使用 arm-none-eabi-g++ 链接成功，但没有 arm-none-eabi-gcc

Question

我正在使用 Launchpad Arm 编译器工具。具体来说，

arm-none-eabi-g++ 和 arm-none-eabi-gcc 来自：

（用于 ARM 嵌入式处理器的 GNU 工具）5.2.1 20151202（发布）[ARM/embedded-5-branch 修订版 231848]

我有一个针对 STM32F103 处理器的简单程序，除了证明我可以编写硬件并调用数学库中的函数之外，没有其他目的。就是这样：

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include "stm32f10x.h"

void hardwareTest(void){
   // Turn on the clock for PortB
   RCC->APB2ENR = RCC_APB2ENR_IOPBEN; // Turn on IO Port B
   // Put PB0 into push pull 50 MHz mode
   GPIOB->CRL = 0x03;
   // Turn PB0 on
   GPIOB->ODR = 1;
}

volatile int x; // force call to sqrt() later

int main(void) {
   x = sqrt(100.0f);
   x = sqrt(x);
   hardwareTest();
   return (x);
}

当我尝试构建它时，我收到一个链接器错误，告诉我有一个未定义的对 sqrt 的引用。构建是使用 arm-none-eabi-gcc 完成的。偶然我发现，如果使用 arm-none-eabi-g++ 完成构建，使用相同的命令行参数，则链接会成功执行。

我写了一个 Makefile 来演示区别：

PROJECT = minimal
SOURCES = src/startup_stm32f10x_hd.s \
          src/system_stm32f10x.c \
          src/main.c
OUTPUT = ./out
print-%:
    @echo '$*=$($*)'

TOOLCHAIN = arm-none-eabi-

CXX = $(TOOLCHAIN)g++
CC = $(TOOLCHAIN)gcc
AR = $(TOOLCHAIN)ar
AS = $(TOOLCHAIN)gcc -c -x assembler-with-cpp
LD =  $(TOOLCHAIN)gcc
OBJCOPY = $(TOOLCHAIN)objcopy
OBJDUMP = $(TOOLCHAIN)objdump
SIZE = $(TOOLCHAIN)size
RM = rm -f

CFLAGS  = -O
CFLAGS += -nostartfiles

CXXFLAGS  = -O
CXXFLAGS += -nostartfiles

ARCH = -mcpu=cortex-m3 -mthumb -DSTM32F10X_HD 
LDFLAGS = 

all: clean $(PROJECT).elf $(PROJECT).gcc $(PROJECT).bin

$(PROJECT).bin: $(PROJECT).elf 
    @echo ' ======== '
    @echo ' Generating binaries'
    $(OBJCOPY) -O binary $(OUTPUT)/$< $(OUTPUT)/$(PROJECT).bin
    $(OBJCOPY) -O ihex   $(OUTPUT)/$< $(OUTPUT)/$(PROJECT).hex
    @echo ' ======== '

$(PROJECT).elf: $(SOURCES)
    @echo ' ======== '
    @echo ' Successful build uses g++'
    @echo ' CXXFLAGS = $(CXXFLAGS)'
    @echo ' LDFLAGS = $(LDFLAGS)'
    @echo ' ARCH = $(ARCH)'
    $(CXX) -o $(OUTPUT)/$@ $(ARCH)  $(CXXFLAGS) $(LDFLAGS) -Wl,-Tld_script/stm32.ld,-lm  $^
    @echo ' ======== '

$(PROJECT).gcc: $(SOURCES)
    @echo ' ======== '
    @echo ' Broken build uses gcc'
    @echo ' CFLAGS = $(CFLAGS)'
    @echo ' LDFLAGS = $(LDFLAGS)'
    @echo ' ARCH = $(ARCH)'
    $(CC) -o $(OUTPUT)/$@ $(ARCH)   $(CFLAGS) $(LDFLAGS) -Wl,-Tld_script/stm32.ld,-lm  $^
    @echo ' ======== '

$(PROJECT).gxx: $(SOURCES)
    @echo ' ======== '
    @echo ' build with g++'
    $(CXX) -o $(OUTPUT)/$@ $(ARCH)  $(CXXFLAGS) $(LDFLAGS) -Wl,-Tld_script/stm32.ld  $^
    @echo ' ======== '

# Program the binary to the board using the builtin serial bootloader
program:
    stm32loader.py -p /dev/ttyUSB0 -ewv $(OUTPUT)/$(PROJECT).bin

# Remove the temporary files
clean:
    @echo ' '
    @echo ' Cleaning up: '
    $(RM) $(OUTPUT)/* *.o *.elf *.bin *.hex *.gcc *.gxx *.g++
    @echo ' ======== '

它给出以下结果：

Cleaning up: 
rm -f ./out/* *.o *.elf *.bin *.hex *.gcc *.gxx *.g++
======== 
======== 
 Successful build uses g++
 CXXFLAGS = -O -nostartfiles
 LDFLAGS = 
 ARCH = -mcpu=cortex-m3 -mthumb -DSTM32F10X_HD 
arm-none-eabi-g++ -o ./out/minimal.elf -mcpu=cortex-m3 -mthumb -DSTM32F10X_HD   -O -nostartfiles  -Wl,-Tld_script/stm32.ld,-lm  src/startup_stm32f10x_hd.s src/system_stm32f10x.c src/main.c
 ======== 
 ======== 
 Broken build uses gcc
 CFLAGS = -O -nostartfiles
 LDFLAGS = 
 ARCH = -mcpu=cortex-m3 -mthumb -DSTM32F10X_HD 
arm-none-eabi-gcc -o ./out/minimal.gcc -mcpu=cortex-m3 -mthumb -DSTM32F10X_HD    -O -nostartfiles  -Wl,-Tld_script/stm32.ld,-lm  src/startup_stm32f10x_hd.s src/system_stm32f10x.c src/main.c
/var/folders/t4/dv7b46055cjgknp4nndn1_zr0000gn/T//ccbl4swG.o: In function `main':
main.c:(.text+0x28): undefined reference to `sqrt'
collect2: error: ld returned 1 exit status
make: *** [minimal.gcc] Error 1
 ======== 
 Generating binaries
arm-none-eabi-objcopy -O binary ./out/minimal.elf ./out/minimal.bin
arm-none-eabi-objcopy -O ihex   ./out/minimal.elf ./out/minimal.hex
make: Target `all' not remade because of errors.

那么谁能告诉我为什么这两个编译器的行为不同？我忽略了什么简单的事情？如果我想使用 arm-none-eabi-gcc，我应该如何确保与 libm 和其他人的正确链接？

我查看了 Freddie Chopin 的 makefile，但它们太复杂了，我无法解开。

Answer 1

C++ 要求数学函数是基本运行时的一部分，而 C 允许它们在库中。 GCC 实现通过在 C++ 构建中自动链接 libm 来实现这一点。

链接阶段还有很多其他的不同；如果使用 C 链接器，C++ 链接将始终失败。

对于 C 链接，使用 C 链接器并指定 -lm 以使 libm 可用。