2017q3 Homework1 (clz)
---

contributed by <`amikai`>

###### tags: `sysprog2017`

# Makefile 研讀
## makefile option
- `-d` 印出 debug 訊息
- `-f file` 當你的 makefile 是隨便取名的時候, 則可以使用此選項, 舉例 make -f my_makefilename

## gcc option
TODO

- `-MMD`
- `-MF`

Q: prequest 不都自己寫好了嗎, 為啥要透過 gcc 產生呢

參考資料:
- https://gcc.gnu.org/onlinedocs/gcc/Preprocessor-Options.html#Preprocessor-Options
- https://www.gnu.org/software/make/manual/html_node/Automatic-Prerequisites.html#Automatic-Prerequisites

## strip string
將前輟 (leading) 和後輟 (trialing) 的空白字元 (whitespace) 刪除, 並字串裡 (不包含前啜後輟) 的一串空白字元替換成空白字元. 因為 `$(strip a b c )` 的結果為 `a b c`

原文:

> Removes leading and trailing whitespace from string and replaces each internal sequence of one or more whitespace characters with a single space. Thus, ‘$(strip a b c )’ results in ‘a b c’.

參考資料: [Functions for String Substitution and Analysis](https://www.gnu.org/software/make/manual/html_node/Text-Functions.html)

## Recipe Echoing

參考資料: https://www.gnu.org/software/make/manual/html_node/Echoing.html

## pattern rule

首先先看看一個簡單的:

%.o : %.c
        $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@ $(SRCS_common)

這裡定義一個 rule 可以讓所有的 x.o 藉由 x.c 創造出來. 當這個 rule 符合時, 此 recipe 會使用 `$@` 和 `$<` 兩個 automatic 變數, 分別替換來源和目的

原文:
> defines a rule that can make any file x.o from x.c. The recipe uses the automatic variables `$@` and `$<` to substitute the names of the target file and the source file in each case where the rule applies

在看老師寫的:

%: $(SRCS_common) %.c clz.h
	$(CC) $(CFLAGS_common) -o $@ \
		-MMD -MF .$@.d \
		-D$(shell echo $(subst .o,,$@)) $(SRCS_common)

在這並沒有寫 file extension 所以下 `make xxx` (xxx 代表任何字) 則會調用到此條 rule

參考資料: [pattern-examples](https://www.gnu.org/software/make/manual/html_node/Pattern-Examples.html#Pattern-Examples)

## PHONY target
TODO

參考資料: [PHONY target - gnu make manual](https://www.gnu.org/software/make/manual/html_node/Phony-Targets.html)

## Substitution Reference
看到老師寫的一個, 有天書的感覺, 我就開了 gnu manual 用 % 字元慢慢搜尋, 原來這招叫做 `Substitution Reference`

deps := $(EXEC:%=.%.o.d)

substitution reference 會將變數得值替換成你指定的值. 而它的形式為 `$(var:a=b)` (或 `${var:a=b}`), 而
`$(var:a=b)` 把 var 裡的值拿出來, 將每個字尾有 a 這個字地替換成字尾有 b 字, 並塞回 var 裡. 原文:

> A substitution reference substitutes the value of a variable with alterations that you specify. It has the form `$(var:a=b)` (or ‘${var:a=b}’) and its meaning is to take the value of the variable var, replace every a at the end of a word with b in that value, and substitute the resulting string.

舉例:

foo := a.o b.o c.o
bar := $(foo:.o=.c)

則 `bar` 為 `a.c b.c c.c`

接下來我們在看這個

EXEC = \
	iteration \
	binary \
	byte \
	recursive \
	harley
    
deps := $(EXEC:%=.%.o.d)

則 `deps` 會被設為 `.iteration.o.d .binary.o.d .byte.o.d .recursive.o.d .harley.o.d`

參考資料:
- [Substitution Reference - gnu make manual](https://www.gnu.org/software/make/manual/html_node/Substitution-Refs.html)
- https://stackoverflow.com/questions/23919200/what-does-the-percent-symbol-mean-in-android-mk

## include

include 這個 directive 告訴 make 先暫時不要繼續讀現在這個 makefile 而是先讀其他的 makefile. 這個 directive 像是以下這樣:

include filenames…

filenames 可以包含 shell filename pattern (這句不是很懂), 如果 filenames 是空的話, 什麼都不會被 include 而且不會有錯誤發生.

原文如下:
The include directive tells make to suspend reading the current makefile and read one or more other makefiles before continuing. The directive is a line in the makefile that looks like this:

include filenames…

filenames can contain shell file name patterns. If filenames is empty, nothing is included and no error is printed.

參考資料: https://www.gnu.org/software/make/manual/html_node/Include.html

# 程式研讀

# 補強工作
- 用機率觀點解釋圖