examples: Add example for the librados C API

Simply copied and modified the C++ API example.

Signed-off-by: Adam Crume <adamcrume@gmail.com>

.gitignore

@@ -81,3 +81,4 @@ GSYMS
 GTAGS
 /examples/librados/.libs/
 /examples/librados/librados_hello_world
+/examples/librados/librados_hello_world_c

examples/librados/Makefile

@@ -1,5 +1,13 @@
-all: hello_world.cc
-	g++ -g -c hello_world.cc -o hello_world.o
-	g++ -g hello_world.o -lrados -o librados_hello_world
+all: librados_hello_world librados_hello_world_c
+
+librados_hello_world: hello_world.cc
+	g++ -g -c hello_world.cc -o hello_world.o $(CFLAGS)
+	g++ -g hello_world.o -lrados -o librados_hello_world $(LDFLAGS)
+
+librados_hello_world_c: hello_world_c.c
+	cc -g -c hello_world_c.c -o hello_world_c.o $(CFLAGS)
+	cc -g hello_world_c.o -lrados -o librados_hello_world_c $(LDFLAGS)
+
 clean:
-	rm hello_world.o librados_hello_world
+	rm hello_world.o librados_hello_world
+	rm hello_world_c.o librados_hello_world_c

examples/librados/hello_world_c.c

@@ -0,0 +1,313 @@
+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+/*
+ * Ceph - scalable distributed file system
+ *
+ * This is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License version 2.1, as published by the Free Software
+ * Foundation.  See file COPYING.
+ * Copyright 2013 Inktank
+ */
+
+// install the librados-dev package to get this
+#include <rados/librados.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int main(int argc, const char **argv)
+{
+  int ret = 0;
+
+  // we will use all of these below
+  const char *pool_name = "hello_world_pool";
+  const char* hello = "hello world!";
+  const char* object_name = "hello_object";
+  rados_ioctx_t io_ctx = NULL;
+  int pool_created = 0;
+
+  // first, we create a Rados object and initialize it
+  rados_t rados = NULL;
+  {
+    ret = rados_create(&rados, "admin"); // just use the client.admin keyring
+    if (ret < 0) { // let's handle any error that might have come back
+      printf("couldn't initialize rados! error %d\n", ret);
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      printf("we just set up a rados cluster object\n");
+    }
+  }
+
+  /*
+   * Now we need to get the rados object its config info. It can
+   * parse argv for us to find the id, monitors, etc, so let's just
+   * use that.
+   */
+  {
+    ret = rados_conf_parse_argv(rados, argc, argv);
+    if (ret < 0) {
+      // This really can't happen, but we need to check to be a good citizen.
+      printf("failed to parse config options! error %d\n", ret);
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      printf("we just parsed our config options\n");
+      // We also want to apply the config file if the user specified
+      // one, and conf_parse_argv won't do that for us.
+      int i;
+      for (i = 0; i < argc; ++i) {
+  	if ((strcmp(argv[i], "-c") == 0) || (strcmp(argv[i], "--conf") == 0)) {
+  	  ret = rados_conf_read_file(rados, argv[i+1]);
+  	  if (ret < 0) {
+  	    // This could fail if the config file is malformed, but it'd be hard.
+	    printf("failed to parse config file %s! error %d\n", argv[i+1], ret);
+  	    ret = EXIT_FAILURE;
+  	    goto out;
+  	  }
+  	  break;
+  	}
+      }
+    }
+  }
+
+  /*
+   * next, we actually connect to the cluster
+   */
+  {
+    ret = rados_connect(rados);
+    if (ret < 0) {
+      printf("couldn't connect to cluster! error %d\n", ret);
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      printf("we just connected to the rados cluster\n");
+    }
+  }
+
+  /*
+   * let's create our own pool instead of scribbling over real data.
+   * Note that this command creates pools with default PG counts specified
+   * by the monitors, which may not be appropriate for real use -- it's fine
+   * for testing, though.
+   */
+  {
+    ret = rados_pool_create(rados, pool_name);
+    if (ret < 0) {
+      printf("couldn't create pool! error %d\n", ret);
+      return EXIT_FAILURE;
+    } else {
+      printf("we just created a new pool named %s\n", pool_name);
+    }
+    pool_created = 1;
+  }
+
+  /*
+   * create an "IoCtx" which is used to do IO to a pool
+   */
+  {
+    ret = rados_ioctx_create(rados, pool_name, &io_ctx);
+    if (ret < 0) {
+      printf("couldn't set up ioctx! error %d\n", ret);
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      printf("we just created an ioctx for our pool\n");
+    }
+  }
+
+  /*
+   * now let's do some IO to the pool! We'll write "hello world!" to a
+   * new object.
+   */
+  {
+    /*
+     * now that we have the data to write, let's send it to an object.
+     * We'll use the synchronous interface for simplicity.
+     */
+    ret = rados_write_full(io_ctx, object_name, hello, strlen(hello));
+    if (ret < 0) {
+      printf("couldn't write object! error %d\n", ret);
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      printf("we just wrote new object %s, with contents '%s'\n", object_name, hello);
+    }
+  }
+
+  /*
+   * now let's read that object back! Just for fun, we'll do it using
+   * async IO instead of synchronous. (This would be more useful if we
+   * wanted to send off multiple reads at once; see
+   * http://ceph.com/docs/master/rados/api/librados/#asychronous-io )
+   */
+  {
+    int read_len = 4194304; // this is way more than we need
+    char* read_buf = malloc(read_len + 1); // add one for the terminating 0 we'll add later
+    if (!read_buf) {
+      printf("couldn't allocate read buffer\n");
+      ret = EXIT_FAILURE;
