Function BufFileReadFromBuffer

src/backend/storage/file/buffile.c:785–815

* BufFileReadFromBuffer * * This function provides a faster implementation of Read which applies * when the data is already in the underlying buffer. * In that case, it returns a pointer to the data in the buffer * If the data is not in the buffer, returns NULL and the caller must * call the regular BufFileRead with a destination buffer. */

 * call the regular BufFileRead with a destination buffer.
 */
void *
BufFileReadFromBuffer(BufFile *file, size_t size)
{
	void	   *result = NULL;

	switch (file->state)
	{
		case BFS_RANDOM_ACCESS:
		case BFS_SEQUENTIAL_READING:
			break;

		case BFS_SEQUENTIAL_WRITING:
		case BFS_COMPRESSED_WRITING:
			elog(ERROR, "cannot read from sequential BufFile before rewinding to start");
			return NULL;

		case BFS_COMPRESSED_READING:
			return NULL;
	}

	if (file->dirty)
		BufFileFlush(file);

	if (file->pos + size < file->nbytes)
	{
		result = file->buffer.data + file->pos;
		file->pos += size;
	}

	return result;
}

/*
 * BufFileWrite