根据我的第一个问题，我尝试在我的应用程序中包含fanotify.h标头。该应用程序基于 fsmon(一个使用 fanotify 系统调用的小应用程序)，我出于我的目的更改了一些代码行并创建了一个 Android.mk makefile：
APP_PLATFORM := android-23
TARGET_PLATFORM := android-23
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
# Enable PIE manually. Will get reset on $(CLEAR_VARS). This
# is what enabling PIE translates to behind the scenes.
LOCAL_CFLAGS += -fPIE -DHAVE_FANOTIFY=1 -DHAVE_SYS_FANOTIFY=0
LOCAL_LDFLAGS += -fPIE -pie
# give module name
LOCAL_MODULE := fsmon
# include fanotify by api 23:
LOCAL_C_INCLUDES := /home/fabian/Android/Sdk/ndk-bundle/platforms/android-23/arch-arm/usr/include
# include fanotify by sysroot ->
# LOCAL_C_INCLUDES := /home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/
# list your C files to compile
LOCAL_SRC_FILES := inotify.c fanotify.c util.c main.c
# this option will build executables instead of building library for android application.
include $(BUILD_EXECUTABLE)
我需要包括linux/inotify.h和linux/fanotify.h标题。
在执行ndk-build时，只有arm64-v8a，mips64和x86_64可执行文件(在libs/xxx中)被创建，导致以下错误消息：
fabian@fabian-ubuntu:~/kernel_android_goldfish/fsmon/fsmon/jni$ ndk-build
[arm64-v8a] Compile : fsmon <= inotify.c
[arm64-v8a] Compile : fsmon <= fanotify.c
[arm64-v8a] Compile : fsmon <= util.c
[arm64-v8a] Compile : fsmon <= main.c
[arm64-v8a] Executable : fsmon
[arm64-v8a] Install : fsmon => libs/arm64-v8a/fsmon
[x86_64] Compile : fsmon <= inotify.c
[x86_64] Compile : fsmon <= fanotify.c
[x86_64] Compile : fsmon <= util.c
[x86_64] Compile : fsmon <= main.c
[x86_64] Executable : fsmon
[x86_64] Install : fsmon => libs/x86_64/fsmon
[mips64] Compile : fsmon <= inotify.c
[mips64] Compile : fsmon <= fanotify.c
[mips64] Compile : fsmon <= util.c
[mips64] Compile : fsmon <= main.c
[mips64] Executable : fsmon
[mips64] Install : fsmon => libs/mips64/fsmon
[armeabi-v7a] Compile thumb : fsmon <= inotify.c
In file included from /home/fabian/kernel_android_goldfish/fsmon/fsmon/jni/inotify.c:9:
In file included from /home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/signal.h:49:
In file included from /home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/linux/signal.h:21:
In file included from /home/fabian/Android/Sdk/ndk-bundle/platforms/android-9/arch-arm/usr/include/asm/signal.h:82:
/home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/asm-generic/signal.h:92:21: error: use of undeclared identifier
'__BITS_PER_LONG'
unsigned long sig[_NSIG_WORDS];
^
/home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/asm-generic/signal.h:25:38: note: expanded from macro '_NSIG_WORDS'
#define _NSIG_WORDS (_KERNEL__NSIG / _NSIG_BPW)
^
/home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/asm-generic/signal.h:24:19: note: expanded from macro '_NSIG_BPW'
#define _NSIG_BPW __BITS_PER_LONG
^
/home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/asm-generic/signal.h:94:3: error: typedef redefinition with different
types ('struct (anonymous struct at /home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/asm-generic/signal.h:91:9)' vs
'unsigned long')
} sigset_t;
^
/home/fabian/Android/Sdk/ndk-bundle/platforms/android-9/arch-arm/usr/include/asm/signal.h:20:23: note: previous definition is here
typedef unsigned long sigset_t;
^
/home/fabian/Android/Sdk/ndk-bundle/platforms/android-9/arch-arm/usr/include/asm/signal.h:84:8: error: redefinition of 'sigaction'
struct sigaction {
^
/home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/asm-generic/signal.h:101:8: note: previous definition is here
struct sigaction {
^
