Exactly one year ago, I started this series on pointers, but what I really wanted to blog about originally was a rather arcane and rarely used keyword that first appeared in the C99 language standard: the ‘restrict’ qualifier. But after trying to digest the formal definition in chapter 6.7.3.1 I decided that taking a little detour would make my and my reader’s life much easier.

Let me set the stage for ‘restrict’ by summarizing what I wrote in episode 3 about the “strict aliasing rule”:

1. The compiler might optimize code involving multiple pointers, provided the pointers are not aliased; that is, they don’t point to the same object or memory.

2. The compiler assumes that pointers to incompatible types never alias.

3. The compiler assumes that pointers to compatible types (same types, apart from CV-qualification and signedness) potentially alias.

Therefore, a function with this signature is eligible for compiler optimization:

whereas this one is not:

This is unfortunate, because most likely, the arrays passed to the second version of ‘transform’ are in completely different, non-overlapping memory regions. But the compiler doesn’t know and hence stubbornly adheres to the strict aliasing rule.

The ‘restrict’ qualifier, which — contrary to the ‘const’ and ‘volatile’ qualifiers — can only be applied to pointers, is a promise given by the programmer to the compiler that pointers don’t alias even though they point to objects of the same type. Therefore, this version of ‘transform’ can be optimized by the compiler:

Let’s put this to the test with the ‘silly’ example from episode 3:

Before knowing about the strict aliasing rule, we were surprised to see that the memory access to ‘x’ in the return statement was not replaced with a simple ‘return 0’. After having learned about the strict alias rule, it’s clear: since ‘x and ‘y’ point to the same type, the compiler must assume that they may point to the same memory location and hence it loads the value pointed to by ‘x’ from memory afresh:

Now, if we tell the compiler that ‘x’ and ‘y’ never point to the same memory location, optimization is possible:

Nice, isn’t it?

If you use the ‘restrict’ qualifier on a pointer, you promise that — at least for the lifetime of the restricted pointer — the object pointed to is only accessed through this pointer. Break that promise and you get undefined behavior. (In the ‘silly3’ example, the lifetime of the pointers ‘x’ and ‘y’ end once the call to ‘silly3’ returns.)

In the C99 language standard, many functions from the standard library have been revised and now make use of the ‘restrict’ keyword. Take ‘memcpy’, for instance:

As everybody knows, ‘memcpy’ can only copy non-overlapping blocks of memory and this fact is nicely highlighted by the use of the ‘restrict’ keyword: during the call to ‘memcpy’ the memory regions src[0] to src[n] as well as dst[0] to dst[n] are exclusively owned and may not be accessed by other pointers. Since ‘memmove’ can copy overlapping blocks of memory (with a little speed penalty, of course), ‘memmove’ consequently doesn’t declare restricted pointers:

Please be aware that ‘restrict’ is not supported by the C++ language standard and it’s unclear whether it ever will be. If you mix C99 and C++ code, you might have to strip the ‘restrict’ keyword from C99 headers to avoid compilation errors:

In general, I’m not a big fan of optimization features that the compiler is free to ignore. If utmost performance is important, you want dependable performance. Most likely, your routine is not on the performance critical path, anyway. If you think it is, carefully profile your code and after you proved that it is, you’d better code that part in assembly language. Without such evidence, sprinkling your code with ‘restrict’ is little short of premature optimization. (I complained here about the unnecessarily overused ‘inline’ keyword for the same reason.)

What I do like about the ‘restrict’ keyword, though, is that by unraveling it, we’ve made a beautiful journey through important everyday programming topics like “pointers vs. arrays”, “type qualifiers”, “pointer conversion rules”, and the “strict aliasing rule”. The journey was the destination.

Sometimes, someone walks up to you and claims that there is a bug in your well-crafted code. Then, after having successfully proved that individual wrong, it occurs to you that there is indeed a bug—albeit a different one! Those are quite humbling experiences, but experiences that we should be most grateful for.

SETTING THE STAGE

This episode was triggered by feedback that I received from a reader regarding a “Dangerously Confusing Interfaces” post. In said post, I advise that instead of accepting a pointer to “uncopied” memory like this:

‘WriteAsync’ should rather take a pointer to an opaque data structure named ‘uncopied_memory’:

“uncopied” memory means that for the sake of efficiency, the called function doesn’t copy the provided data but instead expects you to keep it alive and unchanged while the called function is executed asynchronously. Since the suggested interface change requires an explicit cast to an ‘uncopied_memory’ pointer, it’s a lot less likely that a temporary buffer allocated from the stack is passed accidentally. The idea of the proposed approach is that every call to ‘WriteAsync’ requires an explicit cast that acts as a reminder to the programmer that the buffer’s contents must be preserved.

