The operations performed at link time usually include fixing up the addresses of externally referenced objects and functions, various kinds of cross module checks (e.g. type checks on externally visible identifiers and in some languages instantiation of templates).
Some optimizing compilers delay code generation until link time because it is here that information about a complete program is available to them.
Resolving external variables in a program is also done at link time.
The link-time optimization (LTO), when enabled, occurs at link time.
The definition of a programming language may specify link time requirements that source code must meet to be successfully compiled (e.g. the maximum number of characters in an externally visible identifier that must be considered significant).