An example of C99 strict aliasing issue...

strict-aliasing-bug.c

#include <stdio.h>

struct s1
{
    int i;
};

struct s2
{
    int i;
};

static int __f( struct s1 * s1, struct s2 * s2 );
static int __f( struct s1 * s1, struct s2 * s2 )
{
    if( s1->i == 1 )
    {
        s2->i = 0;
    }
    
    return s1->i;
}

int main( void )
{
    struct s1   s1;
    struct s1 * s1p;
    struct s2 * s2p;
    
    s1.i = 1;
    s1p  = &s1;
    s2p  = ( struct s2 * )&s1;
    
    printf( "%i\n", __f( s1p, s2p ) );
    
    return 0;
}

Weird:

❯❯❯ gcc -std=c99 strict-aliasing-bug.c && ./a.out
0

❯❯❯ gcc -std=c99 -fstrict-aliasing strict-aliasing-bug.c && ./a.out
0

❯❯❯ gcc -std=c99 -fno-strict-aliasing strict-aliasing-bug.c && ./a.out
0