C Language Undefined behavior Modifying a const variable using a pointer
Example
int main (void)
{
const int foo_readonly = 10;
int *foo_ptr;
foo_ptr = (int *)&foo_readonly; /* (1) This casts away the const qualifier */
*foo_ptr = 20; /* This is undefined behavior */
return 0;
}
Quoting ISO/IEC 9899:201x, section 6.7.3 §2:
If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined. [...]
(1) In GCC this can throw the following warning: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
