Makefile 每次都构建目标

Question

我已经编写了一个 makefile 来编译不同目录中的所有源并构建一个目标。即使文件没有更改，当我运行 make 时，它​​也不会重新编译未修改的文件。但是，它总是构建目标。为什么每次运行 make 时都会单独构建目标？

CC = gcc
CFLAGS = $(INCLUDES)
TARGET = FinalBin
OUTDIR := obj

INCLUDES = -Isrc/in
S1 = src/s1
S2 = src/s2

S1_SRC = $(wildcard $(S1)/*.c)
S2_SRC = $(wildcard $(S2)/*.c)
SRCS := $(S1_SRC) \
    $(S2_SRC)

OBJS := $(patsubst %.c,$(OUTDIR)/%.o,$(SRCS))

SRCDIRS := $(S1) $(S2)

all: $(TARGET)

$(TARGET): builddir $(OBJS)
    @echo "Building..." $@
    @$(CC) $(OBJS) $(CFLAGS) -o $@
    @echo "Build Complete!"

$(OUTDIR)/%.o: %.c
    @echo "Compiling.." $(notdir $<)
    @$(CC) $(CFLAGS) -MMD -c $< -o $@

clean:
    -@rm -rf $(OUTDIR) FinalBin
    @echo "Clean complete!"

builddir :
    @$(call create-dir)

define create-dir
    for dir in $(SRCDIRS); \
    do \
        mkdir -p $(OUTDIR)/$$dir; \
    done
endef

-include $(wildcard $(OBJS:.o=.d))

$(TARGET) 每次都是从目标文件构建的。请指导我哪里出错了。

更新：

CC = gcc
CFLAGS = $(INCLUDES)
TARGET = FinalBin
OUTDIR := obj

INCLUDES = -Isrc/in
S1 = src/s1
S2 = src/s2

S1_SRC = $(wildcard $(S1)/*.c)
S2_SRC = $(wildcard $(S2)/*.c)
SRCS := $(S1_SRC) \
    $(S2_SRC)

OBJS := $(patsubst %.c,$(OUTDIR)/%.o,$(SRCS))

SRCDIRS := $(S1) $(S2)
OUTDIRS := $(addprefix $(OUTDIR)/,$(SRCDIRS))

all: $(TARGET)

$(TARGET): $(OBJS)
    @echo "Building..." $@
    @$(CC) $(OBJS) $(CFLAGS) -o $@
    @echo "Build Complete!"

$(OUTDIR)/%.o: $(OUTDIRS) %.c
    @echo "Compiling.." $(notdir $(filter %.c,$^))
    @$(CC) $(CFLAGS) -MMD -c $(filter %.c,$^) -o $@

clean:
    -@rm -rf $(OUTDIR) FinalBin
    @echo "Clean complete!"

$(OUTDIRS):
    mkdir -p $@

builddir :
    @$(call create-dir)

define create-dir
    for dir in $(SRCDIRS); \
    do \
        mkdir -p $(OUTDIR)/$$dir; \
    done
endef

-include $(wildcard $(OBJS:.o=.d))

这每次都会构建所有文件。

最终解决方案：

CC = gcc
CFLAGS = $(INCLUDES)
TARGET = FinalBin
OUTDIR := obj

INCLUDES = -Isrc/in
S1 = src/s1
S2 = src/s2

S1_SRC = $(wildcard $(S1)/*.c)
S2_SRC = $(wildcard $(S2)/*.c)
SRCS := $(S1_SRC) \
    $(S2_SRC)

OBJS := $(patsubst %.c,$(OUTDIR)/%.o,$(SRCS))

SRCDIRS := $(S1) $(S2)
OUTDIRS := $(addprefix $(OUTDIR)/,$(SRCDIRS))

all: $(TARGET)

$(TARGET): $(OUTDIRS) $(OBJS)
    @echo "Building..." $@
    @$(CC) $(OBJS) $(CFLAGS) -o $@
    @echo "Build Complete!"

$(OUTDIR)/%.o: %.c
    @echo "Compiling.." $(notdir $<)
    @$(CC) $(CFLAGS) -MMD -c $< -o $@

clean:
    -@rm -rf $(OUTDIR) FinalBin
    @echo "Clean complete!"

$(OUTDIRS):
    @mkdir -p $@

-include $(wildcard $(OBJS:.o=.d))

最终解决方案按预期工作！

Answer 1

您可以使用 make 的调试输出来查找它认为需要重建的内容：

make -d | grep remake

您可能会看到如下内容：

No need to remake target `Makefile'.
    Must remake target `builddir'.
      No need to remake target `src/s1/x.c'.
    No need to remake target `obj/src/s1/x.o'.
  Must remake target `FinalBin'.
Must remake target `all'.

这说明它一直认为builddir目标需要重做。由于这是$(TARGET) 的依赖项，因此它也重建了后者。

如果您使用规则而不是函数来创建构建目录，make 将知道是否需要创建它们。例如，添加变量$(OUTDIRS)，为其添加规则，并使其成为编译的依赖项：

SRCDIRS := $(S1) $(S2)
OUTDIRS := $(addprefix $(OUTDIR)/, $(SRCDIRS)) # <---------- Add this variable

All: $(TARGET) 

$(TARGET):  $(OBJS)
    @echo "Building..." $@
    @$(CC) $(OBJS) $(CFLAGS) -o $@
    @echo "Build Complete!"

$(OUTDIR)/%.o: $(OUTDIRS) %.c # <--------------------------- Add dependency to $(OUTDIRS)
    @echo "Compiling.." $(notdir $<)
    @$(CC) $(CFLAGS) -MMD -c $< -o $@

clean:
    -@rm -rf $(OUTDIR) FinalBin
    @echo "Clean complete!"

$(OUTDIRS): # <--------------------------------------------- Add rule
    mkdir -p $@

-include $(wildcard $(OBJS:.o=.d))