12.5.5.4 Using Multi-Level Subsections

Subsections can be identified with the base section name and one or more subsection names separated by colons. For example, A:B and A:B:C name subsections of the base section A. In certain places in a linker command file specifying a base name, such as A, selects the section A as well as any subsections of A, such as A:B or A:C:D.

A name such as A:B can specify a (sub)section of that name as well as any (multi-level) subsections beginning with that name, such as A:B:C, A:B:OTHER, etc. All subsections of A:B are also subsections of A. A and A:B are supersections of A:B:C. Among a group of supersections of a subsection, the nearest supersection is the supersection with the longest name. Thus, among {A, A:B} the nearest supersection of A:B:C:D is A:B. With multiple levels of subsections, the constraints are the following:

When specifying input sections within a file (or library unit) the section name selects an input section of the same name and any subsections of that name.
Input sections that are not explicitly allocated are allocated in an existing output section of the same name or in the nearest existing supersection of such an output section. An exception to this rule is that during a partial link (specified by the --relocatable linker option) a subsection is allocated only to an existing output section of the same name.
If no such output section described in 2) is defined, the input section is put in a newly created output section with the same name as the base name of the input section

Consider linking input sections with the following names:
europe:north:norway	europe:central:france	europe:south:spain
europe:north:sweden	europe:central:germany	europe:south:italy
europe:north:finland	europe:central:denmark	europe:south:malta
europe:north:iceland

This SECTIONS specification allocates the input sections as indicated in the comments:

  SECTIONS {
    nordic:  {*(europe:north)
              *(europe:central:denmark)} /* the nordic countries */
    central: {*(europe:central)}         /* france, germany      */
    therest: {*(europe)}                 /* spain, italy, malta  */
  }

This SECTIONS specification allocates the input sections as indicated in the comments:

  SECTIONS {
    islands: {*(europe:south:malta)
              *(europe:north:iceland)}  /* malta, iceland   */
    europe:north:finland : {}           /* finland          */
    europe:north         : {}           /* norway, sweden   */
    europe:central       : {}           /* germany, denmark */
    europe:central:france: {}           /* france           */

    /* (italy, spain) go into a linker-generated output section "europe" */
  }

NOTE

Upward Compatibility of Multi-Level Subsections

Existing linker commands that use the existing single-level subsection features and which do not contain section names containing multiple colon characters continue to behave as before. However, if section names in a linker command file or in the input sections supplied to the linker contain multiple colon characters, some change in behavior could be possible. You should carefully consider the impact of the rules for multiple levels to see if it affects a particular system link.