In computing, a linker or link editor is a computer utility program that takes one or more object files generated by a compiler or an assembler and combines them into a single executable file, library file, or another 'object' file.

Computer programs (i.e. Arduino sketches in this case) typically are composed of several parts or modules; these parts/modules need not all be contained within a single object file, and in such cases refer to each other by means of symbols as addresses into other modules, which are mapped into memory addresses when linked for execution. Typically, an object file can contain three kinds of symbols:

  • defined "external" symbols, sometimes called "public" or "entry" symbols, which allow it to be called by other modules,
  • undefined "external" symbols, which reference other modules where these symbols are defined, and
  • local symbols, used internally within the object file to facilitate relocation.

For most compilers, each object file is the result of compiling one input source code file. When a program comprises multiple object files, the linker combines these files into a unified executable program, resolving the symbols as it goes along.