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Some sections occur in interrelated groups. For example, an out-of-line definition of an inline function might require additional information besides the section containing executable instructions. This additional information can be a read-only data section containing literals referenced, one or more debugging information sections, or other informational sections.
There can be internal references among group sections. However, these references make no sense if one of the sections were removed, or one of the sections were replaced by a duplicate from another object. Therefore, these groups are included, or these groups are 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 the value zero. 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.
Table 12-11 ELF Group Section Flag
GRP_COMDAT is a COMDAT group. This group can 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 is retained by the link-editor. The members of the remaining groups are discarded.
The section header indices in the SHT_GROUP section, identify the sections that make up the group. These sections must have the SHF_GROUP flag set in their sh_flags section header member. If the link-editor decides to remove the section group, the link-editor removes 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. A definition of the same symbol in the object containing the reference must have a separate symbol table entry from the reference. Sections outside of the group can not reference symbols with STB_LOCAL binding for addresses that are contained in the group's sections, including symbols with type STT_SECTION.
Non-symbol references to the sections comprising a group are not allowed 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 defined relative to one of the group's sections can be removed if the group members are discarded. This removal occurs if the symbol table entry is contained in a symbol table section that is not part of the group.