Commit f2c1f240 authored by Sebastian Leske's avatar Sebastian Leske Committed by Brad King

Improve documentation of set command (#13269)

parent b39924cf
......@@ -52,7 +52,7 @@ public:
*/
virtual const char* GetTerseDocumentation() const
{
return "Set a CMAKE variable to a given value.";
return "Set a CMake, cache or environment variable to a given value.";
}
/**
......@@ -63,26 +63,37 @@ public:
return
" set(<variable> <value>\n"
" [[CACHE <type> <docstring> [FORCE]] | PARENT_SCOPE])\n"
"Within CMake sets <variable> to the value <value>. <value> is expanded"
" before <variable> is set to it. If CACHE is present, then the "
"<variable> is put in the cache. <type> and <docstring> are then "
"required. <type> is used by the CMake GUI to choose a widget with "
"which the user sets a value. The value for <type> may be one of\n"
"Within CMake sets <variable> to the value <value>. "
"<value> is expanded before <variable> is set to it. "
"Normally, set will set a regular CMake variable. "
"If CACHE is present, then the <variable> is put in the cache "
"instead, unless it is already in the cache. "
"See section 'Variable types in CMake' below for details of "
"regular and cache variables and their interactions. "
"If CACHE is used, <type> and <docstring> are required. <type> is used "
"by the CMake GUI to choose a widget with which the user sets a value. "
"The value for <type> may be one of\n"
" FILEPATH = File chooser dialog.\n"
" PATH = Directory chooser dialog.\n"
" STRING = Arbitrary string.\n"
" BOOL = Boolean ON/OFF checkbox.\n"
" INTERNAL = No GUI entry (used for persistent variables).\n"
"If <type> is INTERNAL, then the <value> is always written into the "
"cache, replacing any values existing in the cache. If it is not a "
"cache variable, then this always writes into the current makefile. The "
"FORCE option will overwrite the cache value removing any changes by "
"the user.\n"
"If <type> is INTERNAL, the cache variable is marked as internal, "
"and will not be shown to the user in tools like cmake-gui. "
"This is intended for values that should be persisted in the cache, "
"but which users should not normally change. INTERNAL implies FORCE."
"\n"
"Normally, set(...CACHE...) creates cache variables, but does not "
"modify them. "
"If FORCE is specified, the value of the cache variable is set, even "
"if the variable is already in the cache. This should normally be "
"avoided, as it will remove any changes to the cache variable's value "
"by the user.\n"
"If PARENT_SCOPE is present, the variable will be set in the scope "
"above the current scope. Each new directory or function creates a new "
"scope. This command will set the value of a variable into the parent "
"directory or calling function (whichever is applicable to the case at "
"hand).\n"
"hand). PARENT_SCOPE cannot be combined with CACHE.\n"
"If <value> is not specified then the variable is removed "
"instead of set. See also: the unset() command.\n"
" set(<variable> <value1> ... <valueN>)\n"
......@@ -90,7 +101,58 @@ public:
"values.\n"
"<variable> can be an environment variable such as:\n"
" set( ENV{PATH} /home/martink )\n"
"in which case the environment variable will be set.";
"in which case the environment variable will be set.\n"
"*** Variable types in CMake ***\n"
"In CMake there are two types of variables: normal variables and cache "
"variables. Normal variables are meant for the internal use of the "
"script (just like variables in most programming languages); they are "
"not persisted across CMake runs. "
"Cache variables (unless set with INTERNAL) are mostly intended for "
"configuration settings where the first CMake run determines a "
"suitable default value, which the user can then override, by editing "
"the cache with tools such as ccmake or cmake-gui. "
"Cache variables are stored in the CMake cache file, and "
"are persisted across CMake runs. \n"
"Both types can exist at the same time with the same name "
"but different values. "
"When ${FOO} is evaluated, CMake first looks for "
"a normal variable 'FOO' in scope and uses it if set. "
"If and only if no normal variable exists then it falls back to the "
"cache variable 'FOO'.\n"
"Some examples:\n"
"The code 'set(FOO \"x\")' sets the normal variable 'FOO'. It does not "
"touch the cache, but it will hide any existing cache value 'FOO'.\n"
"The code 'set(FOO \"x\" CACHE ...)' checks for 'FOO' in the cache, "
"ignoring any normal variable of the same name. If 'FOO' is in the "
"cache then nothing happens to either the normal variable or the cache "
"variable. If 'FOO' is not in the cache, then it is added to the "
"cache.\n"
"Finally, whenever a cache variable is added or modified by a command, "
"CMake also *removes* the normal variable of the same name from the "
"current scope so that an immediately following evaluation of "
"it will expose the newly cached value.\n"
"Normally projects should avoid using normal and cache variables of "
"the same name, as this interaction can be hard to follow. "
"However, in some situations it can be useful. "
"One example (used by some projects):"
"\n"
"A project has a subproject in its source tree. The child project has "
"its own CMakeLists.txt, which is included from the parent "
"CMakeLists.txt using add_subdirectory(). "
"Now, if the parent and the child project provide the same option "
"(for example a compiler option), the parent gets the first chance "
"to add a user-editable option to the cache. "
"Normally, the child would then use the same value "
"that the parent uses. "
"However, it may be necessary to hard-code the value for the child "
"project's option while still allowing the user to edit the value used "
"by the parent project. The parent project can achieve this simply by "
"setting a normal variable with the same name as the option in a scope "
"sufficient to hide the option's cache variable from the child "
"completely. The parent has already set the cache variable, so the "
"child's set(...CACHE...) will do nothing, and evaluating the option "
"variable will use the value from the normal variable, which hides the "
"cache variable.";
}
cmTypeMacro(cmSetCommand, cmCommand);
......
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