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GNU Linker Scripts

Authors:
Geoffrey Hunter
Geoffrey Hunter
Published On:
Sep 17, 2024
Last Updated:
Sep 17, 2024

Linker scripts use the .ld file extension.

Entry Point

The entry point of a program can be specified in a few different ways1:

  1. Using the ENTRY command.
  2. The value to the start symbol.
  3. The address of the first byte of the .text section.
  4. The address 0.

For example, if we were to use the ENTRY() command:

ENTRY(reset_handler)

Memory Layout

You can use the MEMORY command to describe the size and location of memory “blocks” in the target system. Based on this information, the linker will place requested sections into the correct memory blocks and also raise errors if any block overflows.

A .ld file can contain only one MEMORY command. Inside the command, you can define as many blocks of memory as you want. The syntax is1:

MEMORY
{
  name1 (attr) : ORIGIN = origin1, LENGTH = length1
  ...
}
  • name1 is the name of the memory block, used internally by the linker to refer to that region.
  • attr is a list of attributes for the memory block. The supported attributes are:
    • r for read-only.
    • w for read/write sections.
    • x for executable sections.
    • a for allocatable sections.
    • i for initialized sections.
    • l same as i.
  • origin1 is the starting address of the memory block.
  • length1 is the size (in bytes) of the memory block.

Sections

The following example creates an output file with three sections: .text, .data, and .bss. It takes all input files and takes the corresponding sections from them (.text goes to .text) and places them in the output file:

SECTIONS {
  .text : { *(.text) }
  .data : { *(.data) }
  .bss :  { *(.bss)  *(COMMON) }
}

Footnotes

  1. GNU. Manuals > ld v2.9.1. > Command Language. Retrieved 2024-09-17, from https://ftp.gnu.org/old-gnu/Manuals/ld-2.9.1/html_chapter/ld_3.html. ↩ ↩2