Function BufFileRead

src/backend/storage/file/buffile.c:724–774

* BufFileRead * * Like fread() except we assume 1-byte element size and report I/O errors via * ereport(). */

 * ereport().
 */
size_t
BufFileRead(BufFile *file, void *ptr, size_t size)
{
	size_t		nread = 0;
	size_t		nthistime;

	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");
			break;

		case BFS_COMPRESSED_READING:
			return BufFileLoadCompressedBuffer(file, ptr, size);
	}

	BufFileFlush(file);

	while (size > 0)
	{
		if (file->pos >= file->nbytes)
		{
			/* Try to load more data into buffer. */
			file->curOffset += file->pos;
			file->pos = 0;
			file->nbytes = 0;
			BufFileLoadBuffer(file);
			if (file->nbytes <= 0)
				break;			/* no more data available */
		}

		nthistime = file->nbytes - file->pos;
		if (nthistime > size)
			nthistime = size;
		Assert(nthistime > 0);

		memcpy(ptr, file->buffer.data + file->pos, nthistime);

		file->pos += nthistime;
		ptr = (void *) ((char *) ptr + nthistime);
		size -= nthistime;
		nread += nthistime;
	}

	return nread;
}

/*
 * BufFileReadFromBuffer