Chapter 27. Xref

Xref is a cross reference tool for analyzing dependencies between functions, modules, applications and releases. provides an interface to analyzing all except the releases.

Both predefined checks and custom queries are supported in

27.1. Usage

To run Xref with the default predefined checks:

$ make xref will error out when warnings are found.

The following predefined checks can be used:

  • undefined_function_calls
  • undefined_functions (similar to the previous)
  • locals_not_used (detected by the compiler)
  • exports_not_used
  • deprecated_function_calls (also detected by the compiler)
  • deprecated_functions (similar to the previous)
  • {deprecated_function_calls, Flag}
  • {deprecated_functions, Flag} (similar to the previous) will only run the undefined_function_calls check by default.

To change the check the XREF_CHECKS variable can be used:

$ make xref XREF_CHECKS=exports_not_used

Multiple checks can be run at once. The checks variable must be defined as an Erlang list:

$ make xref XREF_CHECKS="[undefined_function_calls, exports_not_used]" also supports informational analyses. Those will not error out when results are found, since they are not errors. To find all modules that call cowboy_req functions:

$ make xref XREF_CHECKS="{module_use, cowboy_req}"

The following informational checks are supported:

  • {call, MFA} - functions that MFA calls
  • {use, MFA} - functions that call MFA
  • {module_call, Mod} - modules that Mod calls (Mod depends on them)
  • {module_use, Mod} - modules that call Mod (they depend on Mod)
  • {application_call, App} - apps that App calls (App depends on them)
  • {application_use, App} - apps that call App (they depend on App)

The scope might need to be increased in order to obtain the complete results of informational checks. This is especially true for module and applications, with application results being dependent on the applications being added to the scope to be found.

27.2. Queries provides an interface to the Xref query functions. To perform a query, the q variable must be used instead of XREF_CHECKS. For example, to obtain all unresolved calls:

$ make xref q=UC

The query language is documented at the top of the Xref manual page.

27.3. Analysis scope will set the scope of analysis to the current project by default. The scope can be automatically extended to the applications from multi-application repositories, to dependencies and to the built-in Erlang/OTP applications themselves.

To change the scope, the XREF_SCOPE variable can be set. The variable can either be set in your Makefile or from the command line. The following values can be defined:

  • app - the current project
  • apps - applications from multi-application repositories
  • deps - dependencies
  • otp - Erlang/OTP applications

To get the most complete analysis possible they should all be added to the variable:

$ make xref XREF_CHECKS="{application_use, ssl}" XREF_SCOPE="app apps deps otp"
Application ssl is used by:
- my_app
- diameter
- eldap
- ftp
- inets
- ssl

Additional applications can be provided using the XREF_EXTRA_APP_DIRS variable. Note that these applications will need to be compiled before they can be found by Xref.

Similarly, non-application directories can be added using XREF_EXTRA_DIRS. The directory to be provided must be the one that contains the beam files.

27.4. Ignoring warnings

Sometimes it is necessary to ignore warnings because they are expected. This is the case for example when multiple Erlang/OTP versions must be supported and modules or functions have been added or removed. supports both project-wide configuration and Rebar-compatible inline ignores. To ignore warnings for a function in the current module the following line can be added to the source file:

-ignore_xref({log, 1}).

The module name can be specified explicitly:

-ignore_xref({my_mod, log, 1}).

As well as a full module can be ignored:


The ignored functions can be provided as a list:

-ignore_xref([{log, 1}, {pretty_print, 1}]).

The XREF_IGNORE variable can be used to define functions and modules to ignore project-wide. It accepts either MFAs or modules:

XREF_IGNORE = {my_mod, log, 1}

Multiple ignores must be provided as an Erlang list:

XREF_IGNORE = [{my_mod, log, 1}, other_mod]

By default will ignore unused exports for behavior callbacks when the exports_not_used check is run. It is possible to override this behavior, or to ignore the callbacks for queries and other checks, by defining the XREF_IGNORE_CALLBACKS variable:

$ make xref XREF_CHECKS=exports_not_used XREF_IGNORE_CALLBACKS=0