Add the array output C API.

ppapi/api/pp_array_output.idl

+/* Copyright (c) 2012 The Chromium Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+/**
+ * PP_ArrayOutput_GetDataBuffer is a callback function to allocate plugin
+ * memory for an array. It returns the allocated memory or null on failure.
+ *
+ * This function will be called reentrantly. This means that if you call a
+ * function PPB_Foo.GetData(&array_output), GetData will call your
+ * GetDataBuffer function before it returns. 
+ *
+ * This function will be called even when returning 0-length arrays, so be sure
+ * your implementation can support that. You can return NULL for 0 length
+ * arrays and it will not be treated as a failure.
+
+ * You should not perform any processing in this callback, including calling
+ * other PPAPI functions, outside of allocating memory. You should not throw
+ * any exceptions. In C++, this means using "new (nothrow)" or being sure to
+ * catch any exceptions before returning.
+ *
+ * The C++ wrapper provides a convenient templatized implementation around
+ * std::vector which you should generally use instead of coding this
+ * specifically.
+ *
+ * @param user_data The pointer provided in the PP_ArrayOutput structure. This
+ * has no meaning to the browser, it is intended to be used by the
+ * implementation to figure out where to put the data.
+ *
+ * @param element_count The number of elements in the array. This will be 0
+ * if there is no data to return.
+ *
+ * @param element_size The size of each element.

[dmichael (off chromium), 2012/03/07 03:56:41]

" in bytes"

+ *
+ * @return Returns a pointer to the allocated memory. On failure, returns null.
+ * You can also return null if the element_count is 0.
+ */
+typedef mem_t PP_ArrayOutput_GetDataBuffer([inout] mem_t user_data,
+                                           [in] uint32_t element_count,
+                                           [in] uint32_t element_size);
+
+/**
+ * A structure that defines a way for the browser to return arrays of data
+ * to the plugin. The plugin and browser may use different allocators so the
+ * browser can not allocate memory on behalf of the plugin.

[dmichael (off chromium), 2012/03/07 03:56:41]

The last sentence could be confusing, because "so" could be read to mean "so
that" instead of "and therefore". Maybe something like:
"The browser can not allocate memory on behalf of the plugin because the plugin
and browser may use different allocators."

+ *
+ * Array output works by having the browser call to the plugin to allocate a
+ * buffer, and then the browser will copy the contents of the array into that
+ * buffer.
+ *
+ * In C, you would typically implement this as follows:
+ *
+ * <code>

[dmichael (off chromium), 2012/03/07 03:56:41]

Apparently, we're using @code now for blocks of code like this (but <code> for
in-line).  Somehow the doxygen formatting is different.

+ * struct MyArrayOutput {
+ *   void* data;
+ *   int element_count;
+ * };
+ * void* MyGetDataBuffer(void* user_data, uint32_t count, uint32_t size) {
+ *   MyArrayOutput* output = (MyArrayOutput*)user_data;
+ *   output->element_count = count;
+ *   if (size) {
+ *     output->data = malloc(count * size);
+ *     if (!output->data)  // Be careful to set size properly on malloc failure.
+ *       output->element_count = 0;
+ *   } else {
+ *     output->data = NULL;

[dmichael (off chromium), 2012/03/07 03:56:41]

I know size == 0 is a weird case, but shouldn't we still set
output->element_count?

[brettw, 2012/03/07 05:56:20]

It's set above.

+ *   }
+ *   return output->data;
+ * }
+ * void MyFunction() {
+ *   MyArrayOutput array = { NULL, 0 };
+ *   PP_ArrayOutput output = { &MyGetDataBuffer, &array };
+ *   ppb_foo->GetData(&output);
+ * }
+ * </code>
+ */
+struct PP_ArrayOutput {
+  /**
+   * A pointer to the allocation function that the browser implements.
+   */
+  PP_ArrayOutput_GetDataBuffer GetDataBuffer;
+
+  /**
+   * Data that is passed to the allocation function. Typically, this is used
+   * to communicate how the data should be stored.
+   */
+  mem_t user_data;
+};