详解这句代码：srctree := $(if $(KBUILD_SRC),$(KBUILD_SRC),$(CURDIR))

这段代码是一个 makefile 中的语句，主要作用是定义变量 srctree。如果变量 KBUILD_SRC 存在，则将它赋值给 srctree；否则将当前目录的路径赋值给 srctree。

具体来说，这段代码使用了 makefile 中的 if-else 语句。如果 KBUILD_SRC 存在，则 if 语句的结果为真，将 KBUILD_SRC 的值赋给 srctree；否则 if 语句的结果为假，将当前目录的路径赋给 srctree。

这段代码的作用是为了让 makefile 能够在不同的环境中正确地找到源代码树的路径，从而能够顺利地进行编译和构建工作。

相关问题

kbuild-dir := $(if $(filter /%,$(src)),$(src),$(srctree)/$(src))

这段代码是一个Makefile中的语句，用于定义一个变量kbuild-dir的值。

这个语句的作用是判断变量$(src)是否以"/"开头，如果是，则将$(src)的值直接赋给kbuild-dir变量；如果不是，则将$(srctree)/$(src)的值赋给kbuild-dir变量。

其中，$(srctree)是Makefile自带的内置变量，表示当前Makefile所在的目录的绝对路径。$(src)是Makefile中定义的一个变量，表示当前目标所在的源代码目录的相对路径。

kbuild-dir := $(if $(filter /%,$(src)),$(src),$(srctree)/$(src)) explain the grammar points in this sentence

This sentence is a makefile statement that sets the variable `kbuild-dir` based on some conditions.

- The variable `kbuild-dir` is being assigned a value using `:=` which is the immediate assignment operator. This means that the assignment will be evaluated immediately rather than at the time of use.

- The `if` statement is used to check for a condition. In this case, it is checking if the string `'/%'` is contained in the `$(src)` variable. If it is true, then the `$(src)` variable is assigned to `kbuild-dir`.

- If the `if` condition is not true, then the `$(srctree)/$(src)` is assigned to `kbuild-dir`.

- The `$()` syntax is used to reference makefile variables, and in this case, the variables being referenced are `src` and `srctree`.

- The backslash is used to continue the statement on the next line.

Overall, this statement is setting the `kbuild-dir` variable to either the value of `src` or the concatenation of `srctree` and `src`, depending on the condition.