Hadoop通过C的API访问HDFS

在HADOOP_HOM下通过ant compile-c++-libhdfs -Dislibhdfs=true(ant是类似于make的工具，主要应该与java，利用build.xml问价)，产生libhdfs.so

或者在HADOOP_HOME/src/c++/libhdfs执行：①./configure --enable-shared JVM_ARCH=tune=k8 --prefix=`pwd`/nativelib

②make install

在通过Hadoop的C的API 访问HDFS的时候，编译和运行出现了不少问题，在这边，做个总结吧：

系统：Ubuntu11.04，Hadoop-0.20.203.0

样例代码就是参考官方文档中提供到：

 #include "hdfs.h"

 int main(int argc, char **argv) {

     hdfsFS fs = hdfsConnect("default", 0);
     const char* writePath = "/tmp/testfile.txt";
     hdfsFile writeFile = hdfsOpenFile(fs, writePath, O_WRONLY|O_CREAT, 0, 0, 0);
     if(!writeFile) {
           fprintf(stderr, "Failed to open %s for writing!\n", writePath);
           exit(-1);
     }
     char* buffer = "Hello, World!";
     tSize num_written_bytes = hdfsWrite(fs, writeFile, (void*)buffer, strlen(buffer)+1);
     if (hdfsFlush(fs, writeFile)) {
            fprintf(stderr, "Failed to 'flush' %s\n", writePath);
           exit(-1);
     }
    hdfsCloseFile(fs, writeFile);
 }

编译：官网这样描述

See the Makefile for hdfs_test.c in the libhdfs source directory (${HADOOP_HOME}/src/c++/libhdfs/Makefile) or something like:
gcc above_sample.c -I${HADOOP_HOME}/src/c++/libhdfs -L${HADOOP_HOME}/libhdfs -lhdfs -o above_sample

但是我两个方法都试了，不行，后面发现原来是要少了：

 LIB = -L$(HADOOP_INSTALL)/c++/Linux-i386-32/lib/
 libjvm=/usr/lib/jvm/java-6-openjdk/jre/lib/i386/client/libjvm.so

所以完整到makefile就是：

 HADOOP_INSTALL=/home/fzuir/hadoop-0.20.203.0
 PLATFORM=Linux-i386-32
 JAVA_HOME=/usr/lib/jvm/java-6-openjdk/
 CPPFLAGS= -I$(HADOOP_INSTALL)/src/c++/libhdfs
 LIB = -L$(HADOOP_INSTALL)/c++/Linux-i386-32/lib/
 libjvm=/usr/lib/jvm/java-6-openjdk/jre/lib/i386/client/libjvm.so
 LDFLAGS += -lhdfs

 testHdfs: testHdfs.c
     gcc testHdfs.c  $(CPPFLAGS) $(LIB) $(LDFLAGS) $(libjvm) -o testHdfs

 clean:
     rm testHdfs

好了，编译通过，但是运行的时候出现以下错误信息：

Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/commons/configuration/Configuration

...

Call to org.apache.hadoop.fs.Filesystem::get(URI, Configuration) failed!
Exception in thread "main" java.lang.NullPointerException
Call to get configuration object from filesystem failed!

解决方法：将HADOOP_HOME和HADOOP_HOME/lib下所有到jar包加入/usr/lib/jvm/java-6-openjdk/jre/lib/ext/（其实只要需要到加入，但是不知道哪些是需要的）

最后，恭喜你，问题解决了。