简而言之,.cpp 文件中定义的全局 const 变量(数组)从 .o 文件中消失,导致“未定义的引用...”错误。通过在变量的定义中添加“易失性”可以完美解决该问题。关闭编译器优化或“-flto”是没有用的。
编译器:AVR-GCC 11.10,来自 https://github.com/ZakKemble/avr-gcc-build/releases。
环境:PlatformIO。
操作系统:Windows 11 x64。
定义如下:
// file: oled_basic_init_sequence.cpp
#include <Arduino.h>
namespace oled_basic {
const uint8_t _init_sequence_for_ssd1316_4_wire_spi[25] PROGMEM = {
0xae, /*display off*/
0x00, /*set lower column address*/
// ...
};
const uint8_t _init_sequence_for_ssd1306_2_wire_i2c[23] PROGMEM = {
0xAE,
0xD5,
// ...
};
} // namespace oled_basic
有或没有
PROGMEM
并不重要。在.h文件中:
// file: oled_basic_init_sequence.h
#pragma once
#include <stdint.h>
namespace oled_basic {
extern const uint8_t _init_sequence_for_ssd1316_4_wire_spi[25];
extern const uint8_t _init_sequence_for_ssd1306_2_wire_i2c[23];
} // namespace oled_basic
在main.cpp文件中:
#include <Arduino.h>
#include "oled_basic_init_sequence.h"
volatile uint8_t * test = 0;
void setup() {
*test = oled_basic::_init_sequence_for_ssd1316_4_wire_spi[0];
// this doesn't help
*test = pgm_read_byte(&(oled_basic::_init_sequence_for_ssd1316_4_wire_spi[0]));
}
void loop() {
}
编译失败:
Building in release mode
Compiling .pio\build\ATmega128\src\oled_basic_init_sequence.cpp.o
Linking .pio\build\ATmega128\firmware.elf
c:/users/xxxxx/.platformio/packages/toolchain-atmelavr/bin/../lib/gcc/avr/11.1.0/../../../../avr/bin/ld.exe: C:\Users\xxxxx\AppData\Local\Temp\ccelvOYl.ltrans0.ltrans.o: in function `main':
<artificial>:(.text.startup+0x5a): undefined reference to `_ZN10oled_basic37_init_sequence_for_ssd1316_4_wire_spiE'
collect2.exe: error: ld returned 1 exit status
*** [.pio\build\ATmega128\firmware.elf] Error 1
========================================================= [FAILED] Took 3.42 seconds =========================================================
详细构建信息:
avr-g++ -o .pio\build\ATmega128\firmware.elf -mmcu=atmega128 -Os -Wl,--gc-sections -flto -fuse-linker-plugin .pio\build\ATmega128\src\main.cpp.o .pio\build\ATmega128\src\oled_basic_font.cpp.o .pio\build\ATmega128\src\oled_basic_init_sequence.cpp.o -L.pio\build\ATmega128 -Wl,--start-group .pio\build\ATmega128\lib81f\libi2c.a .pio\build\ATmega128\lib685\libSPI.a .pio\build\ATmega128\libe29\libWire.a .pio\build\ATmega128\libFrameworkArduinoVariant.a .pio\build\ATmega128\libFrameworkArduino.a -lm -Wl,--end-group
也许我只是犯了一些低级语法错误,但仍然无法意识到。
包括
oled_basic_init_sequence.h
中的 oled_basic_init_sequence.cpp
。
namespace oled_basic {
const uint8_t _init_sequence_for_ssd1316_4_wire_spi[25] = {
0xae, /*display off*/
0x00, /*set lower column address*/
// ...
};
}
如果没有使用
extern
关键字对变量进行前向声明,此代码将使用 内部链接定义
_init_sequence_for_ssd1316_4_wire_spi
:它不能在其他翻译单元中使用。
通过前向声明,变量将通过外部链接来定义。
namespace oled_basic {
extern const uint8_t _init_sequence_for_ssd1316_4_wire_spi[25];
const uint8_t _init_sequence_for_ssd1316_4_wire_spi[25] = {
0xae, /*display off*/
0x00, /*set lower column address*/
// ...
};
}
我们可以在 compiler explorer 上看到编译器为第二个片段中的变量发出全局符号,但不是第一个片段:
.global oled_basic::_init_sequence_for_ssd1316_4_wire_spi
.section .rodata
.type oled_basic::_init_sequence_for_ssd1316_4_wire_spi, @object
.size oled_basic::_init_sequence_for_ssd1316_4_wire_spi, 25
oled_basic::_init_sequence_for_ssd1316_4_wire_spi:
.string "\256"
.string ""
.zero 22