C Language Aliasing and effective type Changing bytes
Example
Once an object has an effective type, you should not attempt to modify it through a pointer of another type, unless that other type is a character type, char, signed char or unsigned char.
#include <inttypes.h>
#include <stdio.h>
int main(void) {
uint32_t a = 57;
// conversion from incompatible types needs a cast !
unsigned char* ap = (unsigned char*)&a;
for (size_t i = 0; i < sizeof a; ++i) {
/* set each byte of a to 42 */
ap[i] = 42;
}
printf("a now has value %" PRIu32 "\n", a);
}
This is a valid program that prints
a now has value 707406378
This works because:
- The access is made to the individual bytes seen with type
unsigned charso each modification is well defined. - The two views to the object, through
aand through*ap, alias, but sinceapis a pointer to a character type, the strict aliasing rule does not apply. Thus the compiler has to assume that the value ofamay have been changed in theforloop. The modified value ofamust be constructed from the bytes that have been changed. - The type of
a,uint32_thas no padding bits. All its bits of the representation count for the value, here707406378, and there can be no trap representation.
