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[开发工具] GCC对C语言的几个扩展

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 楼主| zhanzr21 发表于 2020-7-19 18:07 | 显示全部楼层 |阅读模式
GCC, C语言, ST, TE, IO
本帖最后由 zhanzr21 于 2020-7-19 18:11 编辑

不管是True Studio, SW4STM32, 还是最新官方出的STM32CubeIDE, 内置编译工具链默认都是arm-none-eabi-gcc. GCC对C语言的标准支持一般是编译器中比较齐全和迅速的, 除此之外GCC还有一些对ISO C标准的若干扩展. 如果暂时不考虑可移植性的话, 可以合理利用这些扩展. 当然是否利用这些扩展是程序员自己决定, 这里简单介绍几个扩展特性, 以供参考.

内嵌函数

即在函数内部定义函数
复制
  1. int main(void)
    

  2. {
    

  3.         /* USER CODE BEGIN 1 */
    

  4.         void nested_func(void) {
    

  5.                 printf("Inside a nested function!\n");
    

  6.                 printf("%s\n", __VERSION__);
    

  7.         }
    

  8. ....
    


  9. 

  10.         printf("Test nested function\n");
    

  11.         nested_func();
    

  12.         printf("\n");
    

  13. .....
参考输出:
复制
  1. Test nested function
    

  2. Inside a nested function!
    

  3. 7.3.1 20180622 (release) [ARM/embedded-7-branch revision 261907]






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 楼主| zhanzr21 发表于 2020-7-19 18:10 | 显示全部楼层
0长度数组

用于在结构体中保留后续扩展空间.

复制
  1. struct msg_header {
    

  2.         uint32_t type;
    

  3.         uint32_t size;
    

  4.         uint8_t content[0];
    

  5. };
    


  6. 


  7. 

  8.         printf("Test zero length of array\n");
    

  9.         {
    

  10.                 struct msg_header *msg;
    

  11.                 int size = 128;
    


  12. 

  13.                 msg = (struct msg_header *) malloc(sizeof (struct msg_header) + size);
    


  14. 

  15.                 msg->type = 0x01;
    

  16.                 msg->size = size;
    


  17. 

  18.                 printf("msg header  is at %p\n", (void *)msg);
    

  19.                 printf("msg content is at %p\n", (void *)msg->content);
    

  20.                 free(msg);
    

  21.         }
    

  22.         printf("\n");
参考输出:
复制
  1. Test zero length of array
    

  2. msg header  is at 0x20000b88
    

  3. msg content is at 0x20000b90




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 楼主| zhanzr21 发表于 2020-7-19 18:13 | 显示全部楼层
范围case

C程序员大多写过这样的代码:
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  1. case 0:
    

  2. case 1:
    

  3. case 2:
    

  4. ....
    

  5. break;
使用这个gcc扩展, 可以更方便.
复制
  1. void ranges(char c) {
    

  2.         switch(c) {
    

  3.         case '0' ... '9':
    

  4.         printf("[%c] is a number.\n", c);
    

  5.         break;
    


  6. 

  7.         case 'a' ... 'z':
    

  8.         printf("[%c] is a lowercase letter.\n", c);
    

  9.         break;
    


  10. 

  11.         case 'A' ... 'Z':
    

  12.         printf("[%c] is an uppercase letter.\n", c);
    

  13.         break;
    


  14. 

  15.         default:
    

  16.                 printf("[%c] is not a valid character!\n", c);
    

  17.                 break;
    

  18.         }
    

  19. }
    


  20. 

  21.         printf("Test case range\n");
    

  22.         {
    

  23.                 ranges('a');
    

  24.                 ranges('8');
    

  25.         }
    

  26.         printf("\n");
参考输出:
复制
  1. Test case range
    

  2. [a] is a lowercase letter.
    

  3. [8] is a number.




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 楼主| zhanzr21 发表于 2020-7-19 18:14 | 显示全部楼层
typeof运行时推断类型

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  1. #define max(a,b) \
    

  2.                 ({ typeof (a) _a = (a); \
    

  3.                 typeof (b) _b = (b); \
    

  4.                 _a > _b ? _a : _b; })
    


  5. 

  6.     printf("Test typeof\n");
    

  7.     {
    

  8.         int x = 1024, y = 4096;
    

  9.         char a = 10, b = 20;
    

  10.         float j = 1.0, k = 2.0;
    


  11. 

  12.         printf("char  max is %d\n", max(a,b));
    

  13.         printf("int   max is %d\n", max(x,y));
    

  14.         printf("float max is %f\n", max(j,k));
    

  15.     }
    

  16.     printf("\n");
参考输出:
复制
  1. Test typeof
    

  2. char  max is 20
    

  3. int   max is 4096
    

  4. float max is 2.000000




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 楼主| zhanzr21 发表于 2020-7-19 18:15 | 显示全部楼层
空结构体

复制
  1. struct empty {
    

  2. };
    


  3. 

