Linking with the mapfile option

Linking with the mapfile option

The ELF linker (ld) automatically maps input sections from object files (.o files) to output segments in executable files (a.out files). The mapfile option to the ld command allows you to change the default mapping provided by the ELF linker.

In particular, the mapfile option allows you to:

declare segments and specify values for segment attributes such as segment type, permissions, addresses, length, and alignment
control mapping of input sections to segments by specifying the attribute values necessary in a section to map to a specific segment (the attributes are section name, section type, and permissions) and by specifying which object file(s) the input sections should be taken from, if necessary
declare a global-absolute symbol that is assigned a value equal to the size of a specified segment (by the linker) and that can be referenced from object files

NOTE: The major purpose of the mapfile option is to allow users of ifiles (an option previously available to ld that used link editor command language directives) to convert to mapfiles. All other facilities previously available for ifiles, other than those mentioned above, are not available with the mapfile option.

CAUTION: When using the mapfile option, be aware that you can easily create a.out files that do not execute. Therefore, the use of the mapfile option is strongly discouraged. ld can produce a correct a.out without the use of the mapfile option. The mapfile option is intended for system programming use, not application programming use.

This appendix describes the structure and syntax of a mapfile and the use of the -M option to the ld command.