Android Mobile Builds For libp2p

cbind/libp2p.nim is the supported C ABI target for mobile builds.

Supported Targets

The Android build uses Android API 23 by default and builds with the NDK clang toolchain from Nix:

Android ABI	Nim CPU	C compiler
`arm64-v8a`	`arm64`	`aarch64-linux-android23-clang`
`x86_64`	`amd64`	`x86_64-linux-android23-clang`

The Nix derivation currently pins Android NDK 27.2.12479018 through androidenv.composeAndroidPackages and uses the same Android clang for Nim-generated C code and the C check harness.

Build Commands

Build a single ABI:

nix build .#cbind-ffi-android-arm64-v8a
nix build .#cbind-ffi-android-x86_64

Build both ABI layouts in one output:

nix build .#cbind-ffi-android

The host C ABI package remains available as:

nix build .#cbind

Artifact Layout

Single-ABI outputs are flat:

result/
  bin/libp2p_android_check
  include/libp2p.h
  include/nim_ffi_cbor.h
  include/nim_ffi_prelude.h
  include/tinycbor/...
  include/cddl_bindings/libp2p.cddl
  lib/liblibp2p.so
  lib/liblibp2p.a
  lib/libc++_shared.so
  nix-support/android-target

The aggregate output nests the same layout by ABI:

result/android/arm64-v8a/...
result/android/x86_64/...

liblibp2p.so and liblibp2p.a intentionally keep the same naming as the host cbind package.

Downstream Linking Notes

Use the generated high-level C helpers in include/libp2p.h:

libp2p_ctx_create
libp2p_ctx_start
libp2p_ctx_stop
libp2p_ctx_destroy

The generated header uses TinyCBOR for request and response encoding. The Nix output installs the required TinyCBOR headers and C sources under include/tinycbor. Downstream C or C++ code that calls the generated helper functions should compile those TinyCBOR .c files into the app or a support library.

For shared-library linking, package the matching ABI's lib/liblibp2p.so and lib/libc++_shared.so with the Android application and load them through the normal Android native library path. For static linking, link lib/liblibp2p.a together with the Android C++ runtime selected by your application.

Android Check Harness

The Nix Android derivation compiles cbind/examples/libp2p_mobile_check.c for each ABI. The harness includes the generated nim-ffi C header, creates a default TCP/Yamux node, starts it, stops it, destroys the context, and fails on callback errors or timeouts.

The derivation only compiles and links the harness. To run it on a device or emulator, build the ABI matching the device and push the shared library and check binary:

nix build .#cbind-ffi-android-arm64-v8a

adb shell 'mkdir -p /data/local/tmp/nim-libp2p'
adb push result/lib/liblibp2p.so /data/local/tmp/nim-libp2p/
adb push result/bin/libp2p_android_check /data/local/tmp/nim-libp2p/
adb shell 'cd /data/local/tmp/nim-libp2p && chmod 755 libp2p_android_check && LD_LIBRARY_PATH=. ./libp2p_android_check'

For an x86_64 emulator, replace the build command with:

nix build .#cbind-ffi-android-x86_64

CI Coverage

.github/workflows/mobile_android.yml builds both Android ABI packages and checks that each output contains:

shared and static liblibp2p libraries
Android libc++_shared.so runtime for the shared object
generated C headers and CDDL
TinyCBOR headers used by the generated C helper layer
the linked Android check executable
Android ELF machine headers matching the selected ABI

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