Linker Script

• Link script file
  • Embedded Programming with the GNU Toolchain
    • 7. Linker Script File
  • ld Documentation for binutils 2.25
    • 3 Linker Scripts
  • vmlinux.lds.S
    • UnderstandingLinkerScript
    • Linux Kernel程式碼分段解析
    • 移植内核必备知识
      • 对linux的early printk的探讨

Linker defined symbol

• Source Code Reference: How to use a linker script defined symbol in source code

PROVIDE

• define a symbol only if it is referenced and is not defined by any object included in the link.

Location Counter

• '.' always contains the current output location counter.
• The location counter holds the VMA value, not the LMA value.
  • Specifying address for a section will change the value of the location counter.
    • The address is an expression for the VMA (the virtual memory address) of the output section.
• It assumes that all assignments or other statements belong to the previous output section, except for the special case of an assignment to '.'.

SECTIONS

Entry Point

• Entry points set by the ENTRY directive
  • You create entry points in objects with the ENTRY directive in an assembler file.
  • Typical use of this directive is to mark code that is entered through the processor exception vectors, such as RESET, IRQ, and FIQ.
  • The directive marks the output code section with an ENTRY keyword that instructs the linker not to remove the section when it performs unused section elimination.
• Initial entry point
  • The initial entry point for an image is a single value that is stored in the ELF header file.
  • You can select one of many possible entry points for an image. An image can have only one entry point.
  • If an embedded image is to be used by a loader, it must have a single initial entry point specified in the header. Use the $--$entry command-line option to select the entry point.
  • If you do not use the —entry option to specify the initial entry point then:
    • If the input objects contain only one entry point set by the ENTRY directive, the linker uses that entry point as the initial entry point for the image.
    • The linker generates an image that does not contain an initial entry point when either:
      • More than one entry point has been specified by using the ENTRY directive.
      • No entry point has been specified by using the ENTRY directive.

Output Sections

section [address] [(type)] :
[AT(lma)]
[ALIGN(section_align)]
[SUBALIGN(subsection_align)]
[constraint]
{
output-section-command
output-section-command
...
} [>region] [AT>lma_region] [:phdr :phdr ...] [=fillexp]

Load Address

section_name : AT (load_address) { * (section_name); }

MEMORY

MEMORY
{
name [(attr)] : ORIGIN = origin, LENGTH = len
...
}

• The expression ‘*(.text)’ means all ‘.text’ input sections in all input files.

*(.text)

• 將作用的函數或資料擺入指定名為"section_name"匯入段。

__attribute__((section("section_name")))

# 變數var將被擺入名為.xdata的匯入段
int var __attribute__((section(".xdata"))) = 0;

# 使函數functionA被擺入名叫.xinit的匯入段
static int __attribute__((section(".xinit"))) functionA(void)
{
.....
}

• Embedded Programming with the GNU Toolchain - 10. C Startup