARD(api)` to
+`1` for every API level (declarations always visible) and makes any symbol newer
+than the toolchain's baked-in min-SDK a **weak** reference resolved by the
+dynamic linker at load time — present on every API-28+ device the artifact
+targets. It is never compiler-predefined, so defining it is clean. The guard
+detects Android via `OS_NAME MATCHES "Android"` (CI passes
+`-DOS_NAME=Linux-Android`) and the compiler path, not `ANDROID_ABI`.
 
 ## OpenCL / Adreno backend on Android
 
@@ -76,7 +107,7 @@ Three places wire it together (mirrors the CUDA classifier pattern):
 Local sanity build:
 ```bash
 .github/dockcross/dockcross-android-arm64 .github/build_opencl_android.sh \
-  "-DANDROID_PLATFORM=android-28 -DOS_NAME=Linux-Android -DOS_ARCH=aarch64 \
+  "-DOS_NAME=Linux-Android -DOS_ARCH=aarch64 \
    -DGGML_OPENCL=ON -DGGML_OPENCL_EMBED_KERNELS=ON \
    -DGGML_OPENCL_USE_ADRENO_KERNELS=ON"
 ```

CMakeLists.txt

@@ -47,14 +47,37 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_L
     link_libraries(stdc++fs)
 endif()
 
-# Android NDK headers gate symbols on __ANDROID_API__:
+# Android bionic gates several libc declarations behind a minimum API level:
 #   >= 24: getifaddrs/freeifaddrs (<ifaddrs.h>), needed by cpp-httplib.
 #   >= 28: posix_spawn / posix_spawnp / posix_spawn_file_actions_* (<spawn.h>),
 #          needed by mtmd-helper.cpp (vendor/sheredom/subprocess.h).
-# NDK unified sysroots (r14b+) include all these symbols in libc regardless of
-# level; the define only controls which declarations are visible in the headers.
-if(ANDROID_ABI)
-    add_compile_definitions(__ANDROID_API__=28)
+# The symbols exist in libc.so at all API levels; bionic only hides the
+# *declarations* below the introducing API via:
+#     #if __BIONIC_AVAILABILITY_GUARD(api)        // <android/versioning.h>
+# which expands to (__ANDROID_MIN_SDK_VERSION__ >= api) UNLESS
+# __ANDROID_UNAVAILABLE_SYMBOLS_ARE_WEAK__ is defined, in which case it is
+# always 1 (all declarations visible, newer-API symbols become weak references
+# resolved at load time — present on every API-28+ device we target).
+#
+# We define __ANDROID_UNAVAILABLE_SYMBOLS_ARE_WEAK__ rather than bumping
+# __ANDROID_API__ because the dockcross-android-arm64 image used by CI does NOT
+# use the Google NDK CMake toolchain: it is a Debian-style cross-clang
+# (/usr/aarch64-linux-android/bin/clang) that (a) never sets the ANDROID /
+# ANDROID_ABI CMake variables and (b) ignores -DANDROID_PLATFORM (CMake reports
+# it as an unused variable). On that toolchain clang predefines __ANDROID_API__
+# from the baked-in target triple, so -D__ANDROID_API__=28 would only clash with
+# the builtin (a -Wmacro-redefined) without moving __ANDROID_MIN_SDK_VERSION__.
+# The weak-symbols macro is never compiler-predefined, so defining it is clean
+# and deterministically exposes the guarded declarations. It also covers the
+# getifaddrs (API 24) case, replacing the old __ANDROID_API__ bump entirely.
+#
+# Detection must not rely on ANDROID_ABI (unset by this toolchain). OS_NAME is
+# passed as -DOS_NAME=Linux-Android by the CI cmake invocation and is already
+# used the same way at the server-models.cpp exclusion below; we also accept the
+# NDK-style ANDROID/ANDROID_ABI signals so a future switch back to the NDK
+# toolchain keeps working.
+if(ANDROID OR ANDROID_ABI OR OS_NAME MATCHES "Android" OR CMAKE_CXX_COMPILER MATCHES "android")
+    add_compile_definitions(__ANDROID_UNAVAILABLE_SYMBOLS_ARE_WEAK__)
 endif()
 
 set(LLAMA_BUILD_COMMON ON)