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| This page describes a proposal for a formal backwards/forwards compatibility feature.
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| =Motivating Examples=
| | This page has moved [https://gitlab.kitware.com/cmake/community/wikis/doc/cmake/dev/Policies-Design-Discussion here]. |
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| ==The ADD_DEFINITIONS Command==
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| Consider code such as
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| add_definitions("-DFOO=\\\"hello\\ world\\\"")
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| which tries to add the option
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| -DFOO=\"hello\ world\"
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| to the compile command line. The code works in CMake 2.4's <code>Unix Makefiles</code> generator and produces a definition as if
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| #define FOO "hello world"
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| appeared in the source code. It works only because of the way the makefile generator happens to place the definition string in the command line. It may not work with the VS IDE generators.
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| In CMake HEAD we provide the <code>COMPILE_DEFINITIONS</code> directory property so that one may write
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| set_property(DIRECTORY PROPERTY COMPILE_DEFINITIONS "FOO=\"hello world\"")
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| to get the correct behavior on all generators. Since CMake HEAD contains the appropriate escaping support it is desirable to allow the user to write
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| add_definitions("-DFOO=\"hello world\"")
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| and get the expected behavior. Unfortunately if we were to start escaping the definitions given to <code>add_definitions</code> we would break compatibility with projects that are already adding their own escapes. In hindsight we should have either supported escapes from the beginning or make the command give an error that the definition value is not supported, but it is too late now.
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| A similar problem appears in the <code>add_custom_command</code> command where support for properly escaping all arguments was added late. The solution currently used by that command is to require the user to add a <code>VERBATIM</code> argument to the command to get proper escaping. Using that solution for <code>add_definitions</code> would make the user write
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| add_definitions(VERBATIM "-DFOO=\"hello world\"")
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| just to get CMake to do the right thing. For compatibility CMake would have to implement the wrong behavior by default forever.
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| ==Missing Link Directories==
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| Projects used to be able to write this (wrong) code and it would work by accident:
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| add_executable(myexe myexe.c)
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| target_link_libraries(myexe /path/to/libA.so B)
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| where "<code>B</code>" is meant to link "<code>/path/to/libB.so</code>". This code is incorrect
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| because it asks CMake to link to <code>B</code> but does not provide the proper
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| linker search path for it. It used to work by accident because the
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| <code>-L/path/to</code> would get added as part of the implementation of linking to
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| A. The correct code would be
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| link_directories(/path/to)
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| add_executable(myexe myexe.c)
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| target_link_libraries(myexe /path/to/libA.so B)
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| or even better
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| add_executable(myexe myexe.c)
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| target_link_libraries(myexe /path/to/libA.so /path/to/libB.so)
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| Currently we provide the <code>CMAKE_OLD_LINK_PATHS</code> variable to allow projects or users to quickly work around the problem. Full compatibility would require us to support thie behavior by default forever. That would allow new projects to be written with the same bug.
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| An alternative is to require all libraries to be linked via full path (where target names are expanded automatically). Whenever a non-full-path is given we could produce a warning that tells the user to start using <code>find_library</code> or something like that but then implement the old linker search path computation for compatibility. It is desirable to let projects that have been updated for newer CMake versions tell CMake that they know what they are doing and to not warn or use the compatibility behavior.
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| =Proposed Solution=
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| We propose the following solution to this problem.
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| Each change that introduces a compatibility issue is assigned a new identification number (like CMP_00001). Then we try to detect cases in user code where it '''might''' be an issue and deal with it. We can maintain in the implementation of CMake a mapping from feature id to a rule to deal with the issue when encountered. The possible means of dealing with such cases are:
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| {| border="1"
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| |- bgcolor="#abcdef"
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| ! Rule !! Behavior !! Meaning !! Default?
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| | OLD || old || Project suppresses the diagnostic || never
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| |-
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| | NEW || new || Project declares the case has been resolved || never
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| | WARN || old || Emit warning when case is encountered || most cases
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| |-
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| | REQUIRED || new || Must use the new behavior || future CMake release, compatibility can be dropped but still checked for
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| |-
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| | REQUIRED_NO_CHECK || new || Must use the new behavior || future CMake release, compatibility is dropped as well as the code to check if the file uses this issue
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| |}
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| Several releases after a compatibility issue has been introduced we can remove implementation of support for the old behavior and set it to "REQUIRED" internally that requires a project to declare the issue NEW or to be of a CMake version such that we know it would have generated warning. See the feature lifespan example.
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| ==Proposed CMAKE_POLICY Command==
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| We will introduce a new command for projects to use for setting the rule for each policy.
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| The signature
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| cmake_policy(<OLD|NEW> [policy-id1 [policy-id2 [...]]])
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| sets a list of policies to use OLD or NEW behavior. It is an error to specify a policy id that does not exist (because it might refer to a policy id introduced in a future version of CMake).
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| The signature
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| cmake_policy(VERSION <major[.minor[.patch]]>)
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| sets all rules for policies introduced in the specified version or below to NEW and all other rules to WARN. It also requires the version of CMake running to be at least that high.
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| The <code>cmake_minimum_required(VERSION)</code> command can do something similar (but use of new commands versus fixing other issues are distinct cases so the defaults may need to be different).
