Using sizeof on VLAs gives an incorrect warning.

[neo313]

Using sizeof operator on variable-length arrays gives an incorrect Expression with no effect removed. warning.

Code: [Select]


#include <stdlib.h>
#include <string.h>
#include <stdio.h>

int main( void )
{
const size_t size = 12 ;
char a[size] ;     //warnings are not present if size is constant: char a[12] ;

size_t a_size = sizeof( a ) ;

int n = snprintf( a , a_size , "Copy text ABCDEFG." ) ;
printf("%s\n%d\n" , a , n ) ;


n = snprintf( a , sizeof( a ) , "Copy text ABCDEFG." ) ;
printf("%s\n%d\n" , a , n ) ;

printf( "%zu" , sizeof( a ) ) ;

return 0 ;
}

Gives warnings:
main.c(11): warning #2046: Expression with no effect removed.
main.c(17): warning #2046: Expression with no effect removed.
main.c(20): warning #2046: Expression with no effect removed.

Level 2 warnings must be used. Optimization or other project options don't have any effect. I get this result on 32bit RC5 and RC4 version.

[frankie]

It seems a compiler development debug info promoted to warning.
Substantially it advices that the compiler doesn't need to use the sizeof operator because the size is in a variable, when it is declared as static size the compiler must use the sizeof operator to retrieve the size ... 
If you look to assembly in debug you will see that in the first case it uses the content of variable 'size' as array size, in the other case it uses a constant value presumably coming from the internal sizeof operation.

[Pelle]

For a VLA, any side-effects of the *runtime* expression must be evaluated. The compiler function removing expressions without side-effects can either be silent or issue warnings. In this case the quick fix is to use the silent mode (and possibly teaching that function new tricks in the future).