GNU Makefile中的條件控制結構

在常見的程式設計語言中，使用條件控制結構諸如if ... else if ... else...是很尋常的事情，那麼在GNU Makefile中如何使用呢？

ifeq
ifneq

例如：foo.sh

1 #!/bin/bash
 2 
 3 ARCH=$(uname -p)
 4 if [[ $ARCH == "x86_64" ]]; then
 5     ARCH32="i686"
 6     ARCH64="x86_64"
 7 elif [[ $ARCH == "ppc64le" ]]; then
 8     ARCH32=""
 9     ARCH64="ppc64le"
10 else
11     ARCH32=$ARCH
12     ARCH64=""
13 fi
14 
15 if [[ -n $ARCH32 ]]; then
16     OUTPUT+=" ARCH32=$ARCH32"
17 fi
18 
19 if [[ -n $ARCH64 ]]; then
20     OUTPUT+=" ARCH64=$ARCH64"
21 fi
22 
23 echo $OUTPUT

将上述的foo.sh用Makefile實作就是：

1 ARCH = $(shell uname -p)
 2 
 3 ifeq ($(ARCH), x86_64)
 4        ARCH32 = i686
 5        ARCH64 = x86_64
 6 else ifeq ($(ARCH), ppc64le)
 7        ARCH32 =
 8        ARCH64 = ppc64le
 9 else
10        ARCH32 = $(ARCH)
11        ARCH64 =
12 endif
13 
14 ifneq ($(ARCH32), )
15     OUTPUT += ARCH32=$(ARCH32)
16 endif
17 
18 ifneq ($(ARCH64), )
19     OUTPUT += ARCH64=$(ARCH64)
20 endif
21 
22 all: foo
23 foo:
24     @echo $(OUTPUT)

運作foo.sh 和 Makefile 結果如下：

$ uname -p
x86_64
$ bash foo.sh
ARCH32=i686 ARCH64=x86_64
$ make -f Makefile
ARCH32=i686 ARCH64=x86_64

由此可見，

ifeq($(VAR), ) 等價于bash中的 [[ -z $VAR ]]
ifneq($(VAR), ) 等價于bash中的 [[ -n $VAR ]]

那麼，在GNU Makefile中使用條件控制結構有什麼用？很簡單，對不同的平台提供不同的編譯選項或者安裝包支援。例如：

一個針對不同的平台提供不同的安裝包的Makefile

1 ARCH = $(shell uname -p)
 2 
 3 ifeq ($(ARCH), x86_64)
 4        ARCH32 = i686
 5        ARCH64 = x86_64
 6 else ifeq ($(ARCH), ppc64)
 7        ARCH32 =
 8        ARCH64 = ppc64
 9 else ifeq ($(ARCH), ppc64le)
10        ARCH32 =
11        ARCH64 = ppc64le
12 else ifeq ($(ARCH), s390x)
13        ARCH32 =
14        ARCH64 = s390x
15 else ifeq ($(ARCH), aarch64)
16        ARCH32 =
17        ARCH64 = aarch64
18 else
19        ARCH32 = $(ARCH)
20        ARCH64 =
21 endif
22 
23 ifneq ($(ARCH32), )
24        CPKGS32 = libgcc.$(ARCH32)
25        CPKGS32 += glibc.$(ARCH32)
26 endif
27 ifneq ($(ARCH64), )
28        CPKGS64 = libgcc.$(ARCH64)
29        CPKGS64 += glibc.$(ARCH64)
30 endif
31 CPKGS = $(CPKGS32) $(CPKGS64)
32 
33 all: foo
34 foo:
35     @echo $(CPKGS)

參考資料：

7.2 Syntax of Conditionals