Add io_pread().

It will be used by --list.

src/xz/file_io.c

@@ -812,6 +812,31 @@ io_read(file_pair *pair, io_buf *buf_union, size_t size)
 }
 
 
+extern bool
+io_pread(file_pair *pair, io_buf *buf, size_t size, off_t pos)
+{
+	// Using lseek() and read() is more portable than pread() and
+	// for us it is as good as real pread().
+	if (lseek(pair->src_fd, pos, SEEK_SET) != pos) {
+		message_error(_("%s: Error seeking the file: %s"),
+				pair->src_name, strerror(errno));
+		return true;
+	}
+
+	const size_t amount = io_read(pair, buf, size);
+	if (amount == SIZE_MAX)
+		return true;
+
+	if (amount != size) {
+		message_error(_("%s: Unexpected end of file"),
+				pair->src_name);
+		return true;
+	}
+
+	return false;
+}
+
+
 static bool
 is_sparse(const io_buf *buf)
 {

src/xz/file_io.h

@@ -23,6 +23,7 @@
 /// Use an union to make sure that the buffer is properly aligned.
 typedef union {
 	uint8_t u8[IO_BUFFER_SIZE];
+	uint32_t u32[IO_BUFFER_SIZE / sizeof(uint32_t)];
 	uint64_t u64[IO_BUFFER_SIZE / sizeof(uint64_t)];
 } io_buf;
 
@@ -97,6 +98,22 @@ extern void io_close(file_pair *pair, bool success);
 extern size_t io_read(file_pair *pair, io_buf *buf, size_t size);
 
 
+/// \brief      Read from source file from given offset to a buffer
+///
+/// This is remotely similar to standard pread(). This uses lseek() though,
+/// so the read offset is changed on each call.
+///
+/// \param      pair    Seekable source file
+/// \param      buf     Destination buffer
+/// \param      size    Amount of data to read
+/// \param      pos     Offset relative to the beginning of the file,
+///                     from which the data should be read.
+///
+/// \return     On success, false is returned. On error, error message
+///             is printed and true is returned.
+extern bool io_pread(file_pair *pair, io_buf *buf, size_t size, off_t pos);
+
+
 /// \brief      Writes a buffer to the destination file
 ///
 /// \param      pair    File pair having the destination file open for writing