Why Can a char* Alias Other Object Pointers but Not the Other Way Around?

Why the Strict Aliasing Rule Allows char* to Alias Other Object Pointers

The strict aliasing rule, as mentioned in the accepted answer to "What is the strict aliasing rule?", states that pointers of different types may not be used to access the same memory location. However, an exception to this rule exists for char*.

The Aliasing Exception for char*

Despite the strict aliasing rule, it is allowed to use a char to alias a pointer of any other object type. This is possible because a char can access the memory at any address with a single-byte granularity. Therefore, a char* can correctly read the bytes that compose an object of any other type.

One-Way Aliasing

The aliasing exception for char is one-way. This means that while a char can alias a pointer of any other object type, the reverse is not true. A pointer of another object type, such as struct something, cannot be used to alias a char.

Reason for the Asymmetry

The reason for this asymmetry lies in the potential for data corruption. If a pointer of another object type was allowed to alias a char, it would be possible to read the bytes of the char using the more permissive alignment rules of the object pointer. This could lead to unpredictable and incorrect behavior, as the bytes of the char* may not be correctly aligned for the object type.

Example

To illustrate the exception, consider the following example:

char* charptr;
struct something* structptr;
charptr = (char*) structptr; // Allowed
structptr = (struct something*) charptr; // Not allowed

In this example, it is allowed to assign the value of structptr to charptr because charptr can safely alias the bytes of the struct. However, the opposite assignment is not permitted, as structptr cannot correctly access the bytes of the char* with its more restrictive alignment rules.

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