avio: Add an internal function for reading without copying

As long as there is enough contiguous data in the avio buffer,
just return a pointer to it instead of copying it to the caller
provided buffer.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/avio_internal.h

@@ -37,6 +37,23 @@ int ffio_init_context(AVIOContext *s,
                   int64_t (*seek)(void *opaque, int64_t offset, int whence));
 
 
+/**
+ * Read size bytes from AVIOContext, returning a pointer.
+ * Note that the data pointed at by the returned pointer is only
+ * valid until the next call that references the same IO context.
+ * @param s IO context
+ * @param buf pointer to buffer into which to assemble the requested
+ *    data if it is not available in contiguous addresses in the
+ *    underlying buffer
+ * @param size number of bytes requested
+ * @param data address at which to store pointer: this will be a
+ *    a direct pointer into the underlying buffer if the requested
+ *    number of bytes are available at contiguous addresses, otherwise
+ *    will be a copy of buf
+ * @return number of bytes read or AVERROR
+ */
+int ffio_read_indirect(AVIOContext *s, unsigned char *buf, int size, unsigned char **data);
+
 /**
  * Read size bytes from AVIOContext into buf.
  * This reads at most 1 packet. If that is not enough fewer bytes will be

libavformat/aviobuf.c

@@ -490,6 +490,18 @@ int avio_read(AVIOContext *s, unsigned char *buf, int size)
     return size1 - size;
 }
 
+int ffio_read_indirect(AVIOContext *s, unsigned char *buf, int size, unsigned char **data)
+{
+    if (s->buf_end - s->buf_ptr >= size && !s->write_flag) {
+        *data = s->buf_ptr;
+        s->buf_ptr += size;
+        return size;
+    } else {
+        *data = buf;
+        return avio_read(s, buf, size);
+    }
+}
+
 int ffio_read_partial(AVIOContext *s, unsigned char *buf, int size)
 {
     int len;