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229<h1>C API to HDFS: libhdfs</h1>
230<div id="minitoc-area">
231<ul class="minitoc">
232<li>
233<a href="#C+API+to+HDFS%3A+libhdfs">C API to HDFS: libhdfs</a>
234</li>
235<li>
236<a href="#The+APIs">The APIs</a>
237</li>
238<li>
239<a href="#A+sample+program">A sample program</a>
240</li>
241<li>
242<a href="#How+to+link+with+the+library">How to link with the library</a>
243</li>
244<li>
245<a href="#Common+problems">Common problems</a>
246</li>
247<li>
248<a href="#libhdfs+is+thread+safe">libhdfs is thread safe</a>
249</li>
250</ul>
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252
253<a name="N10019"></a><a name="C+API+to+HDFS%3A+libhdfs"></a>
254<h2 class="h3">C API to HDFS: libhdfs</h2>
255<div class="section">
256<p>
257libhdfs is a JNI based C api for Hadoop's DFS. It provides C apis to a subset of the HDFS APIs to manipulate DFS files and the filesystem. libhdfs is part of the hadoop distribution and comes pre-compiled in ${HADOOP_HOME}/libhdfs/libhdfs.so .
258</p>
259</div>
260
261<a name="N10023"></a><a name="The+APIs"></a>
262<h2 class="h3">The APIs</h2>
263<div class="section">
264<p>
265The libhdfs APIs are a subset of: <a href="api/org/apache/hadoop/fs/FileSystem.html">hadoop fs APIs</a>. 
266</p>
267<p>
268The header file for libhdfs describes each API in detail and is available in ${HADOOP_HOME}/src/c++/libhdfs/hdfs.h
269</p>
270</div>
271
272<a name="N10034"></a><a name="A+sample+program"></a>
273<h2 class="h3">A sample program</h2>
274<div class="section">
275<pre class="code">
276#include "hdfs.h"
277
278int main(int argc, char **argv) {
279
280    hdfsFS fs = hdfsConnect("default", 0);
281    const char* writePath = "/tmp/testfile.txt";
282    hdfsFile writeFile = hdfsOpenFile(fs, writePath, O_WRONLY|O_CREAT, 0, 0, 0);
283    if(!writeFile) {
284          fprintf(stderr, "Failed to open %s for writing!\n", writePath);
285          exit(-1);
286    }
287    char* buffer = "Hello, World!";
288    tSize num_written_bytes = hdfsWrite(fs, writeFile, (void*)buffer, strlen(buffer)+1);
289    if (hdfsFlush(fs, writeFile)) {
290           fprintf(stderr, "Failed to 'flush' %s\n", writePath);
291          exit(-1);
292    }
293   hdfsCloseFile(fs, writeFile);
294}
295
296</pre>
297</div>
298
299
300<a name="N1003F"></a><a name="How+to+link+with+the+library"></a>
301<h2 class="h3">How to link with the library</h2>
302<div class="section">
303<p>
304See the Makefile for hdfs_test.c in the libhdfs source directory (${HADOOP_HOME}/src/c++/libhdfs/Makefile) or something like:
305gcc above_sample.c -I${HADOOP_HOME}/src/c++/libhdfs -L${HADOOP_HOME}/libhdfs -lhdfs -o above_sample
306</p>
307</div>
308
309<a name="N10049"></a><a name="Common+problems"></a>
310<h2 class="h3">Common problems</h2>
311<div class="section">
312<p>
313The most common problem is the CLASSPATH is not set properly when calling a program that uses libhdfs. Make sure you set it to all the hadoop jars needed to run Hadoop itself. Currently, there is no way to programmatically generate the classpath, but a good bet is to include all the jar files in ${HADOOP_HOME} and ${HADOOP_HOME}/lib as well as the right configuration directory containing hdfs-site.xml
314</p>
315</div>
316
317<a name="N10053"></a><a name="libhdfs+is+thread+safe"></a>
318<h2 class="h3">libhdfs is thread safe</h2>
319<div class="section">
320<p>Concurrency and Hadoop FS "handles" - the hadoop FS implementation includes a FS handle cache which caches based on the URI of the namenode along with the user connecting. So, all calls to hdfsConnect will return the same handle but calls to hdfsConnectAsUser with different users will return different handles.  But, since HDFS client handles are completely thread safe, this has no bearing on concurrency.
321</p>
322<p>Concurrency and libhdfs/JNI - the libhdfs calls to JNI should always be creating thread local storage, so (in theory), libhdfs should be as thread safe as the underlying calls to the Hadoop FS.
323</p>
324</div>
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