As per the link C Switch-case curly braces after every case , the below code should not compile since the variable is declared within switch without braces.
case 2:
int s = 0;
printf("enter side value:");
scanf("%d", &s);
printf("area of square is %d", squarearea(s));
break;
However lot of online c editors ( https://www.onlinegdb.com/online_c_compiler , https://www.programiz.com/c-programming/online-compiler/ ) are allowing this program to compile and run .
Why is it so ? These online compilers are claiming to use gcc but then why is this difference in behavior?
Testing with Compiler Explorer suggests GCC changed in version 11 to accept labeled declarations, which is an extension to the current C standard language.
In the OnlineGDB compiler you link to, adding the extra switch -pedantic produces a warning message, confirming that the compiler is treating this as an extension to the language but knows it is not part of the C standard.
GCC documents this: “ISO C2X allows labels to be placed before declarations and at the end of a compound statement. As an extension, GNU C also allows all this in C90 mode.” (“ISO C2X” means the upcoming ISO C standard, likely to be C 2023.)