讀書心得 Managing Projects with GNU Make (2-2) Automatic Variables

本文出自 Managing Projects with GNU Make 3rd-Robert Mecklenburg(2005)

Chp2. Rules

Automatic Variables:

為了方便編寫 makefile，make 定義了一些 Automatic Variables (自動變數)

Automatic Variable	Description
$＠	代表 target 的字串
$%	The filename element of an archive member specification.
$<	第一個 prerequisite 字串
$?	The names of all prerequisites that are newer than the target, separated by spaces.
$^	所有 prerequisites 用空白分隔並去除掉重複的 prerequisite
$+	Similar to $^, this is the names of all the prerequisites separated by spaces, except that $+ includes duplicates. This variable was created for specific situations such as arguments to linkers where duplicate values have meaning.
$*	The stem of the target filename. A stem is typically a filename without its suffix.

來看個例子吧：

count_words: count_words.o counter.o lexer.o -lfl
        gcc $^ -o $＠  # $^: 所有 prerequisites (count_words.c)
                       # $＠: 代表 target 的字串 (count_words)
        # 查看 $^ 和 ＄＠ 的方法
        # ＠echo ＄^
        # ＠echo ＄＠
count_words.o: count_words.c
        gcc -c $<    # $<: 第一個 prerequisite (count_words.c)
counter.o: counter.c
        gcc -c $<
lexer.o: lexer.c
        gcc -c $<
lexer.c: lexer.l
        flex -t $< > $@