Consider an allocated byte array: casting its address to unsigned* is permitted as a conversion, but reading through *p violates the effective-type rule. TAKEAWAY: Objects must be accessed through their effective type or through a pointer to a character type. section 12.6
The core access rules are:
- A variable or compound literal has the effective type specified by its declaration and cannot be accessed through an unrelated non-character type. section 12.6
- Any object may be viewed through a character type, but an unsigned-char array cannot be read through another type. section 12.6
- A union object's effective type is the union type; accessing a member is allowed when its byte representation forms a valid value of the access type. section 12.6
- Allocated storage begins uninitialized and typeless; assignments to its elements provide effective types and values. section 13.1
A cast does not authorize access
unsigned char A[sizeof(unsigned)] = { 9 }; unsigned* p = (unsigned*)A; printf("value %u\n", *p); // invalid access The cast creates an unsigned* value, but A remains an unsigned-char array. Dereferencing p accesses the bytes through neither the effective type nor a character type, leaving the program state undefined. section 12.6
unsigned char A[sizeof(unsigned)] = { 9 };
// Valid but useless, as most casts are
unsigned* p = (unsigned*)A;
// Error: access with a type that is neither the effective type nor a
// character type
printf("value \%u\n", *p);
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Inspect the complete example showing a valid pointer conversion followed by an invalid dereference. section 12.6
