[Linker Script Tutorial] $1- What is Linker Scripts

Every link is controlled by a linker script. This script is written in the linker command language.
The main purpose of the linker script is to describe how the sections in the input files should be mapped into the output file, and to control the memory layout of the output file. Most linker scripts do nothing more than this. However, when necessary, the linker script can also direct the linker to perform many other operations, using the commands described below.
The linker always uses a linker script. If you do not supply one yourself, the linker will use a default script that is compiled into the linker executable. You can use the ‘--verbose’ command-line option to display the default linker script. Certain command-line options, such as ‘-r’ or ‘-N’, will affect the default linker script.
You may supply your own linker script by using the ‘-T’ command line option. When you do this, your linker script will replace the default linker script.
You may also use linker scripts implicitly by naming them as input files to the linker, as though they were files to be linked.

Content of Tutorial:

• Basic Script Concepts:		Basic Linker Script Concepts
• Script Format:		Linker Script Format
• Simple Example:		Simple Linker Script Example
• Simple Commands:		Simple Linker Script Commands
• Assignments:		Assigning Values to Symbols
• SECTIONS:		SECTIONS Command
• MEMORY:		MEMORY Command
• PHDRS:		PHDRS Command
• VERSION:		VERSION Command
• Expressions:		Expressions in Linker Scripts
• Implicit Linker Scripts:		Implicit Linker Scripts

We need to define some basic concepts and vocabulary in order to describe the linker script language.
The linker combines input files into a single output file. The output file and each input file are in a special data format known as an object file format. Each file is called an object file. The output file is often called an executable, but for our purposes we will also call it an object file. Each object file has, among other things, a list of sections. We sometimes refer to a section in an input file as an input section; similarly, a section in the output file is an output section.
Each section in an object file has a name and a size. Most sections also have an associated block of data, known as the section contents. A section may be marked as loadable, which means that the contents should be loaded into memory when the output file is run. A section with no contents may be allocatable, which means that an area in memory should be set aside, but nothing in particular should be loaded there (in some cases this memory must be zeroed out). A section which is neither loadable nor allocatable typically contains some sort of debugging information.
Every loadable or allocatable output section has two addresses. The first is the VMA, or virtual memory address. This is the address the section will have when the output file is run. The second is the LMA, or load memory address. This is the address at which the section will be loaded. In most cases the two addresses will be the same. An example of when they might be different is when a data section is loaded into ROM, and then copied into RAM when the program starts up (this technique is often used to initialize global variables in a ROM based system). In this case the ROM address would be the LMA, and the RAM address would be the VMA.
You can see the sections in an object file by using the objdump program with the ‘-h’ option.
Every object file also has a list of symbols, known as the symbol table. A symbol may be defined or undefined. Each symbol has a name, and each defined symbol has an address, among other information. If you compile a C or C++ program into an object file, you will get a defined symbol for every defined function and global or static variable. Every undefined function or global variable which is referenced in the input file will become an undefined symbol.
You can see the symbols in an object file by using the nm program, or by using the objdump program with the ‘-t’ option.