如何修复GCC中的“此条件具有相同的分支”错误？

huangapple
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英文:

How to fix "this condition has identical branches" in GCC?

问题

  1. #include <stdio.h>
  3. struct file_format_a_header 
  4. {
  5.     int i;
  6. };
  8. struct file_format_b_header 
  9. {
  10.     int i;
  11. };
  13. int main()
  14. {
  15.     int type = 1;
  16.     size_t len = 0;
  18.     if (1 == type)
  19.     {
  20.         len = sizeof(struct file_format_a_header ); // sizeof file_format_a_header and file_format_b_header  is SAME but they are different structures
  21.     }
  22.     else
  23.     {
  24.         len = sizeof(struct file_format_b_header );
  25.     }
  27.     printf("len = %zu\n", len);
  28.     return 0;
  29. }
  1. <source>: In function 'main':
  2. <source>:18:8: error: this condition has identical branches [-Werror=duplicated-branches]
  3.    18 |     if (1 == type)
  4.       |        ^
  5. cc1: all warnings being treated as errors
  6. ASM generation compiler returned: 1
  7. <source>: In function 'main':
  8. <source>:18:8: error: this condition has identical branches [-Werror=duplicated-branches]
  9.    18 |     if (1 == type)
  10.       |        ^
  11. cc1: all warnings being treated as errors
  12. Execution build compiler returned: 1

GCC开关：

-std=gnu11
-Wall
-Werror
-Wextra
-Wduplicated-branches

问题：是否有办法解决这个GCC警告？

英文:

https://godbolt.org/z/YK5PPvz7d

  1. #include &lt;stdio.h&gt;
  3. struct file_format_a_header 
  4. {
  5.     int i;
  6. };
  8. struct file_format_b_header 
  9. {
  10.     int i;
  11. };
  13. int main()
  14. {
  15.     int type = 1;
  16.     size_t len = 0;
  18.     if (1 == type)
  19.     {
  20.         len = sizeof(struct file_format_a_header ); // sizeof file_format_a_header and file_format_b_header  is SAME but they are different structures
  21.     }
  22.     else
  23.     {
  24.         len = sizeof(struct file_format_b_header );
  25.     }
  27.     printf(&quot;len = %zu\n&quot;, len);
  28.     return 0;
  29. }
  32. &lt;source&gt;: In function &#39;main&#39;:
  33. &lt;source&gt;:18:8: error: this condition has identical branches [-Werror=duplicated-branches]
  34.    18 |     if (1 == type)
  35.       |        ^
  36. cc1: all warnings being treated as errors
  37. ASM generation compiler returned: 1
  38. &lt;source&gt;: In function &#39;main&#39;:
  39. &lt;source&gt;:18:8: error: this condition has identical branches [-Werror=duplicated-branches]
  40.    18 |     if (1 == type)
  41.       |        ^
  42. cc1: all warnings being treated as errors
  43. Execution build compiler returned: 1

GCC switches:

  1. -std=gnu11
  2. -Wall
  3. -Werror
  4. -Wextra
  5. -Wduplicated-branches

Question> Is there a way that I can fix this GCC warning?

答案1

得分: 3

gcc可能会在这里生成警告，因为sizeof操作符的结果是一个编译时常量（除非其操作数是变长数组）。

因此，它实际上看到了这样的情况：

  1. if (1 == type)
  2. {
  3.     len = 4;
  4. }
  5. else
  6. {
  7.     len = 4;
  8. }

即使涉及的两个结构体具有不同的成员名称、不同的成员类型或不同数量的成员，只要它们的大小相同，这种情况也会发生。

您可以通过在特定范围内添加pragma来忽略特定的警告来解决此问题：

  1. #pragma GCC diagnostic push
  2. #pragma GCC diagnostic ignored "-Wduplicated-branches"
  3. if (1 == type)
  4. {
  5.     len = sizeof(struct ABC);
  6. }
  7. else
  8. {
  9.     len = sizeof(struct abc);
  10. }
  11. #pragma GCC diagnostic pop

这样可以抑制特定警告。

英文:

gcc is likely generating a warning here because the result of the sizeof operator is a compile time constant (unless its operand is a variable length array).

So effectively it sees this:

  1.     if (1 == type)
  2.     {
  3.         len = 4;
  4.     }
  5.     else
  6.     {
  7.         len = 4;
  8.     }

This is happening even if the two structs in question have differently named members, differing member types, or a differing number of members, as long as the sizes are the same.

You can work around this by adding pragma to ignore this specific warning in a particular scope:

  1. #pragma GCC diagnostic push
  2. #pragma GCC diagnostic ignored &quot;-Wduplicated-branches&quot;
  3.     if (1 == type)
  4.     {
  5.         len = sizeof(struct ABC);
  6.     }
  7.     else
  8.     {
  9.         len = sizeof(struct abc);
  10.     }
  11. #pragma GCC diagnostic pop

huangapple
  • 本文由 发表于 2023年6月9日 03:17:49
  • 转载请务必保留本文链接：https://go.coder-hub.com/76435062.html
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