Improve documentation of set command (#13269)
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@ -52,7 +52,7 @@ public:
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*/
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virtual const char* GetTerseDocumentation() const
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{
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return "Set a CMAKE variable to a given value.";
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return "Set a CMake, cache or environment variable to a given value.";
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}
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/**
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@ -63,26 +63,37 @@ public:
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return
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" set(<variable> <value>\n"
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" [[CACHE <type> <docstring> [FORCE]] | PARENT_SCOPE])\n"
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"Within CMake sets <variable> to the value <value>. <value> is expanded"
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" before <variable> is set to it. If CACHE is present, then the "
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"<variable> is put in the cache. <type> and <docstring> are then "
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"required. <type> is used by the CMake GUI to choose a widget with "
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"which the user sets a value. The value for <type> may be one of\n"
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"Within CMake sets <variable> to the value <value>. "
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"<value> is expanded before <variable> is set to it. "
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"Normally, set will set a regular CMake variable. "
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"If CACHE is present, then the <variable> is put in the cache "
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"instead, unless it is already in the cache. "
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"See section 'Variable types in CMake' below for details of "
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"regular and cache variables and their interactions. "
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"If CACHE is used, <type> and <docstring> are required. <type> is used "
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"by the CMake GUI to choose a widget with which the user sets a value. "
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"The value for <type> may be one of\n"
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" FILEPATH = File chooser dialog.\n"
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" PATH = Directory chooser dialog.\n"
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" STRING = Arbitrary string.\n"
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" BOOL = Boolean ON/OFF checkbox.\n"
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" INTERNAL = No GUI entry (used for persistent variables).\n"
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"If <type> is INTERNAL, then the <value> is always written into the "
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"cache, replacing any values existing in the cache. If it is not a "
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"cache variable, then this always writes into the current makefile. The "
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"FORCE option will overwrite the cache value removing any changes by "
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"the user.\n"
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"If <type> is INTERNAL, the cache variable is marked as internal, "
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"and will not be shown to the user in tools like cmake-gui. "
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"This is intended for values that should be persisted in the cache, "
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"but which users should not normally change. INTERNAL implies FORCE."
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"\n"
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"Normally, set(...CACHE...) creates cache variables, but does not "
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"modify them. "
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"If FORCE is specified, the value of the cache variable is set, even "
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"if the variable is already in the cache. This should normally be "
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"avoided, as it will remove any changes to the cache variable's value "
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"by the user.\n"
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"If PARENT_SCOPE is present, the variable will be set in the scope "
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"above the current scope. Each new directory or function creates a new "
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"scope. This command will set the value of a variable into the parent "
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"directory or calling function (whichever is applicable to the case at "
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"hand).\n"
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"hand). PARENT_SCOPE cannot be combined with CACHE.\n"
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"If <value> is not specified then the variable is removed "
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"instead of set. See also: the unset() command.\n"
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" set(<variable> <value1> ... <valueN>)\n"
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@ -90,7 +101,58 @@ public:
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"values.\n"
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"<variable> can be an environment variable such as:\n"
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" set( ENV{PATH} /home/martink )\n"
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"in which case the environment variable will be set.";
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"in which case the environment variable will be set.\n"
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"*** Variable types in CMake ***\n"
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"In CMake there are two types of variables: normal variables and cache "
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"variables. Normal variables are meant for the internal use of the "
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"script (just like variables in most programming languages); they are "
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"not persisted across CMake runs. "
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"Cache variables (unless set with INTERNAL) are mostly intended for "
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"configuration settings where the first CMake run determines a "
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"suitable default value, which the user can then override, by editing "
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"the cache with tools such as ccmake or cmake-gui. "
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"Cache variables are stored in the CMake cache file, and "
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"are persisted across CMake runs. \n"
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"Both types can exist at the same time with the same name "
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"but different values. "
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"When ${FOO} is evaluated, CMake first looks for "
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"a normal variable 'FOO' in scope and uses it if set. "
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"If and only if no normal variable exists then it falls back to the "
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"cache variable 'FOO'.\n"
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"Some examples:\n"
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"The code 'set(FOO \"x\")' sets the normal variable 'FOO'. It does not "
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"touch the cache, but it will hide any existing cache value 'FOO'.\n"
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"The code 'set(FOO \"x\" CACHE ...)' checks for 'FOO' in the cache, "
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"ignoring any normal variable of the same name. If 'FOO' is in the "
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"cache then nothing happens to either the normal variable or the cache "
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"variable. If 'FOO' is not in the cache, then it is added to the "
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"cache.\n"
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"Finally, whenever a cache variable is added or modified by a command, "
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"CMake also *removes* the normal variable of the same name from the "
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"current scope so that an immediately following evaluation of "
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"it will expose the newly cached value.\n"
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"Normally projects should avoid using normal and cache variables of "
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"the same name, as this interaction can be hard to follow. "
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"However, in some situations it can be useful. "
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"One example (used by some projects):"
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"\n"
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"A project has a subproject in its source tree. The child project has "
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"its own CMakeLists.txt, which is included from the parent "
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"CMakeLists.txt using add_subdirectory(). "
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"Now, if the parent and the child project provide the same option "
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"(for example a compiler option), the parent gets the first chance "
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"to add a user-editable option to the cache. "
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"Normally, the child would then use the same value "
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"that the parent uses. "
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"However, it may be necessary to hard-code the value for the child "
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"project's option while still allowing the user to edit the value used "
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"by the parent project. The parent project can achieve this simply by "
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"setting a normal variable with the same name as the option in a scope "
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"sufficient to hide the option's cache variable from the child "
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"completely. The parent has already set the cache variable, so the "
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"child's set(...CACHE...) will do nothing, and evaluating the option "
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"variable will use the value from the normal variable, which hides the "
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"cache variable.";
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}
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cmTypeMacro(cmSetCommand, cmCommand);
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