For instance, if you wanted to pass a structure that I used in the previous installment of this series to ‘WriteAsync’, you would do it like this:

But back to the question. What the reader was worried about is that since ‘measurements_t’ and ‘uncopied_memory’ are by no means compatible, wouldn’t a cast to an ‘uncopied_memory’ pointer constitute a violation of the “strict aliasing rule“?

Actually, when it comes to the “strict aliasing rule,” the fact that these structs have incompatible members doesn’t really matter—even if you accessed the stored value through a pointer to a struct with an identical set of members you would be in trouble; if the tag names of the structs are different, it already counts as a violation of the “strict aliasing rule.”

The key word here is access. If you just create a pointer to incompatible types, everything is fine. Within ‘WriteAsync’ you just cast the received ‘uncopied_memory’ pointer into a ‘uint8_t’ pointer and access the provided data byte-wise, which is always safe, as you know (if you didn’t know, go back and read my previous post).

So far, so good. We don’t access stored memory through incompatible pointers; we only do pointer conversion, which is always safe, isn’t it? I replied to my reader that everything was fine, there was no violation of the “strict aliasing rule.”

Nevertheless, I couldn’t rid myself of this nagging feeling about whether the conversion/cast is really always safe.

POINTER CONVERSION RULES

The venerable book “The C Programming Language” by Brian Kernighan and Dennies Ritchie has this to say on pointer conversions:

A pointer to one type may be converted to a pointer to another type. The resulting pointer may cause addressing exceptions if the subject pointer does not refer to an object suitably aligned in storage. It is guaranteed that a pointer to an object may be converted to a pointer to an object whose type requires less or equally strict storage alignment and back again without change; the notion of alignment” is implementation-dependent, but objects of the char types have least strict alignment requirements. As described in Par.A.6.8, a pointer may also be converted to type void * and back again without change.

Let me paraphrase: pointer conversion is safe provided the alignment requirements of the target type are less or equal to the alignment requirements of the source type. The converted pointer can be converted back to the original pointer without problems.

Though, the statement “The resulting pointer may cause addressing exceptions” is not clear to me. What does it mean? If the target type has stricter alignment requirements, do you get “addressing exceptions” when you create the pointer or when you access memory through it? Let’s assume that we are on a typical platform where objects of type ‘double’ are aligned on an 8-byte boundary and ‘chars’ have no alignment requirements (‘chars’ are aligned on a 1-byte boundary, so to speak.):

The conversion (1) is 100% safe and so is the corresponding read-access (2): the alignment requirements of type ‘char’ are less than the alignment requirements of type ‘double’. (4) is 100% unsafe, but what about (3)? Aren’t we just creating a pointer? To find out, I had to dig deep into my copy of the C99 language standard. Eventually, I found what I call the “pointer conversion rule”:

6.3.2.3/7 A pointer to an object or incomplete type may be converted to a pointer to a different object or incomplete type. If the resulting pointer is not correctly aligned for the pointed-to type, the behavior is undefined. Otherwise, when converted back again, the result shall compare equal to the original pointer. When a pointer to an object is converted to a pointer to a character type, the result points to the lowest addressed byte of the object.

There you have it and much more precise than the paragraph from “The C Programming Language.” Believe it or not—statement (3), the sheer pointer conversion already gets you into the realm of undefined behavior. Who knew?

So what does this mean regarding the conversion/cast from a ‘measurements_t’ pointer to an ‘uncopied_memory’ pointer? As we know from the standard, it would be safe if the alignment requirements for ‘uncopied_memory’ were less or equal to the alignment requirements of ‘measurements_t’.

In the previous example, we had to deal with primitive types (‘char’, ‘double’) whose alignment requirements can easily be determined. In order to find out about the alignment requirements for structs, we need to dive once more into the C99 standard document:

6.7.2.1/13 A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning.

Meditate on this for a while. Paraphrased, this all means that the alignment requirements of a struct are the same as the alignment requirements of a struct’s first member. So the question boils down to this: Are the alignment requirements of a ‘void’ pointer (‘uncopied_memory’s first member) less or equal to the alignment requirements of a ‘char’ (‘measurements_t’s first member)?

Of course, they’re not! A pointer type (like void*) is more or less just an integer type in disguise that is capable of holding all the addresses of your system and as such, pointer types have the same alignment requirements as regular integer types. On a 32-bit platform, pointers typically comprise 4 bytes. Thus, on typical 32-bit platforms, they will need to be aligned on 4-byte boundaries.

By contrast, a character (like the first element of measurements_t) comprises exactly one byte and thus has no alignment requirement—it can be stored at any address in memory.

Since the alignment requirements of the first element of ‘uncopied_memory’ are stronger than the alignment requirements of ‘measurements_t’, we can conclude that my advice to cast to ‘uncopied_memory’ may yield undefined behavior. Not because of the “strict aliasing rule,” but because of a violation of the “pointer conversion rules.”

