Section Groups

Some sections occur in interrelated groups. For example, an out-of-line definition of an inline function might require, in addition to the section containing its executable instructions, a read-only data section containing literals referenced, one or more debugging information sections and other informational sections. Furthermore, there may be internal references among these sections that would not make sense if one of the sections were removed or replaced by a duplicate from another object. Therefore, such groups must be included or omitted from the linked object as a unit.

A section of type SHT_GROUP defines such a grouping of sections. The name of a symbol from one of the containing object's symbol tables provides a signature for the section group. The section header of the SHT_GROUP section specifies the identifying symbol entry. The sh_link member contains the section header index of the symbol table section that contains the entry. The sh_info member contains the symbol table index of the identifying entry. The sh_flags member of the section header contains 0. The name of the section (sh_name) is not specified.

The section data of a SHT_GROUP section is an array of Elf32_Word entries. The first entry is a flag word. The remaining entries are a sequence of section header indices.

The following flag is currently defined:

Table 7-16 ELF Section Group Flag

Name	Value
GRP_COMDAT	0x1

GRP_COMDAT

This is a COMDAT group. It may duplicate another COMDAT group in another object file, where duplication is defined as having the same group signature. In such cases, only one of the duplicate groups will be retained by the link-editor, and the members of the remaining groups will be discarded.

The section header indices in the SHT_GROUP section identify the sections that make up the group. Each such section must have the SHF_GROUP flag set in its sh_flags section header member. If the link-editor decides to remove the section group, it will remove all members of the group.

To facilitate removing a group without leaving dangling references and with only minimal processing of the symbol table, the following rules are followed:

• References to the sections comprising a group from sections outside of the group must be made through symbol table entries with STB_GLOBAL or STB_WEAK binding and section index SHN_UNDEF. If there is a definition of the same symbol in the object containing the references, it must have a separate symbol table entry from the references. Sections outside of the group may not reference symbols with STB_LOCAL binding for addresses contained in the group's sections, including symbols with type STT_SECTION.

• There may not be non-symbol references to the sections comprising a group from outside the group. For example, you cannot use a group member's section header index in an sh_link or sh_info member.

• A symbol table entry that is defined relative to one of the group's sections and that is contained in a symbol table section that is not part of the group, will be removed if the group members are discarded.