GNU Makefile中的条件控制结构

在常见的编程语言中，使用条件控制结构诸如if ... else if ... else...是很寻常的事情，那么在GNU Makefile中如何使用呢？

ifeq
ifneq

例如：foo.sh

 #!/bin/bash

 ARCH=$(uname -p)

 if [[ $ARCH == "x86_64" ]]; then

     ARCH32="i686"

     ARCH64="x86_64"

 elif [[ $ARCH == "ppc64le" ]]; then

     ARCH32=""

     ARCH64="ppc64le"

 else

     ARCH32=$ARCH

     ARCH64=""

 fi

 if [[ -n $ARCH32 ]]; then

     OUTPUT+=" ARCH32=$ARCH32"

 fi

 if [[ -n $ARCH64 ]]; then

     OUTPUT+=" ARCH64=$ARCH64"

 fi

 echo $OUTPUT

将上述的foo.sh用Makefile实现就是：

 ARCH = $(shell uname -p)

 ifeq ($(ARCH), x86_64)

        ARCH32 = i686

        ARCH64 = x86_64

 else ifeq ($(ARCH), ppc64le)

        ARCH32 =

        ARCH64 = ppc64le

 else

        ARCH32 = $(ARCH)

        ARCH64 =

 endif

 ifneq ($(ARCH32), )

     OUTPUT += ARCH32=$(ARCH32)

 endif

 ifneq ($(ARCH64), )

     OUTPUT += ARCH64=$(ARCH64)

 endif

 all: foo

 foo:

     @echo $(OUTPUT)

运行foo.sh 和 Makefile 结果如下：

$ uname -p

x86_64

$ bash foo.sh

ARCH32=i686 ARCH64=x86_64

$ make -f Makefile

ARCH32=i686 ARCH64=x86_64

由此可见，

ifeq($(VAR), ) 等价于bash中的 [[ -z $VAR ]]
ifneq($(VAR), ) 等价于bash中的 [[ -n $VAR ]]

那么，在GNU Makefile中使用条件控制结构有什么用？很简单，对不同的平台提供不同的编译选项或者安装包支持。例如：

一个针对不同的平台提供不同的安装包的Makefile

 ARCH = $(shell uname -p)

 ifeq ($(ARCH), x86_64)

        ARCH32 = i686

        ARCH64 = x86_64

 else ifeq ($(ARCH), ppc64)

        ARCH32 =

        ARCH64 = ppc64

 else ifeq ($(ARCH), ppc64le)

        ARCH32 =

        ARCH64 = ppc64le

 else ifeq ($(ARCH), s390x)

        ARCH32 =

        ARCH64 = s390x

 else ifeq ($(ARCH), aarch64)

        ARCH32 =

        ARCH64 = aarch64

 else

        ARCH32 = $(ARCH)

        ARCH64 =

 endif

 ifneq ($(ARCH32), )

        CPKGS32 = libgcc.$(ARCH32)

        CPKGS32 += glibc.$(ARCH32)

 endif

 ifneq ($(ARCH64), )

        CPKGS64 = libgcc.$(ARCH64)

        CPKGS64 += glibc.$(ARCH64)

 endif

 CPKGS = $(CPKGS32) $(CPKGS64)

 all: foo

 foo:

     @echo $(CPKGS)

参考资料：

7.2 Syntax of Conditionals

makefile

GNU Makefile中的条件控制结构