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

June 8, 2023

#include <stdio.h>

struct file_format_a_header 
{
    int i;
};

struct file_format_b_header 
{
    int i;
};

int main()
{
    int type = 1;
    size_t len = 0;

    if (1 == type)
    {
        len = sizeof(struct file_format_a_header ); // sizeof file_format_a_header and file_format_b_header  is SAME but they are different structures
    }
    else
    {
        len = sizeof(struct file_format_b_header );
    }

    printf("len = %zu\n", len);
    return 0;
}


<source>: In function 'main':
<source>:18:8: error: this condition has identical branches [-Werror=duplicated-branches]
   18 |     if (1 == type)
      |        ^
cc1: all warnings being treated as errors
ASM generation compiler returned: 1
<source>: In function 'main':
<source>:18:8: error: this condition has identical branches [-Werror=duplicated-branches]
   18 |     if (1 == type)
      |        ^
cc1: all warnings being treated as errors
Execution build compiler returned: 1

GCC switches:

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

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

>Solution :

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:

    if (1 == type)
    {
        len = 4;
    }
    else
    {
        len = 4;
    }

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:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wduplicated-branches"
    if (1 == type)
    {
        len = sizeof(struct ABC);
    }
    else
    {
        len = sizeof(struct abc);
    }
#pragma GCC diagnostic pop