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authorSebastian Leske <Sebastian.Leske@sleske.name>2012-06-11 19:34:00 (GMT)
committerBrad King <brad.king@kitware.com>2012-06-18 14:02:13 (GMT)
commitf2c1f2402ee230bef6ebd3867aaa6f761466b61c (patch)
treebeb9eb38e29cdcfe3b66cb035577f2e79bcf30a9 /Source/cmSetCommand.h
parentb39924cf222e0c2e958a780dfe77ca60592d2afd (diff)
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Improve documentation of set command (#13269)
Diffstat (limited to 'Source/cmSetCommand.h')
-rw-r--r--Source/cmSetCommand.h88
1 files changed, 75 insertions, 13 deletions
diff --git a/Source/cmSetCommand.h b/Source/cmSetCommand.h
index 66b129e..2dd80e3 100644
--- a/Source/cmSetCommand.h
+++ b/Source/cmSetCommand.h
@@ -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);