MantisBT - CMake
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IDProjectCategoryView StatusDate SubmittedLast Update
0015231CMakeCMakepublic2014-10-31 12:312016-06-10 14:31
ReporterCharles Karney 
Assigned ToTamas Kenez 
PrioritynormalSeverityfeatureReproducibilityN/A
StatusclosedResolutionmoved 
PlatformOSOS Version
Product VersionCMake 3.0.2 
Target VersionFixed in Version 
Summary0015231: find_package: should be a simple way to alter the order of the config/module lookups
DescriptionAs we are integrating cmake more and more into our environment, it is
clear how much superior the config-mode for find_package is compared to
the module-mode. As a result, our top-level CMakeLists.txt contains
several occurrences of the following pattern

  find_package (PROJ4 CONFIG QUIET)
  if (NOT PROJ4_FOUND)
    find_package (PROJ4 MODULE REQUIRED)
  endif ()

Request 1: Add

  MODULE_FIRST
  CONFIG_FIRST

options to find_package, meaning do the module lookup first (the default
but see next) or the config lookup first. (These options complement the
MODULE and CONFIG options.) So the find_package invocation above
becomes

  find_package (PROJ4 CONFIG_FIRST REQUIRED)

Request 2: Add a global variable, e.g., CMAKE_FIND_PACKAGE_PREFER_CONFIG
(default OFF) to specify the default lookup order. Thus if I've worked
to get all my dependent packages with config-mode support, I can invoke
cmake with

  cmake -DCMAKE_FIND_PACKAGE_PREFER_CONFIG=ON ..

without having to edit the CMakeLists.txt file.

Finally a comment: the advice (not sure where from) that our current
pattern can be shortened to

  find_package (PROJ4 NO_MODULE QUIET)
  find_package (PROJ4 MODULE)

seems not to hold in all cases. If you think this should work, let me
know and I will look for a counter-example.
Steps To Reproduce
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Issue History
Date ModifiedUsernameFieldChange
2014-10-31 12:31Charles KarneyNew Issue
2014-10-31 12:50Brad KingNote Added: 0037122
2014-10-31 13:13Charles KarneyNote Added: 0037123
2014-10-31 13:30Brad KingNote Added: 0037124
2015-04-16 16:10Brad KingNote Added: 0038536
2015-04-20 10:09Brad KingAssigned To => Tamas Kenez
2015-04-20 10:09Brad KingStatusnew => assigned
2016-06-10 14:29Kitware RobotNote Added: 0042656
2016-06-10 14:29Kitware RobotStatusassigned => resolved
2016-06-10 14:29Kitware RobotResolutionopen => moved
2016-06-10 14:31Kitware RobotStatusresolved => closed

Notes
(0037122)
Brad King   
2014-10-31 12:50   
There is no "order" in a single call to find_package(). If the mode is not explicitly specified then it uses module mode if and only if there is a Find module in the CMAKE_MODULE_PATH or builtin to CMake. It will always take two calls to try both modes. The order of the calls is up to the project.

A common way to achieve config-then-module is to have the second call at the top of the Find<pkg>.cmake module. The find_package command documents how it forwards component and version arguments automatically. This way the application code still only has one find_package call.

I don't think

  find_package (PROJ4 NO_MODULE QUIET)
  find_package (PROJ4 MODULE)

is clean because the Find module will be loaded by the second call regardless of whether the first call succeeds. The "if (NOT PROJ4_FOUND)" condition is needed.
(0037123)
Charles Karney   
2014-10-31 13:13   
Yes, I'm aware of the trick of FindXXX.cmake doing a config lookup first
(from FindBoost.cmake). However our problem is that we have our own
builds of standard libraries like JPEG and ZLIB which we want found via
the config lookup. We don't want to be modifying the system FindXXX
modules and providing overriding FindXXX modules seems to be heavy
handed.
(0037124)
Brad King   
2014-10-31 13:30   
FWIW here are some approaches you could use for now with existing CMake releases:

- Provide your own FindXXX modules that do the NO_MODULE call and then include() the upstream module if config mode did not find anything.

- Use a my_find_package macro that contains two calls to find_package internally.

Functionality like the request 1 and request 2 in the description is a reasonable proposal for future behavior. However, it would be better discussed on the developer's list:

 http://www.cmake.org/mailman/listinfo/cmake-developers [^]

with a broader audience. I think the main challenge will be how to format error messages when neither mode finds anything for a REQUIRED package. It took years to get the current messages to a state that minimizes confusion among users that do not understand the two modes.
(0038536)
Brad King   
2015-04-16 16:10   
A relevant mailing list thread:

 [CMake 0015231]: find_package: should be a simple way to alter the order of the config/module lookups
 http://thread.gmane.org/gmane.comp.programming.tools.cmake.devel/12975 [^]
(0042656)
Kitware Robot   
2016-06-10 14:29   
Resolving issue as `moved`.

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