To solve the problem, the type of the ‘dummy’ member of ‘uncopied_memory’ needs to be changed to ‘char’, a type that has the weakest alignment requirements. I have updated the “Dangerously Confusing Interfaces” post accordingly.

One of the lesser-known secrets of the C programming language is the so-called “strict aliasing rule”. This is a shame, because failing to adhere to it takes you (along with your code) straight into the realm of undefined behavior. As no one in their right mind wants to go there, let’s shed some light on it!

POINTER ALIASING DEFINED

First of all, we have to clarify what “aliasing” really means, or rather aliasing of pointers. Take a look at this example:

Here, ‘p1’ and ‘p2’ are aliased to the same object ‘value’; that is, they point to the same object. If you update ‘value’ through ‘p1’:

a read through ‘p2’ will reflect this change:

Because of the possibility of aliasing, a C compiler is prevented from applying certain optimizations. Consider:

You might think that any decent compiler would generate simplified code equivalent to this:

It’s not a matter of decency — the compiler just can’t do this optimization! Here’s the assembly output that clearly shows that the return value is loaded from memory:

The optimization is not possible because the caller could call ‘silly’ like so:

In this case, ‘x’ and ‘y’ are aliased to the same ‘value’, which means ‘silly’ must return 1 not 0. Consequently, ‘*x’ must be read from memory, every time. Period.

ROOM FOR IMPROVEMENT

If you think about it, even though it may happen, pointer aliasing won’t happen very often in practice. Why waste so much potential for optimization for the uncommon case? Most likely, the folks from the C standards committee had the same line of thinking. They introduced rules that state when pointer aliasing must not happen. Enter the strict aliasing rule.

To facilitate compiler optimization, the strict aliasing rule demands that (in simple words) pointers to incompatible types never alias. Pointers to compatible types (like the two ‘int’ pointers ‘x’ and ‘y’ in ‘silly’) are assumed to (potentially) alias. Let’s make the pointer types incompatible (‘short*’ vs. ‘int*’):

As you can see, this time no load from memory is performed — 0 is returned instead. The optimization is possible because the compiler assumes that aliasing is not allowed in this case.

VIOLATIONS

But what happens if pointers to incompatible types nevertheless alias? After all, this can happen quite easily. Maybe not in the ‘silly’ example, but in real-world production code:

In an attempt to convert data stored in a buffer (maybe read over a network connection) into a high-level structure, a pointer to ‘struct measurements_t’ is aliased with a pointer to a ‘uint8_t’. Since both types are incompatible (pointer to struct vs. pointer to ‘uint8_t’) this code is a violation of the strict aliasing rule. Experienced C developers most likely recognized immediately that this code yields undefined behavior, but they would have probably attributed it to struct padding and alignment issues. The real reason, as we know by now, is a violation of the strict aliasing rule.

THE FINE PRINT

So what exactly is the strict aliasing rule and what does “type compatibility” mean? Here’s an excerpt from the ISO C99, standard, chapter 6.5:

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

Such Standardeese is often hard to digest, so let me try to clarify it a bit. Aliased pointer access is fine if:

1. The pointed-at types are identical. Note that typedefs are just type aliases and don’t introduce new types:

2. The pointed-at types are identical apart from the “signed-ness” (e. g. ‘int’ vs. ‘unsigned int’).
3. The pointed-at types are identical apart from qualification (e. g. ‘const int’ vs. ‘int’).
4. The rule “an aggregate or union type that includes one of the aforementioned types among its members” is highly confusing and probably doesn’t mean much. Check this out for details.
5. The pointed-at types are different, but the pointed-at type through which the access is made is a pointer to character:

Conversely, aliased pointer access is not defined if the pointed-at types are fundamentally different. Note that this includes pointers to structs that are identically defined but have different tag names:

CONCLUSION

The strict aliasing rule was introduced to give the compiler vendors some leeway regarding optimizations. By default, the compiler assumes that pointers to (loosely speaking) incompatible types never alias. As a consequence, you, the programmer, have to make sure that this rule is obeyed.

Here’s some disquieting news: a lot of existing code isn’t conforming to the strict aliasing rule, but the code works (or appears to work) fine anyway. As an example, the ‘convert’ function above, which aliases a struct to an array of bytes might work fine on an Intel x86-based platform, which supports unaligned memory access. However, if you use ‘convert’ on an (older) ARM-based platform, you might get a “bus error” exception that could crash your system. In other cases, nonconforming code just works by coincident, with a particular compiler, or a particular compiler version at a particular optimization level.

To me, knowing about the strict aliasing rule is as important for every systems developer as knowing about the other systems programming “secrets” like alignment, struct padding, and endianness.

FURTHER READING

I’m indebted to the authors of the following two blog post. From the first post, I shamelessly stole the idea for the ‘silly’ function, which the author originally called ‘foo’, which I found silly :-) The second post gives a much more detailed coverage of the strict aliasing rule and also discusses the interesting technique of “casting through a union”.