What is the strict aliasing rule?

When asking about common undefined behavior in C, souls more enlightened than I referred to the strict aliasing rule.
What are they talking about?

A typical situation you encounter strict aliasing problems is when overlaying a struct (like a device/network msg) onto a buffer of the word size of your system (like a pointer to uint32_t s or uint16_t s). When you overlay a struct onto such a buffer, or a buffer onto such a struct through pointer casting you can easily violate strict aliasing rules.

So in this kind of setup, if I want to send a message to something I'd have to have two incompatible pointers pointing to the same chunk of memory. I might then naively code something like this:

typedef struct Msg
{
    unsigned int a;
    unsigned int b;
} Msg;

void SendWord(uint32_t);

int main(void)
{
    // Get a 32-bit buffer from the system
    uint32_t* buff = malloc(sizeof(Msg));

    // Alias that buffer through message
    Msg* msg = (Msg*)(buff);

    // Send a bunch of messages    
    for (int i =0; i < 10; ++i)
    {
        msg->a = i;
        msg->b = i+1;
        SendWord(buff[0]);
        SendWord(buff[1]);   
    }
}

The strict aliasing rule makes this setup illegal: dereferencing a pointer that aliases another of an incompatible type is undefined behavior. Unfortunately, you can still code this way, maybe get some warnings, have it compile fine, only to have weird unexpected behavior when you run the code.

(GCC appears somewhat inconsistent in its ability to give aliasing warnings, sometimes giving us a friendly warning and sometimes not.)

To see why this behavior is undefined, we have to think about what the strict aliasing rule buys the compiler. Basically, with this rule, it doesn't have to think about inserting instructions to refresh the contents of buff every run of the loop. Instead, when optimizing, with some annoyingly unenforced assumptions about aliasing, it can omit those instructions, load buff[0] and buff[1 ] into CPU registers once before the loop is run, and speed up the body of the loop. Before strict aliasing was introduced, the compiler had to live in a state of paranoia that the contents of buff could change at anytime from anywhere by anybody. So to get an extra performance edge, and assuming most people don't type-pun pointers, the strict aliasing rule was introduced.

Keep in mind, if you think the example is contrived, this might even happen if you're passing a buffer to another function doing the sending for you, if instead you have.

void SendMessage(uint32_t* buff, size_t size32)
{
    for (int i = 0; i < size32; ++i) 
    {
        SendWord(buff[i]);
    }
}

And rewrote our earlier loop to take advantage of this convenient function

for (int i = 0; i < 10; ++i)
{
    msg->a = i;
    msg->b = i+1;
    SendMessage(buff, 2);
}

The compiler may or may not be able to or smart enough to try to inline SendMessage and it may or may not decide to load or not load buff again. If SendMessage is part of another API that's compiled separately, it probably has instructions to load buff's contents. Then again, maybe you're in C++ and this is some templated header only implementation that the compiler thinks it can inline. Or maybe it's just something you wrote in your .c file for your own convenience. Anyway undefined behavior might still ensue. Even when we know some of what's happening under the hood, it's still a violation of the rule so no well defined behavior is guaranteed. So just by wrapping in a function that takes our word delimited buffer doesn't necessarily help.

So how do I get around this?

union {
    Msg msg;
    unsigned int asBuffer[sizeof(Msg)/sizeof(unsigned int)];
};

Beginner beware

This is only one potential minefield when overlaying two types onto each other. You should also learn about endianness, word alignment, and how to deal with alignment issues through packing structs correctly.

The best explanation I have found is by Mike Acton, Understanding Strict Aliasing. It's focused a little on PS3 development, but that's basically just GCC.

From the article:

"Strict aliasing is an assumption, made by the C (or C++) compiler, that dereferencing pointers to objects of different types will never refer to the same memory location (ie alias each other.)"

So basically if you have an int* pointing to some memory containing an int and then you point a float* to that memory and use it as a float you break the rule. If your code does not respect this, then the compiler's optimizer will most likely break your code.

The exception to the rule is a char* , which is allowed to point to any type.

This is the strict aliasing rule, found in section 3.10 of the C++03 standard (other answers provide good explanation, but none provided the rule itself):

If a program attempts to access the stored value of an object through an lvalue of other than one of the following types the behavior is undefined:

C++11 and C++14 wording (changes emphasized):

If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:

Two changes were small: glvalue instead of lvalue, and clarification of the aggregate/union case.

The third change makes a stronger guarantee (relaxes the strong aliasing rule): The new concept of similar types that are now safe to alias.

Also the C wording (C99; ISO/IEC 9899:1999 6.5/7; the exact same wording is used in ISO/IEC 9899:2011 §6.5 ¶7):

An object shall have its stored value accessed only by an lvalue expression that has one of the following types 73) or 88):

73) or 88) The intent of this list is to specify those circumstances in which an object may or may not be aliased.

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