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| The signature
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| cmake_policy(<PUSH|POP>)
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| pushes or pops the current policy state on a stack. The stack is automatically pushed/popped when entering a new directory (under <code>add_subdirectory</code> for example). Within a directory the number of PUSH and POP calls must match or it is an error. This signature is useful in a .cmake script meant for inclusion in other projects. It could write
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| cmake_policy(PUSH)
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| cmake_policy(NEW CMP_00001)
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| ... code using CMP_00001 ...
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| cmake_policy(POP)
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| The command could also provide an alias for each policy id that is more human readable:
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| cmake_policy(NEW CMP_ESCAPE_DEFINITIONS)
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| ==Examples==
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| Let's define a few example policy ids
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| {| border="1"
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| ! Id !! Alias !! Description
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| | CMP_00001 || CMP_ESCAPE_DEFINITIONS || Enable proper escaping of definitions added with ADD_DEFINITIONS
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| |-
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| | CMP_00002 || CMP_ESCAPE_CUSTOM_COMMANDS || Enable proper escaping of custom command lines with ADD_CUSTOM_COMMAND, make VERBATIM argument an error
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| |-
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| | CMP_00003 || CMP_NO_AUTOMATIC_LINK_PATHS || Disable generation of compatibility -L options on link lines
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| |}
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| The code
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| add_definitions("-DFOO=\\\"hello\\ world\\\"")
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| can now produce a warning CMP_00001 that CMake does not know whether to escape the definition (only when a non-trivial value is given).
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| The project may suppress the warning in their release branch:
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| project(FOO)
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| cmake_policy(OLD CMP_ESCAPE_DEFINITIONS)
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| ...
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| add_definitions("-DFOO=\\\"hello\\ world\\\"")
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| or fix the code:
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| project(FOO)
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| cmake_policy(NEW CMP_ESCAPE_DEFINITIONS)
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| ...
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| add_definitions("-DFOO=\"hello world\"")
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| Either way CMake knows exactly how to handle the code and does not need to warn.
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| Similarly, the code
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| add_custom_command(OUTPUT foo.txt COMMAND mycmd -with -some -arguments -out foo.txt)
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| may now produce a warning CMP_00002 that CMake does not know whether to escape the command.
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| The project may write
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| project(FOO)
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| cmake_policy(NEW CMP_ESCAPE_CUSTOM_COMMANDS)
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| ...
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| add_custom_command(OUTPUT foo.txt COMMAND mycmd -with -some -arguments -out foo.txt)
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| to get rid of the warning and use proper escaping.
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| The code
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| target_link_libraries(mytarget /path/to/libA.so B)
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| may now produce a warning CMP_00003 that library B may not have the correct link directory specified but then generate -L/path/to anyway. The project may write
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| project(FOO)
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| cmake_policy(NEW CMP_NO_AUTOMATIC_LINK_PATHS)
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| ...
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| link_directories(/path/to)
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| target_link_libraries(mytarget /path/to/libA.so B)
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| Once the project has fixed all issues related to upgrading to CMake 2.6 it can just write
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| project(FOO)
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| cmake_policy(VERSION 2.6)
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| to get all the updated behavior. Future versions of CMake (> 2.6) may then warn on their new policies.
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| ==Feature Lifespan Example==
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| Using the CMP_00001 || CMP_ESCAPE_DEFINITIONS example above let's examine how it would work. In CMake 2.4 definitions had to be escaped by the user which probably does not work on MSVC etc. So in 2.6 we want to start switching over to having CMake handle the escaping.
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| * CMake 2.4: old behavior, issue CMP_0001 does not even exist
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| * CMake 2.6: issue CMP_0001 is created and set to WARN and version number 2.6. By default old projects will still work but will receive a warning if they use this feature. The policy can be either made OLD <code>cmake_policy(OLD CMP_0001)</code> with no other changes to the CMakeLists file or the issue can be fixed by changing their code and setting it to NEW <code>cmake_policy(NEW CMP_0001)</code>. Or they can ignore it and just live with the warnings.
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| * CMake 2.8: Nothing changes, stays as a warning
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| * CMake 2.10: We change the default value of issue CMP_0001 to REQUIRED. Now if the user's CMakeLists file sets the issue to OLD or does not set a policy at all CMake will produce an error indicating that that is no longer an option for policy CMP_0001 and the issue must be fixed or an earlier version of CMake used. Ther user list file will have to set the policy to NEW explicitly or via <code>cmake_policy(VERSION 2.6)</code> (or higher).
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| * CMake 2.12: We change the default value of issue CMP_0001 to REQUIRED_NO_CHECK. We do not check to see if they are using the feature at all. That code has been removed. The user must specify NEW for the policy explicitly or via <code>cmake_policy(VERSION 2.6)</code> (or higher) in order to use this version of CMake (whether or not the feature is used).
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| ==Discussion==
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| This solution has the following advantages:
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| * All compatibility issues are documented and given unique identifiers
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| ** Documentation can include association with CMake version ranges (when introduced, when REQUIRED, etc.)
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| * We can be more aggressive about detecting cases without worrying about false positives because it is just a warning
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| * When generating a warning or error message we can reference the feature id and tell the user what to do
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| * Projects can be converted to newer CMake features incrementally
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| * Project authors will be motivated by the warnings to update their code, but it will build for users without extra work
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| * Code written that does not hit one of these issues will work without any special declarations
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| ** The hello-world example does not need to show version specification
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| * We have an exit-path to remove support for a compatibility feature eventually
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| ** Make projects set the rule to NEW explicitly or via VERSION specification
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| * Any compatibility bug introduced accidentally and reported by a user can be fixed in CMake by converting it to one of these issues
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