更新時傳遞的變量改變

Question

我的makefile目標：

ifndef VEC_LEN 
     VEC_LEN = 1 
endif 

my_target: a.c 
     gcc a.c -DVEC_LEN=$(VEC_LEN) 

有沒有辦法告訴做出my_target應更新時VEC_LEN 改變？

更新：

我的腳本現在這個樣子（和他們的工作）： 的Makefile

SHELL := /bin/bash 

# Define the answer if not defined yet 
ANSWERTOLIFETHEUNIVERSEANDEVERYTHING ?= 42 

# Update the header file if the answer has changed 
# := always executes the shell command, = does not! Quote from http://www.gnu.org/software/make/manual/make.html: 
#  immediate = deferred 
#  immediate := immediate 
DUMMY := $(shell ./updateAnswer.sh $(ANSWERTOLIFETHEUNIVERSEANDEVERYTHING) >logMakefile.txt) 

answer : answer.h 
    echo "Updated!" 
    touch answer 

updateAnswer.sh

#!/bin/bash 

# Check if the definition of the answer has changed in the header file 
# If yes, re-write it. If not, do not touch it to avoid an updated timestamp. 
if grep -q "ANSWER ${1}" answer.h 
then 
    echo "ANSWER unchanged, is still ${1}." 
else 
    echo "#define ANSWER ${1}" >answer.h 
    echo 'Answer has changed:' 
    cat answer.h 
fi 

輸出示例：

simon@x220:~$ make 
echo "Updated!" 
Updated! 
touch answer 
simon@x220:~$ make 
make: `answer' is up to date. 
simon@x220:~$ make ANSWERTOLIFETHEUNIVERSEANDEVERYTHING=3 
echo "Updated!" 
Updated! 
touch answer 
simon@x220:~$ make ANSWERTOLIFETHEUNIVERSEANDEVERYTHING=3 
make: `answer' is up to date. 

Answer 1

假設語言C， 我認爲最簡單的方法可能是：

1. 準備vec_len.h具有線#define VEC_LEN 1
2. 交流再添#include "vec_len.h"
3. 如果VEC_LEN的價值有重寫vec_len.h
4. 使用通常的.c構建文件和頭文件依賴項

編輯：
雖然這是一個有點天真的方式，做你的 的情況下改變工作？

準備一個腳本（define.sh）像下面這樣：

#!/usr/bin/bash 
echo '#define VEC_LEN' $1 > vec_len.h 

，並在生成文件的開頭添加以下行：

VEC_LEN ?= 1 
DUMMY := $(shell define.sh $(VEC_LEN)) 

Answer 2

我想這會做到這一點，在makefile中：

-include old_vec_len 

VEC_LEN ?= 1 

ifneq ($(VEC_LEN),$(OLD_VEC_LEN)) 
target: marker 
endif 

target: 
    @echo run_script_to_make_target with VEC_LEN=$(VEC_LEN) 

.PHONY:marker 
marker: 
    @echo OLD_VEC_LEN=$(VEC_LEN) > old_vec_len