+      goto out;
+    }
+    // allocate the completion from librados
+    rados_completion_t read_completion;
+    ret = rados_aio_create_completion(NULL, NULL, NULL, &read_completion);
+    if (ret < 0) {
+      printf("couldn't create completion! error %d\n", ret);
+      ret = EXIT_FAILURE;
+      free(read_buf);
+      goto out;
+    } else {
+      printf("we just created a new completion\n");
+    }
+    // send off the request.
+    ret = rados_aio_read(io_ctx, object_name, read_completion, read_buf, read_len, 0);
+    if (ret < 0) {
+      printf("couldn't start read object! error %d\n", ret);
+      ret = EXIT_FAILURE;
+      free(read_buf);
+      rados_aio_release(read_completion);
+      goto out;
+    }
+    // wait for the request to complete, and check that it succeeded.
+    rados_aio_wait_for_complete(read_completion);
+    ret = rados_aio_get_return_value(read_completion);
+    if (ret < 0) {
+      printf("couldn't read object! error %d\n", ret);
+      ret = EXIT_FAILURE;
+      free(read_buf);
+      rados_aio_release(read_completion);
+      goto out;
+    } else {
+      read_buf[ret] = 0; // null-terminate the string
+      printf("we read our object %s, and got back %d bytes with contents\n%s\n", object_name, ret, read_buf);
+    }
+
+    free(read_buf);
+    rados_aio_release(read_completion);
+  }
+
+  /*
+   * We can also use xattrs that go alongside the object.
+   */
+  {
+    const char* version = "1";
+    ret = rados_setxattr(io_ctx, object_name, "version", version, strlen(version));
+    if (ret < 0) {
+      printf("failed to set xattr version entry! error %d\n", ret);
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      printf("we set the xattr 'version' on our object!\n");
+    }
+  }
+
+  /*
+   * And if we want to be really cool, we can do multiple things in a single
+   * atomic operation. For instance, we can update the contents of our object
+   * and set the version at the same time.
+   */
+  {
+    const char* content = "v2";
+    rados_write_op_t write_op = rados_create_write_op();
+    if (!write_op) {
+      printf("failed to allocate write op\n");
+      ret = EXIT_FAILURE;
+      goto out;
+    }
+    rados_write_op_write_full(write_op, content, strlen(content));
+    const char* version = "2";
+    rados_write_op_setxattr(write_op, "version", version, strlen(version));
+    ret = rados_write_op_operate(write_op, io_ctx, object_name, NULL, 0);
+    if (ret < 0) {
+      printf("failed to do compound write! error %d\n", ret);
+      ret = EXIT_FAILURE;
+      rados_release_write_op(write_op);
+      goto out;
+    } else {
+      printf("we overwrote our object %s with contents\n%s\n", object_name, content);
+    }
+    rados_release_write_op(write_op);
+  }
+
+  /*
+   * And to be even cooler, we can make sure that the object looks the
+   * way we expect before doing the write! Notice how this attempt fails
+   * because the xattr differs.
+   */
+  {
+    rados_write_op_t failed_write_op = rados_create_write_op();
+    if (!failed_write_op) {
+      printf("failed to allocate write op\n");
+      ret = EXIT_FAILURE;
+      goto out;
+    }
+    const char* content = "v2";
+    const char* version = "2";
+    const char* old_version = "1";
+    rados_write_op_cmpxattr(failed_write_op, "version", LIBRADOS_CMPXATTR_OP_EQ, old_version, strlen(old_version));
+    rados_write_op_write_full(failed_write_op, content, strlen(content));
+    rados_write_op_setxattr(failed_write_op, "version", version, strlen(version));
+    ret = rados_write_op_operate(failed_write_op, io_ctx, object_name, NULL, 0);
+    if (ret < 0) {
+      printf("we just failed a write because the xattr wasn't as specified\n");
+    } else {
+      printf("we succeeded on writing despite an xattr comparison mismatch!\n");
+      ret = EXIT_FAILURE;
+      rados_release_write_op(failed_write_op);
+      goto out;
+    }
+    rados_release_write_op(failed_write_op);
+
+    /*
+     * Now let's do the update with the correct xattr values so it
+     * actually goes through
+     */
+    content = "v3";
+    old_version = "2";
+    version = "3";
+    rados_write_op_t update_op = rados_create_write_op();
+    if (!failed_write_op) {
+      printf("failed to allocate write op\n");
+      ret = EXIT_FAILURE;
+      goto out;
+    }
+    rados_write_op_cmpxattr(update_op, "version", LIBRADOS_CMPXATTR_OP_EQ, old_version, strlen(old_version));
+    rados_write_op_write_full(update_op, content, strlen(content));
+    rados_write_op_setxattr(update_op, "version", version, strlen(version));
+    ret = rados_write_op_operate(update_op, io_ctx, object_name, NULL, 0);
+    if (ret < 0) {
+      printf("failed to do a compound write update! error %d\n", ret);
+      ret = EXIT_FAILURE;
+      rados_release_write_op(update_op);
+      goto out;
+    } else {
+      printf("we overwrote our object %s following an xattr test with contents\n%s\n", object_name, content);
+    }
+    rados_release_write_op(update_op);
+  }
+
+  ret = EXIT_SUCCESS;
+
+ out:
+  if (io_ctx) {
+    rados_ioctx_destroy(io_ctx);
+  }
+
+  if (pool_created) {
+    /*
+     * And now we're done, so let's remove our pool and then
+     * shut down the connection gracefully.
+     */
+    int delete_ret = rados_pool_delete(rados, pool_name);
+    if (delete_ret < 0) {
+      // be careful not to
+      printf("We failed to delete our test pool!\n");
+      ret = EXIT_FAILURE;
+    }
+  }
+
+  rados_shutdown(rados);
+
+  return ret;
+}