Internal map structure

One of the most important data structures in the ELF-based ld is the map structure. A default map structure, corresponding to the model default mapfile mentioned above, is used by ld when the command is executed. Then, if the mapfile option is used, ld parses the mapfile to augment and/or override certain values in the default map structure.

A typical (although somewhat simplified) map structure is illustrated in ``Simple map structure''. The ``Entrance Criteria'' boxes correspond to the information in the default mapping directives and the ``Segment Attribute Descriptors'' boxes correspond to the information in the default segment declarations. The ``Output Section Descriptors'' boxes give the detailed attributes of the sections that fall under each segment. The sections themselves are in circles.

Simple map structure

ld performs the following steps when mapping sections to segments:

1. When a section is read in, ld checks the list of Entrance Criteria looking for a match. (All specified criteria must match):

   • In ``Simple map structure'', for a section to fall into the text segment it must have a section_type value of $PROGBITS and have a section_flags value of ?A!W. It need not have the name .text since no name is specified in the Entrance Criteria. The section may be either X or !X (in the section_flags value) since nothing was specified for the execute bit in the Entrance Criteria.

   • If no Entrance Criteria match is found, the section is placed at the end of the a.out file after all other segments. No program header entry is created for this information. See ``ELF object files'' for information on program headers.
   
   

2. When the section falls into a segment, ld checks the list of existing Output Section Descriptors in that segment as follows:

   • If the section attribute values match those of an existing Output Section Descriptor exactly, the section is placed at the end of the list of sections associated with that Output Section Descriptor.

     For instance, a section with a section_name value of .data1, a section_type value of $PROGBITS, and a section_flags value of ?AWX falls into the second Entrance Criteria box in ``Simple map structure'', placing it in the data segment. The section matches the second Output Section Descriptor box exactly (.data1, $PROGBITS, ?AWX) and is added to the end of the list associated with that box. The .data1 sections from fido.o, rover.o, and sam.o illustrate this point.

   • If no matching Output Section Descriptor is found, but other Output Section Descriptors of the same section_type exist, a new Output Section Descriptor is created with the same attribute values as the section and that section is associated with the new Output Section Descriptor. The Output Section Descriptor (and the section) are placed after the last Output Section Descriptor of the same section_type. The .data2 section in ``Simple map structure'' was placed in this manner.
     

   • If no other Output Section Descriptors of the indicated section_type exist, a new Output Section Descriptor is created and the section is placed so as to maintain the following section_type order:

     $DYNAMIC
     $PROGBITS
     $SYMTAB
     $STRTAB
     $RELA
     $REL
     $HASH
     $NOTE
     $NOBITS
     

     The .bss section in ``Simple map structure'' illustrates this point.

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     NOTE: If the input section has a user-defined section_type value (that is, between SHT_LOUSER and SHT_HIUSER) it is treated as a $PROGBITS section. No method exists for naming this section_type value in the mapfile, but these sections can be redirected using the other attribute value specifications (section_flags, section_name) in the entrance criteria. For more information, see ``Section header''.

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3. If a segment contains no sections after all of the command line object files and libraries have been read in, no program header entry is produced for that segment.