如何在JNI编程中使用logCat

首先声明，这个是转 http://blog.csdn.net/zgjxwl/article/details/6234025 这篇文章的，但是没有全摘下来，我只摘了自己需要的地方

Android NDK发布后，java+C的编程方式成为android上性能编程的首选。当然，也支持其他语言，如C++等，只要符合JNI规则即可。

在android编程java代码中，我们知道，可以使用Log.v等一些将日志输出到logcat，然后我们就可以看到日志输出信息。当然，也可以在

shell 里使用adb logcat来查看日志信息。对于java代码可以这样查看日志信息，但java调用的C函数呢，是否也可以将日志输出到logcat里呢？这就要看ANDROID NDK是否支持了。以往，在JNI编程中，调试Native Interface Method比较困难，往往都是采用打log的方式将日志输出到文件。今天，在目录

/build/platforms/android-8/arch-arm/usr/include/android/log.h

下发现android NDK提供的头文件，打开瞧瞧

/* * Copyright (C) 2009 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */#ifndef _ANDROID_LOG_H#define _ANDROID_LOG_H/****************************************************************** * * IMPORTANT NOTICE: * *   This file is part of Android's set of stable system headers *   exposed by the Android NDK (Native Development Kit) since *   platform release 1.5 * *   Third-party source AND binary code relies on the definitions *   here to be FROZEN ON ALL UPCOMING PLATFORM RELEASES. * *   - DO NOT MODIFY ENUMS (EXCEPT IF YOU ADD NEW 32-BIT VALUES) *   - DO NOT MODIFY CONSTANTS OR FUNCTIONAL MACROS *   - DO NOT CHANGE THE SIGNATURE OF FUNCTIONS IN ANY WAY *   - DO NOT CHANGE THE LAYOUT OR SIZE OF STRUCTURES *//* * Support routines to send messages to the Android in-kernel log buffer, * which can later be accessed through the 'logcat' utility. * * Each log message must have *   - a priority *   - a log tag *   - some text * * The tag normally corresponds to the component that emits the log message, * and should be reasonably small. * * Log message text may be truncated to less than an implementation-specific * limit (e.g. 1023 characters max). * * Note that a newline character ("/n") will be appended automatically to your * log message, if not already there. It is not possible to send several messages * and have them appear on a single line in logcat. * * PLEASE USE LOGS WITH MODERATION: * *  - Sending log messages eats CPU and slow down your application and the *    system. * *  - The circular log buffer is pretty small (<64KB), sending many messages *    might push off other important log messages from the rest of the system. * *  - In release builds, only send log messages to account for exceptional *    conditions. * * NOTE: These functions MUST be implemented by /system/lib/liblog.so */#include <stdarg.h>#ifdef __cplusplusextern "C" {#endif/* * Android log priority values, in ascending priority order. */typedef enum android_LogPriority {    ANDROID_LOG_UNKNOWN = 0,    ANDROID_LOG_DEFAULT,    /* only for SetMinPriority() */    ANDROID_LOG_VERBOSE,    ANDROID_LOG_DEBUG,    ANDROID_LOG_INFO,    ANDROID_LOG_WARN,    ANDROID_LOG_ERROR,    ANDROID_LOG_FATAL,    ANDROID_LOG_SILENT,     /* only for SetMinPriority(); must be last */} android_LogPriority;/* * Send a simple string to the log. */int __android_log_write(int prio, const char *tag, const char *text);/* * Send a formatted string to the log, used like printf(fmt,...) */int __android_log_print(int prio, const char *tag,  const char *fmt, ...)#if defined(__GNUC__)    __attribute__ ((format(printf, 3, 4)))#endif    ;/* * A variant of __android_log_print() that takes a va_list to list * additional parameters. */int __android_log_vprint(int prio, const char *tag,                         const char *fmt, va_list ap);/* * Log an assertion failure and SIGTRAP the process to have a chance * to inspect it, if a debugger is attached. This uses the FATAL priority. */void __android_log_assert(const char *cond, const char *tag,  const char *fmt, ...)    #if defined(__GNUC__)    __attribute__ ((noreturn))    __attribute__ ((format(printf, 3, 4)))#endif    ;#ifdef __cplusplus}#endif#endif /* _ANDROID_LOG_H */

请仔细阅读这个头文件，我们会发现，android NDK完全支持JNI本地方法调试。它提供4个函数供我们使用，如下

/* * Send a simple string to the log. */int __android_log_write(int prio, const char *tag, const char *text);/* * Send a formatted string to the log, used like printf(fmt,...) */int __android_log_print(int prio, const char *tag,  const char *fmt, ...)/* * A variant of __android_log_print() that takes a va_list to list * additional parameters. */int __android_log_vprint(int prio, const char *tag,                         const char *fmt, va_list ap);/* * Log an assertion failure and SIGTRAP the process to have a chance * to inspect it, if a debugger is attached. This uses the FATAL priority. */void __android_log_assert(const char *cond, const char *tag,  const char *fmt, ...)  

....................

Android.mk

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE     := PerformanceTest

LOCAL_SRC_FILES :=  src/com_jni_AccessFiledPerformanceTest.c

LOCAL_LDLIBS := -L$(SYSROOT)/usr/lib -llog

include $(BUILD_SHARED_LIBRARY)

 

注意：

LOCAL_LDLIBS := -L$(SYSROOT)/usr/lib -llog

这句很重要

如果在上面的mk文件里没有写LOCAL_LDLIBS := -L$(SYSROOT)/usr/lib -llog这句，则在编译链接的时候会报错误