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