In file included from /home/fabian/kernel_android_goldfish/fsmon/fsmon/jni/inotify.c:9:
In file included from /home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/signal.h:49:
In file included from /home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/linux/signal.h:21:
/home/fabian/Android/Sdk/ndk-bundle/platforms/android-9/arch-arm/usr/include/asm/signal.h:97:16: error: redefinition of
'sigaltstack'
typedef struct sigaltstack {
^
/home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/asm-generic/signal.h:111:16: note: previous definition is here
typedef struct sigaltstack {
^
In file included from /home/fabian/kernel_android_goldfish/fsmon/fsmon/jni/inotify.c:9:
In file included from /home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/signal.h:49:
In file included from /home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/linux/signal.h:21:
/home/fabian/Android/Sdk/ndk-bundle/platforms/android-9/arch-arm/usr/include/asm/signal.h:101:3: error: typedef redefinition with
different types ('struct (anonymous struct at
/home/fabian/Android/Sdk/ndk-bundle/platforms/android-9/arch-arm/usr/include/asm/signal.h:97:16)' vs 'struct sigaltstack')
} stack_t;
^
/home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/asm-generic/signal.h:116:3: note: previous definition is here
} stack_t;
^
In file included from /home/fabian/kernel_android_goldfish/fsmon/fsmon/jni/inotify.c:9:
/home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/signal.h:71:9: warning: 'NSIG' macro redefined [-Wmacro-redefined]
#define NSIG _NSIG
^
/home/fabian/Android/Sdk/ndk-bundle/platforms/android-9/arch-arm/usr/include/asm/signal.h:19:9: note: previous definition is here
#define NSIG 32
^
In file included from /home/fabian/kernel_android_goldfish/fsmon/fsmon/jni/inotify.c:9:
/home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/signal.h:79:9: warning: 'SIGRTMIN' macro redefined [-Wmacro-redefined]
#define SIGRTMIN (__libc_current_sigrtmin())
^
/home/fabian/Android/Sdk/ndk-bundle/platforms/android-9/arch-arm/usr/include/asm/signal.h:58:9: note: previous definition is here
#define SIGRTMIN 32
^
In file included from /home/fabian/kernel_android_goldfish/fsmon/fsmon/jni/inotify.c:9:
/home/fabian/Android/Sdk/ndk-bundle/sysroot/usr/include/signal.h:80:9: warning: 'SIGRTMAX' macro redefined [-Wmacro-redefined]
#define SIGRTMAX (__libc_current_sigrtmax())
^
/home/fabian/Android/Sdk/ndk-bundle/platforms/android-9/arch-arm/usr/include/asm/signal.h:59:9: note: previous definition is here
#define SIGRTMAX _NSIG
^
5 errors generated.
make: *** [/home/fabian/kernel_android_goldfish/fsmon/fsmon/obj/local/armeabi-v7a/objs/fsmon/inotify.o] Error 1
inotify.c的原因包括重新包含linux/signal.h的signal.h标头，这反过来又是出现错误的platforms/android-9/arch-arm/usr/include/asm/signal.h。(突然来自安卓-9)。
我无法弄清楚问题出在哪里。我试图包含来自 sysroot 的包含文件夹 - 但出现了类似的错误。
源文件： inotify.c fanotify.c
您不应手动指向LOCAL_C_INCLUDES中的 NDK 标头。这应该自动添加，指向正确的版本。但是，实际上没有任何内容使用此处APP_PLATFORM或TARGET_PLATFORM变量。
而是在文件jni/Application.mk中添加APP_PLATFORM := android-23。这应该使其在所有架构中使用正确的平台标头进行构建。
除了 @mstorsjo 的答案之外，您还可以在 NDK r14 及更高版本中使用统一标头。这些是标头的一种新形式，将始终是最新的，而不是为每个 API 级别提供一组不同的标头。
与增加APP_PLATFORM相比，这样做的优势在于您的APP_PLAFORM不能高于您的minSdkVersion(请参阅我们的常见问题文档)。由于 fanotify 是在相当旧的内核版本中引入的(fanotify_init为 2.6.36)，即使旧版本的 libc 中不存在该函数，您仍然应该能够使用syscall(__NR_fanotify_init)调用它。尽管如此，ENOSYS总是有可能的。
默认情况下，在 r14 中，您仍然会获得旧形式的标头。新的"统一标头"具有您正在寻找的标头。如果要尝试统一标头，请在 Application.mk 中设置APP_UNIFIED_HEADERS := true(可以在上面的链接中找到其他构建系统的设置)。
在 r15(第一个测试版即将推出)中，默认值已更改为新标头，并且禁用它们的选项已更改(请参阅 r15 中的同一文档)。