IS_ORDER Attribute

The link-editor normally places output sections into the segment in the order they are encountered. Similarly, the input sections that make up the output section are placed in the order they are encountered. The IS_ORDER attribute can be used to alter this default placement of input sections. IS_ORDER specifies a space separated list of entrance criterion names (assign_name). Sections matched by one of these entrance criteria are placed at the head of the output section, sorted in the order given by IS_ORDER. Sections matched by entrance criteria not found in the IS_ORDER list are placed following the sorted sections, in the order they are encountered.

When the “=” form of assignment is used, the previous value of IS_ORDER for the given segment is discarded, and replaced with the new list. The “+=” form of IS_ORDER concatenates the new list to the end of the existing list.

The IS_ORDER attribute is of particular interest when used in conjunction with the -xF option to the compilers. When a file is compiled with the -xF option, each function in that file is placed in a separate section with the same attributes as the text section. These sections are called .text%function_name.

For example, a file containing three functions, main(), foo() and bar(), when compiled with the -xF option, yields a relocatable object file with text for the three functions being placed in sections called .text%main, .text%foo, and .text%bar. When the link-editor places these sections into the output, the % and anything following the % are removed. Hence, all three of these functions will be placed in the .text output section. The IS_ORDER attribute can be used to force them to be placed in a specific order within the .text output section relative to each other.

Consider the following user-defined mapfile.

        $mapfile_version 2
                LOAD_SEGMENT text {
                ASSIGN_SECTION text_bar  { IS_NAME = .text%bar };
                ASSIGN_SECTION text_main { IS_NAME = .text%main };
                ASSIGN_SECTION text_foo  { IS_NAME = .text%foo };
                IS_ORDER = text_foo text_bar text_main;
        };

No matter the order in which these three functions are found in the source code, or encountered by the link-editor, their order in the output object text segment will be foo(), bar(), and main().