  4.         printf("Test empty structure\n");
    

  5.         printf("sizeof struct empty is %u\n", sizeof(struct empty));
    

  6.         printf("\n");
参考输出:
复制
  1. Test empty structure
    

  2. sizeof struct empty is 0




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 楼主| zhanzr21 发表于 2020-7-19 18:17 | 显示全部楼层
二进制立即数, 三元操作符简化写法


这是两个扩展,用一个例子表达:
复制
  1.         printf("binary immediate and ternary operator\n");
    

  2.         {
    

  3.                 int a = 10;
    

  4.                 int x = 0;
    


  5. 

  6.                 a = x ? : 0b0011;
    


  7. 

  8.                 printf("a is %d.\n", a);
    

  9.         }
    

  10.         printf("\n");
参考输出:
复制
  1. binary immediate and ternary operator
    

  2. a is 3.




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 楼主| zhanzr21 发表于 2020-7-19 18:22 | 显示全部楼层
以上介绍的介绍的几个GCC扩展特性, 共程序员们参考, 是否利用扩展, 取决于自己的选择.
事实上很多厂商提供的驱动代码中也有利用一些GCC扩展, 有些GCC扩展特性应用很广泛, clang, armcc等编译器也能支持. 以STM32CubeIDE为例, 可以打开-pedantic选项, 看整个工程应用了哪些gcc扩展.
pedantic_warning_enable.png
打开这个选项之后, 重新build工程, 可以看到对于以上故意利用扩展的例子, 编译器会给出详细的warning.
复制
  1. ../Core/Src/main.c:97:10: warning: ISO C forbids zero-size array 'content' [-Wpedantic]
    

  2.   uint8_t content[0];
    

  3.           ^~~~~~~
    

  4. ../Core/Src/main.c: In function 'ranges':
    

  5. ../Core/Src/main.c:102:2: warning: range expressions in switch statements are non-standard [-Wpedantic]
    

  6.   case '0' ... '9':
    

  7.   ^~~~
    

  8. ../Core/Src/main.c:103:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
    

  9.   printf("[%c] is a number.\n", c);
    

  10.   ^~~~~~
    

  11. ../Core/Src/main.c:103:2: warning: incompatible implicit declaration of built-in function 'printf'
    

  12. ../Core/Src/main.c:103:2: note: include '<stdio.h>' or provide a declaration of 'printf'
    

  13. ../Core/Src/main.c:106:2: warning: range expressions in switch statements are non-standard [-Wpedantic]
    

  14.   case 'a' ... 'z':
    

  15.   ^~~~
    

  16. ../Core/Src/main.c:110:2: warning: range expressions in switch statements are non-standard [-Wpedantic]
    

  17.   case 'A' ... 'Z':
    

  18.   ^~~~
    

  19. ../Core/Src/main.c: At top level:
    

  20. ../Core/Src/main.c:125:8: warning: struct has no members [-Wpedantic]
    

  21.  struct empty {
    

  22.         ^~~~~
    

  23. ../Core/Src/main.c: In function 'main':
    

  24. ../Core/Src/main.c:137:2: warning: ISO C forbids nested functions [-Wpedantic]
    

  25.   void nested_func(void) {
    

  26.   ^~~~
    

  27. ../Core/Src/main.c: In function 'nested_func':
    

  28. ../Core/Src/main.c:138:3: warning: incompatible implicit declaration of built-in function 'printf'
    

  29.    printf("Inside a nested function!\n");
    

  30.    ^~~~~~
    

  31. ../Core/Src/main.c:138:3: note: include '<stdio.h>' or provide a declaration of 'printf'
    

  32. ../Core/Src/main.c: In function 'main':
    

  33. ../Core/Src/main.c:169:2: warning: incompatible implicit declaration of built-in function 'printf'
    

  34.   printf("BOR: %u\n", __HAL_RCC_GET_FLAG(RCC_FLAG_BORRST));
    

  35.   ^~~~~~
    

  36. ../Core/Src/main.c:169:2: note: include '<stdio.h>' or provide a declaration of 'printf'
    

  37. ../Core/Src/main.c:193:31: warning: implicit declaration of function 'malloc' [-Wimplicit-function-declaration]
    

  38.    msg = (struct msg_header *) malloc(sizeof (struct msg_header) + size);
    

  39.                                ^~~~~~
    

  40. ../Core/Src/main.c:193:31: warning: incompatible implicit declaration of built-in function 'malloc'
    

  41. ../Core/Src/main.c:193:31: note: include '<stdlib.h>' or provide a declaration of 'malloc'
    

