4 The basic idea was inspired by Make. When we look at Make, we notice sort of
5 two languages in one. One language describes dependency graphs consisting of
6 targets and prerequisites. The other is a macro language for performing textual
9 There is clear distinction between the two language stages. For example, you
10 can write a makefile like follows:
17 $(CC) -o $(APP) $(SRC)
19 The macro language replaces the variable references with their expanded form,
20 and handles as if the source file were input like follows:
25 Then, Make analyzes the dependency graph and determines the targets to be
28 The idea is quite similar in Kconfig - it is possible to describe a Kconfig
34 def_bool $(shell, $(srctree)/scripts/gcc-check-foo.sh $(CC))
36 The macro language in Kconfig processes the source file into the following
42 Then, Kconfig moves onto the evaluation stage to resolve inter-symbol
43 dependency as explained in kconfig-language.txt.
49 Like in Make, a variable in Kconfig works as a macro variable. A macro
50 variable is expanded "in place" to yield a text string that may then be
51 expanded further. To get the value of a variable, enclose the variable name in
52 $( ). The parentheses are required even for single-letter variable names; $X is
53 a syntax error. The curly brace form as in ${CC} is not supported either.
55 There are two types of variables: simply expanded variables and recursively
58 A simply expanded variable is defined using the := assignment operator. Its
59 righthand side is expanded immediately upon reading the line from the Kconfig
62 A recursively expanded variable is defined using the = assignment operator.
63 Its righthand side is simply stored as the value of the variable without
64 expanding it in any way. Instead, the expansion is performed when the variable
67 There is another type of assignment operator; += is used to append text to a
68 variable. The righthand side of += is expanded immediately if the lefthand
69 side was originally defined as a simple variable. Otherwise, its evaluation is
72 The variable reference can take parameters, in the following form:
74 $(name,arg1,arg2,arg3)
76 You can consider the parameterized reference as a function. (more precisely,
77 "user-defined function" in contrast to "built-in function" listed below).
79 Useful functions must be expanded when they are used since the same function is
80 expanded differently if different parameters are passed. Hence, a user-defined
81 function is defined using the = assignment operator. The parameters are
82 referenced within the body definition with $(1), $(2), etc.
84 In fact, recursively expanded variables and user-defined functions are the same
85 internally. (In other words, "variable" is "function with zero argument".)
86 When we say "variable" in a broad sense, it includes "user-defined function".
92 Like Make, Kconfig provides several built-in functions. Every function takes a
93 particular number of arguments.
95 In Make, every built-in function takes at least one argument. Kconfig allows
96 zero argument for built-in functions, such as $(fileno), $(lineno). You could
97 consider those as "built-in variable", but it is just a matter of how we call
98 it after all. Let's say "built-in function" here to refer to natively supported
101 Kconfig currently supports the following built-in functions.
105 The "shell" function accepts a single argument that is expanded and passed
106 to a subshell for execution. The standard output of the command is then read
107 and returned as the value of the function. Every newline in the output is
108 replaced with a space. Any trailing newlines are deleted. The standard error
109 is not returned, nor is any program exit status.
113 The "info" function takes a single argument and prints it to stdout.
114 It evaluates to an empty string.
116 - $(warning-if,condition,text)
118 The "warning-if" function takes two arguments. If the condition part is "y",
119 the text part is sent to stderr. The text is prefixed with the name of the
120 current Kconfig file and the current line number.
122 - $(error-if,condition,text)
124 The "error-if" function is similar to "warning-if", but it terminates the
125 parsing immediately if the condition part is "y".
129 The 'filename' takes no argument, and $(filename) is expanded to the file
134 The 'lineno' takes no argument, and $(lineno) is expanded to the line number
141 Kconfig adopts Make-like macro language, but the function call syntax is
144 A function call in Make looks like this:
146 $(func-name arg1,arg2,arg3)
148 The function name and the first argument are separated by at least one
149 whitespace. Then, leading whitespaces are trimmed from the first argument,
150 while whitespaces in the other arguments are kept. You need to use a kind of
151 trick to start the first parameter with spaces. For example, if you want
152 to make "info" function print " hello", you can write like follows:
155 space := $(empty) $(empty)
156 $(info $(space)$(space)hello)
158 Kconfig uses only commas for delimiters, and keeps all whitespaces in the
159 function call. Some people prefer putting a space after each comma delimiter:
161 $(func-name, arg1, arg2, arg3)
163 In this case, "func-name" will receive " arg1", " arg2", " arg3". The presence
164 of leading spaces may matter depending on the function. The same applies to
165 Make - for example, $(subst .c, .o, $(sources)) is a typical mistake; it
166 replaces ".c" with " .o".
168 In Make, a user-defined function is referenced by using a built-in function,
171 $(call my-func,arg1,arg2,arg3)
173 Kconfig invokes user-defined functions and built-in functions in the same way.
174 The omission of 'call' makes the syntax shorter.
176 In Make, some functions treat commas verbatim instead of argument separators.
177 For example, $(shell echo hello, world) runs the command "echo hello, world".
178 Likewise, $(info hello, world) prints "hello, world" to stdout. You could say
179 this is _useful_ inconsistency.
181 In Kconfig, for simpler implementation and grammatical consistency, commas that
182 appear in the $( ) context are always delimiters. It means
184 $(shell, echo hello, world)
186 is an error because it is passing two parameters where the 'shell' function
187 accepts only one. To pass commas in arguments, you can use the following trick:
190 $(shell, echo hello$(comma) world)
196 A variable (or function) cannot be expanded across tokens. So, you cannot use
197 a variable as a shorthand for an expression that consists of multiple tokens.
205 range $(RANGE_MIN) $(RANGE_MAX)
207 But, the following does not work:
215 A variable cannot be expanded to any keyword in Kconfig. The following does
224 Obviously from the design, $(shell command) is expanded in the textual
225 substitution phase. You cannot pass symbols to the 'shell' function.
226 The following does not work as expected.
230 default "-mbig-endian" if CPU_BIG_ENDIAN
231 default "-mlittle-endian" if CPU_LITTLE_ENDIAN
233 config CC_HAS_ENDIAN_FLAG
234 def_bool $(shell $(srctree)/scripts/gcc-check-flag ENDIAN_FLAG)
236 Instead, you can do like follows so that any function call is statically
239 config CC_HAS_ENDIAN_FLAG
241 default $(shell $(srctree)/scripts/gcc-check-flag -mbig-endian) if CPU_BIG_ENDIAN
242 default $(shell $(srctree)/scripts/gcc-check-flag -mlittle-endian) if CPU_LITTLE_ENDIAN