Mapping directives

A mapping directive tells ld how to map input sections to segments. Basically, you name the segment that you are mapping to and indicate what the attributes of a section must be in order to map into the named segment. The set of section_attribute_values that a section must have to map into a specific segment is called the entrance criteria for that segment. In order to be placed in a specified segment of the a.out, a section must meet the entrance criteria for a segment exactly.

A mapping directive has the following syntax:

segment_name: {section_attribute_value} [: {file_name}+];

For a segment_name, you specify any number of section_attribute_values in any order, each separated by a space. (At most one section attribute value is allowed for each section attribute.) You can also specify that the section must come from a certain .o file(s) via the file_name substitutable. The section attributes and their valid values are as follows:

section_name:

any valid section name

section_type:

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

section_flags:

?[[!]A][[!]W][[!]X]

• You must choose at most one section_type from the section_types listed above. The section_types listed above are built-in types. For more information on section_types, see ``Section header''.

• The section_flags values are A for allocatable, W for writable, or X for executable. If an individual flag is preceded by an exclamation mark (!), the linker checks to make sure that the flag is not set. No spaces are allowed between the question mark, exclamation point(s), and the individual flags that make up the section_flags value.

• file_name may be any valid file name and can be of the form archive_name(component_name), for example:
  
  /usr/lib/usr/libc.a(printf.o)

  A file name may be of the form file_name (see next bullet item). Note that ld does not check the syntax of file names.

• If a file_name is of the form file_name, ld simulates a basename(C) on the file name from the command line and uses that to match against the mapfile file_name. In other words, the file_name from the mapfile only needs to match the last part of the file name from the command line. (See ``Mapping example'' for more information.)

• If you use the -l option on the cc or ld command line, and the library after the -l option is in the current directory, you must precede the library with ``./'' (or the entire path name) in the mapfile in order to create a match.
  

• More than one directive line may appear for a particular output segment, for example, the following set of directives is valid:

  S1 : $PROGBITS;
  S1 : $NOBITS;
  

  Entering more than one mapping directive line for a segment is the only way to specify multiple values of a section attribute.

• A section can match more than one entrance criteria. In this case, the first segment encountered in the mapfile with that entrance criteria is used, for example, if a mapfile reads:

  S1 : $PROGBITS;
  S2 : $PROGBITS;
  

  the $PROGBITS sections are mapped to segment S1.