lavu: add a new API for reference-counted data buffers.

1 files changed, 197 insertions, 0 deletions

diff --git a/libavutil/buffer.h b/libavutil/buffer.h
new file mode 100644
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--- /dev/null
+++ b/libavutil/buffer.h
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+/*
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file
+ * @ingroup lavu_buffer
+ * refcounted data buffer API
+ */
+
+#ifndef AVUTIL_BUFFER_H
+#define AVUTIL_BUFFER_H
+
+#include <stdint.h>
+
+/**
+ * @defgroup lavu_buffer AVBuffer
+ * @ingroup lavu_data
+ *
+ * @{
+ * AVBuffer is an API for reference-counted data buffers.
+ *
+ * There are two core objects in this API -- AVBuffer and AVBufferRef. AVBuffer
+ * represents the data buffer itself; it is opaque and not meant to be accessed
+ * by the caller directly, but only through AVBufferRef. However, the caller may
+ * e.g. compare two AVBuffer pointers to check whether two different references
+ * are describing the same data buffer. AVBufferRef represents a single
+ * reference to an AVBuffer and it is the object that may be manipulated by the
+ * caller directly.
+ *
+ * There are two functions provided for creating a new AVBuffer with a single
+ * reference -- av_buffer_alloc() to just allocate a new buffer, and
+ * av_buffer_create() to wrap an existing array in an AVBuffer. From an existing
+ * reference, additional references may be created with av_buffer_ref().
+ * Use av_buffer_unref() to free a reference (this will automatically free the
+ * data once all the references are freed).
+ *
+ * The convention throughout this API and the rest of Libav is such that the
+ * buffer is considered writable if there exists only one reference to it (and
+ * it has not been marked as read-only). The av_buffer_is_writable() function is
+ * provided to check whether this is true and av_buffer_make_writable() will
+ * automatically create a new writable buffer when necessary.
+ * Of course nothing prevents the calling code from violating this convention,
+ * however that is safe only when all the existing references are under its
+ * control.
+ *
+ * @note Referencing and unreferencing the buffers is thread-safe and thus
+ * may be done from multiple threads simultaneously without any need for
+ * additional locking.
+ *
+ * @note Two different references to the same buffer can point to different
+ * parts of the buffer (i.e. their AVBufferRef.data will not be equal).
+ */
+
+/**
+ * A reference counted buffer type. It is opaque and is meant to be used through
+ * references (AVBufferRef).
+ */
+typedef struct AVBuffer AVBuffer;
+
+/**
+ * A reference to a data buffer.
+ *
+ * The size of this struct is not a part of the public ABI and it is not meant
+ * to be allocated directly.
+ */
+typedef struct AVBufferRef {
+    AVBuffer *buffer;
+
+    /**
+     * The data buffer. It is considered writable if and only if
+     * this is the only reference to the buffer, in which case
+     * av_buffer_is_writable() returns 1.
+     */
+    uint8_t *data;
+    /**
+     * Size of data in bytes.
+     */
+    int      size;
+} AVBufferRef;
+
+/**
+ * Allocate an AVBuffer of the given size using av_malloc().
+ *
+ * @return an AVBufferRef of given size or NULL when out of memory
+ */
+AVBufferRef *av_buffer_alloc(int size);
+
+/**
+ * Same as av_buffer_alloc(), except the returned buffer will be initialized
+ * to zero.
+ */
+AVBufferRef *av_buffer_allocz(int size);
+
+/**
+ * Always treat the buffer as read-only, even when it has only one
+ * reference.
+ */
+#define AV_BUFFER_FLAG_READONLY (1 << 0)
+
+/**
+ * Create an AVBuffer from an existing array.
+ *
+ * If this function is successful, data is owned by the AVBuffer. The caller may
+ * only access data through the returned AVBufferRef and references derived from
+ * it.
+ * If this function fails, data is left untouched.
+ * @param data   data array
+ * @param size   size of data in bytes
+ * @param free   a callback for freeing data
+ * @param opaque parameter to be passed to free
+ * @param flags  a combination of AV_BUFFER_FLAG_*
+ *
+ * @return an AVBufferRef referring to data on success, NULL on failure.
+ */
+AVBufferRef *av_buffer_create(uint8_t *data, int size,
+                              void (*free)(void *opaque, uint8_t *data),
+                              void *opaque, int flags);
+
+/**
+ * Default free callback, which calls av_free() on the buffer data.
+ * This function is meant to be passed to av_buffer_create(), not called
+ * directly.
+ */
+void av_buffer_default_free(void *opaque, uint8_t *data);
+
+/**
+ * Create a new reference to an AVBuffer.
+ *
+ * @return a new AVBufferRef referring to the same AVBuffer as buf or NULL on
+ * failure.
+ */
+AVBufferRef *av_buffer_ref(AVBufferRef *buf);
+
+/**
+ * Free a given reference and automatically free the buffer if there are no more
+ * references to it.
+ *
+ * @param buf the reference to be freed. The pointer is set to NULL on return.
+ */
+void av_buffer_unref(AVBufferRef **buf);
+
+/**
+ * @return 1 if the caller may write to the data referred to by buf (which is
+ * true if and only if buf is the only reference to the underlying AVBuffer).
+ * Return 0 otherwise.
+ * A positive answer is valid until av_buffer_ref() is called on buf.
+ */
+int av_buffer_is_writable(const AVBufferRef *buf);
+
+/**
+ * Create a writable reference from a given buffer reference, avoiding data copy
+ * if possible.
+ *
+ * @param buf buffer reference to make writable. On success, buf is either left
+ *            untouched, or it is unreferenced and a new writable AVBufferRef is
+ *            written in its place. On failure, buf is left untouched.
+ * @return 0 on success, a negative AVERROR on failure.
+ */
+int av_buffer_make_writable(AVBufferRef **buf);
+
+/**
+ * Reallocate a given buffer.
+ *
+ * @param buf  a buffer reference to reallocate. On success, buf will be
+ *             unreferenced and a new reference with the required size will be
+ *             written in its place. On failure buf will be left untouched. *buf
+ *             may be NULL, then a new buffer is allocated.
+ * @param size required new buffer size.
+ * @return 0 on success, a negative AVERROR on failure.
+ *
+ * @note the buffer is actually reallocated with av_realloc() only if it was
+ * initially allocated through av_buffer_realloc(NULL) and there is only one
+ * reference to it (i.e. the one passed to this function). In all other cases
+ * a new buffer is allocated and the data is copied.
+ */
+int av_buffer_realloc(AVBufferRef **buf, int size);
+
+/**
+ * @}
+ */
+
+#endif /* AVUTIL_BUFFER_H */