3.2. Linker

The linker (in our case, GNU ld) is responsible for taking the raw machine code from the compiler/assembler and creating a valid ELF file. ELF files consist of several different sections and have many different abilities. In other words, laying out an ELF file can be quite involved considering all the options that are available and all the information that needs to be stored. The linker, therefore, uses a script which helps guide how the output file is laid out. If you don't supply a script, there is an implicit default one provided for you.

The different sections and their attributes are used by, for example, the operating system's loader (when you want to execute an application). For example, some sections contain executable code that needs to be loaded into memory, other sections don't contain any data at all, but rather they instruct the loader to allocate some memory for the application to use (sometimes this memory even needs to be explicitly zeroed). Some sections aren't used by the loader at all, for example, sections that contain debugging information are of no use to the loader but are used by a debugger instead.

By default, executable code you write ends up in a section called .text, initialised data in a .data section, read-only data in .rodata, uninitialised data in .bss...