6、实现baremetal编译

最终希望的 baremetal 目录应包括src、obj、output，其中src目录下源码和头文件可以随意放置。如下：

├── Makefile
├── obj
│   ├── board
│   │   └── prog1.o
│   ├── common
│   │   └── prog2.o
│   └── main.o
├── output
│   └── target
├── README.md
└── src
        ├── board
        │   ├── prog1.c
        │   └── prog1.h
        ├── common
        │   ├── prog2.c
        │   └── prog2.h
        └── main.c

Makefile需要实现自动找到所有的 .c文件，编译成 .o 文件，并最后链接生成 binary 放入 output。这样可以在任意目录增加 c文件或者头文件，都会被编译进去。

基于上一个例子修改如下：

# ------------------------
# Generic Makefile
# ------------------------

# Project name
Target = target

# Compile command and flag
CC = gcc
CFLAG = -Wall

# Linker command and flag
LINKER = gcc
LFLAG =

# Dir
SRC_DIR = src
OBJ_DIR = obj
BIN_DIR = output

# Source directorys, find all source directory ,like src/board src/common
SRC_DIRS        = $(shell find src -maxdepth 3 -type d)

# OBJ_DIRS, change src to obj, match the source directorys, like obj/board obj/common
OBJ_DIRS        := $(foreach dir,$(SRC_DIRS),$(subst src,obj,$(dir)))

# INCLUDES, add source directorys to include, like -Isrc/board -Isrc/common
INCLUDES        = $(foreach dir, $(SRC_DIRS),-I$(dir))

# Source files, c srouce files and asmeble source files. like src/board/test.c src/board/test.S
C_SRC   += $(foreach dir,$(SRC_DIRS),$(wildcard $(dir)/*.c))
S_SRC   := $(foreach dir,$(SRC_DIRS),$(wildcard $(dir)/*.S))

# OBJ files, object files. like obj/board/test.o obj/board/test.o
OBJ_S_FILES     := $(foreach file,$(S_SRC),$(patsubst %.S,%.o,$(subst src,obj,$(file))))
OBJ_C_FILES     := $(foreach file,$(C_SRC),$(patsubst %.c,%.o,$(subst src,obj,$(file))))
OBJ_FILES       := $(OBJ_S_FILES) $(OBJ_C_FILES)

# 1. Create obj directorys and bin directory
# 2. Comple all OBJ_FILES, from .c .S to .o
# 3. Link all .o to binary target
$(BIN_DIR)/$(Target) : $(OBJ_DIRS) $(BIN_DIR) $(OBJ_FILES)
        $(LINKER) -o $@ $(LFLAG) $(OBJ_FILES)
        @echo "Linking complete!"

# Compile .c to .o
obj/%.o : src/%.c
        @echo Compiling $< to $@
        $(CC) $(CFLAG) $(INCLUDES) -c $< -o $@


PHONY: clean
clean :
        rm -rf $(BIN_DIR) $(OBJ_DIR)
        @echo "Cleanup complete!"

$(OBJ_DIRS):
        mkdir -p $@

$(BIN_DIR):
        mkdir -p $@

注解

大部分都通过注释说明了，其中有几个makefile专有命令说明下 。

$(foreach <var>,<list>,<text>)

names := a b c d
files := $(foreach n,$(names),$(n).o)

$(files)的值是“a.o b.o c.o d.o”

$(subst <from>,<to>,<text>)
把text中的from 替换为 to

$(subst ee,EE,feet on the street)，
“fEEt on the strEEt”

$(patsubst %.c,%.o,x.c.c bar.c)
把字串“x.c.c bar.c”符合模式[%.c]的单词替换成[%.o]，返回结果是“x.c.o bar.o”