C语言 通过命令行附加到 GNU make 变量

Append to GNU make variables via command line

提问by Michael Koval

I am using a GNU-make Makefile to build a C project with several targets (all, clean, and a few project specific targets). In the process of debugging, I would like to append some flags to a single build without permanently editing the Makefile (e.g. add debugging symbols or set a preprocessor flag).

我使用的是GNU-化妆的Makefile建立与几个目标（一个C项目all，clean以及一些项目的具体目标）。在调试过程中，我想在不永久编辑 Makefile 的情况下将一些标志附加到单个构建中（例如添加调试符号或设置预处理器标志）。

In the past, I have done that as follows (using the debugging symbols example):

过去，我是这样做的（使用调试符号示例）：

make target CFLAGS+=-g

Unfortunately, this is not appending to the CFLAGSvariable, but instead, clearing it and stopping it from compiling. Is there a clean way of doing this without defining some sort of dummy variable appended to the end of CFLAGSand LDFLAGS?

不幸的是，这不是附加到CFLAGS变量，而是清除它并阻止它编译。有没有一种干净的方法来做到这一点，而无需定义某种附加到CFLAGSand末尾的虚拟变量LDFLAGS？

回答by Carl Norum

Check out the override directive. You will probably need to modify the makefile once, but it should do what you want.

查看覆盖指令。您可能需要修改 makefile 一次，但它应该可以满足您的需求。

Example makefile:

示例生成文件：

override CFLAGS += -Wall

app: main.c
    gcc $(CFLAGS) -o app main.c 

Example command lines:

示例命令行：

$ make
gcc -Wall -o app main.c 
$ make CFLAGS=-g
gcc -g -Wall -o app main.c 

回答by aib

For the record, @Carl Norum's answer prependsthe variable, from the command line perspective.

为了记录在案，@Carl Norum时的回答预先考虑的变量，在命令行的观点。

I needed a way to actually append and came up with:

我需要一种方法来实际追加并提出：

override CFLAGS := -Wall $(CFLAGS)

回答by Jér?me Pouiller

There are two ways to pass variables to make:

传递变量给make有两种方式：

• Using command line arguments:

  make VAR=value
  

• Using environment:

  export VAR=var; make
  

  or (better because it change environment only for current command)

  VAR=var make
  

• 使用命令行参数：

  make VAR=value
  

• 使用环境：

  export VAR=var; make
  

  或（更好，因为它只为当前命令改变环境）

  VAR=var make
  

They are slightly different. The first one is stronger. It mean you know what you want. The second may be considered like a hint. Difference between them is about operators =and +=(without override). These operators are ignored when a variable is defined on command line, but are not ignored when variable is defined in environment. Thus, I suggest you to have a Makefile with:

它们略有不同。第一个更强。这意味着你知道你想要什么。第二个可以被认为是一个提示。它们之间的区别在于运算符=和+=（没有override）。当在命令行上定义变量时，这些运算符会被忽略，但在环境中定义变量时不会被忽略。因此，我建议你有一个 Makefile ：

CC ?= gcc
CFLAGS += -Wall
INTERNAL_VARS = value

and call it with:

并调用它：

CFLAGS=-g make

Notice, if you want to withdraw -Wall, you can use:

注意，如果你想提现-Wall，你可以使用：

make CFLAGS=

Please don't use overridekeyword, else you won't have any way to change a variable affected with override.

请不要使用override关键字，否则您将无法更改受override.

回答by sdaau

Just a note, as I got confused - let this be file testmake:

只是一个说明，因为我很困惑 - 让这成为文件testmake：

$(eval $(info A: CFLAGS here is $(CFLAGS)))

override CFLAGS += -B

$(eval $(info B: CFLAGS here is $(CFLAGS)))

CFLAGS += -C

$(eval $(info C: CFLAGS here is $(CFLAGS)))

override CFLAGS += -D

$(eval $(info D: CFLAGS here is $(CFLAGS)))

CFLAGS += -E

$(eval $(info E: CFLAGS here is $(CFLAGS)))

Then:

然后：

$ make -f testmake
A: CFLAGS here is 
B: CFLAGS here is -B
C: CFLAGS here is -B
D: CFLAGS here is -B -D
E: CFLAGS here is -B -D
make: *** No targets.  Stop.
$ make -f testmake CFLAGS+=-g
A: CFLAGS here is -g
B: CFLAGS here is -g -B
C: CFLAGS here is -g -B
D: CFLAGS here is -g -B -D
E: CFLAGS here is -g -B -D
make: *** No targets.  Stop.

With the overridedirectives deleted from the testmakefile:

随着override从已删除的指令testmake文件：

$ make -f testmake
A: CFLAGS here is 
B: CFLAGS here is -B
C: CFLAGS here is -B -C
D: CFLAGS here is -B -C -D
E: CFLAGS here is -B -C -D -E
make: *** No targets.  Stop.
$ make -f testmake CFLAGS+=-g
A: CFLAGS here is -g
B: CFLAGS here is -g
C: CFLAGS here is -g
D: CFLAGS here is -g
E: CFLAGS here is -g
make: *** No targets.  Stop.

So,

所以，

• if a variable used overrideonce, it can only be appended with another statement with override(the normal assignments will be ignored);
• when there's been no overrideat all; trying to append (as in +=) from the command line overwrites every instance of that variable.

• 如果一个变量使用override一次，它只能附加另一个语句 with override（正常的赋值将被忽略）；
• 当根本没有的override时候；尝试+=从命令行追加（如）会覆盖该变量的每个实例。