  42. ../Core/Src/main.c:200:3: warning: implicit declaration of function 'free' [-Wimplicit-function-declaration]
    

  43.    free(msg);
    

  44.    ^~~~
    

  45. ../Core/Src/main.c:200:3: warning: incompatible implicit declaration of built-in function 'free'
    

  46. ../Core/Src/main.c:200:3: note: include '<stdlib.h>' or provide a declaration of 'free'
    

  47. ../Core/Src/main.c:121:3: warning: ISO C forbids braced-groups within expressions [-Wpedantic]
    

  48.    ({ typeof (a) _a = (a); \
    

  49.    ^
    

  50. ../Core/Src/main.c:219:31: note: in expansion of macro 'max'
    

  51.    printf("char  max is %d\n", max(a,b));
    

  52.                                ^~~
    

  53. ../Core/Src/main.c:121:3: warning: ISO C forbids braced-groups within expressions [-Wpedantic]
    

  54.    ({ typeof (a) _a = (a); \
    

  55.    ^
    

  56. ../Core/Src/main.c:220:31: note: in expansion of macro 'max'
    

  57.    printf("int   max is %d\n", max(x,y));
    

  58.                                ^~~
    

  59. ../Core/Src/main.c:121:3: warning: ISO C forbids braced-groups within expressions [-Wpedantic]
    

  60.    ({ typeof (a) _a = (a); \
    

  61.    ^
    

  62. ../Core/Src/main.c:221:31: note: in expansion of macro 'max'
    

  63.    printf("float max is %f\n", max(j,k));
    

  64.                                ^~~
    

  65. ../Core/Src/main.c:236:11: warning: ISO C forbids omitting the middle term of a ?: expression [-Wpedantic]
    

  66.    a = x ? : 0b0011;
    

  67.            ^
    

  68. ../Core/Src/main.c:236:13: warning: binary constants are a GCC extension
    

  69.    a = x ? : 0b0011;
    

  70.              ^~~~~~
    

  71. ../Core/Src/main.c:243:28: warning: format '%u' expects argument of type 'unsigned int', but argument 2 has type 'uint32_t {aka long unsigned int}' [-Wformat=]
    

  72.   printf("Started ADC DMA, %u\n", SystemCoreClock);
    

  73.                            ~^
    

  74.                            %lu
    

  75. arm-none-eabi-gcc "../Core/Src/stm32g4xx_it.c" -mcpu=cortex-m4 -std=gnu11 -g3 -DSTM32G431xx -DARM_MATH_DSP -DUSE_NUCLEO_64 -DUSE_HAL_DRIVER -DDEBUG -c -I../Drivers/STM32G4xx_HAL_Driver/Inc/Legacy -I"F:/stm32_ide_prj/stm32g431_t1_stm32ide/Core/Src/STM32G4xx_Nucleo" -I../Drivers/CMSIS/Include -I../Drivers/CMSIS/Device/ST/STM32G4xx/Include -I../Core/Inc -I../Drivers/STM32G4xx_HAL_Driver/Inc -O0 -ffunction-sections -fdata-sections -Wall -pedantic -fstack-usage -MMD -MP -MF"Core/Src/stm32g4xx_it.d" -MT"Core/Src/stm32g4xx_it.o" --specs=nano.specs -mfpu=fpv4-sp-d16 -mfloat-abi=hard -mthumb -o "Core/Src/stm32g4xx_it.o"
    

  76. arm-none-eabi-gcc "../Core/Src/syscalls.c" -mcpu=cortex-m4 -std=gnu11 -g3 -DSTM32G431xx -DARM_MATH_DSP -DUSE_NUCLEO_64 -DUSE_HAL_DRIVER -DDEBUG -c -I../Drivers/STM32G4xx_HAL_Driver/Inc/Legacy -I"F:/stm32_ide_prj/stm32g431_t1_stm32ide/Core/Src/STM32G4xx_Nucleo" -I../Drivers/CMSIS/Include -I../Drivers/CMSIS/Device/ST/STM32G4xx/Include -I../Core/Inc -I../Drivers/STM32G4xx_HAL_Driver/Inc -O0 -ffunction-sections -fdata-sections -Wall -pedantic -fstack-usage -MMD -MP -MF"Core/Src/syscalls.d" -MT"Core/Src/syscalls.o" --specs=nano.specs -mfpu=fpv4-sp-d16 -mfloat-abi=hard -mthumb -o "Core/Src/syscalls.o"
    

  77. arm-none-eabi-gcc "../Core/Src/sysmem.c" -mcpu=cortex-m4 -std=gnu11 -g3 -DSTM32G431xx -DARM_MATH_DSP -DUSE_NUCLEO_64 -DUSE_HAL_DRIVER -DDEBUG -c -I../Drivers/STM32G4xx_HAL_Driver/Inc/Legacy -I"F:/stm32_ide_prj/stm32g431_t1_stm32ide/Core/Src/STM32G4xx_Nucleo" -I../Drivers/CMSIS/Include -I../Drivers/CMSIS/Device/ST/STM32G4xx/Include -I../Core/Inc -I../Drivers/STM32G4xx_HAL_Driver/Inc -O0 -ffunction-sections -fdata-sections -Wall -pedantic -fstack-usage -MMD -MP -MF"Core/Src/sysmem.d" -MT"Core/Src/sysmem.o" --specs=nano.specs -mfpu=fpv4-sp-d16 -mfloat-abi=hard -mthumb -o "Core/Src/sysmem.o"
    

  78. arm-none-eabi-gcc "../Core/Src/system_stm32g4xx.c" -mcpu=cortex-m4 -std=gnu11 -g3 -DSTM32G431xx -DARM_MATH_DSP -DUSE_NUCLEO_64 -DUSE_HAL_DRIVER -DDEBUG -c -I../Drivers/STM32G4xx_HAL_Driver/Inc/Legacy -I"F:/stm32_ide_prj/stm32g431_t1_stm32ide/Core/Src/STM32G4xx_Nucleo" -I../Drivers/CMSIS/Include -I../Drivers/CMSIS/Device/ST/STM32G4xx/Include -I../Core/Inc -I../Drivers/STM32G4xx_HAL_Driver/Inc -O0 -ffunction-sections -fdata-sections -Wall -pedantic -fstack-usage -MMD -MP -MF"Core/Src/system_stm32g4xx.d" -MT"Core/Src/system_stm32g4xx.o" --specs=nano.specs -mfpu=fpv4-sp-d16 -mfloat-abi=hard -mthumb -o "Core/Src/system_stm32g4xx.o"
    

  79. arm-none-eabi-gcc "../Core/Src/usart.c" -mcpu=cortex-m4 -std=gnu11 -g3 -DSTM32G431xx -DARM_MATH_DSP -DUSE_NUCLEO_64 -DUSE_HAL_DRIVER -DDEBUG -c -I../Drivers/STM32G4xx_HAL_Driver/Inc/Legacy -I"F:/stm32_ide_prj/stm32g431_t1_stm32ide/Core/Src/STM32G4xx_Nucleo" -I../Drivers/CMSIS/Include -I../Drivers/CMSIS/Device/ST/STM32G4xx/Include -I../Core/Inc -I../Drivers/STM32G4xx_HAL_Driver/Inc -O0 -ffunction-sections -fdata-sections -Wall -pedantic -fstack-usage -MMD -MP -MF"Core/Src/usart.d" -MT"Core/Src/usart.o" --specs=nano.specs -mfpu=fpv4-sp-d16 -mfloat-abi=hard -mthumb -o "Core/Src/usart.o"
    

  80. ../Core/Src/syscalls.c:41:1: warning: file-scope declaration of 'stack_ptr' specifies 'register' [-Wpedantic]
    

  81.  register char * stack_ptr asm("sp");
    

  82.  ^~~~~~~~
    

  83. ../Core/Src/sysmem.c:30:1: warning: file-scope declaration of 'stack_ptr' specifies 'register' [-Wpedantic]
    

  84.  register char * stack_ptr asm("sp");
    

  85.  ^~~~~~~~
    


  86.



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 楼主| zhanzr21 发表于 2020-7-19 18:24 | 显示全部楼层
除此之外, 还可以完全禁止gcc扩展, 这样工程就需要改很多代码, 因为厂商提供的驱动库也会依赖某些gcc扩展特性.
pedantic_warning_error_enable.png
复制
  1. ../Drivers/STM32G4xx_HAL_Driver/Src/stm32g4xx_ll_adc.c:1336:0: error: ISO C forbids an empty translation unit [-Wpedantic]
    

  2. ../Drivers/STM32G4xx_HAL_Driver/Src/stm32g4xx_ll_pwr.c:86:0: error: ISO C forbids an empty translation unit [-Wpedantic]
    

  3. make: *** [Drivers/STM32G4xx_HAL_Driver/Src/subdir.mk:133: Drivers/STM32G4xx_HAL_Driver/Src/stm32g4xx_ll_pwr.o] Error 1
    

  4. make: *** Waiting for unfinished jobs....
    

  5. make: *** [Drivers/STM32G4xx_HAL_Driver/Src/subdir.mk:131: Drivers/STM32G4xx_HAL_Driver/Src/stm32g4xx_ll_adc.o] Error 1
如果要完全禁止gcc扩展特性, 以上错误必须修复.

本贴完, 谢谢阅读!


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天灵灵地灵灵 发表于 2020-7-20 23:56 | 显示全部楼层
给力，以前好像真的没这功能。
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