[deathanatos]

I don't think most C programmers share your depth of the language. I tried hard to explain strict aliasing once, and utterly failed. The dev was convinced that he knew the exact behavior of the platform, and that it was fine. Yet people constantly find examples where we "know" what the compiler will do, and it does something completely different.

[DSMan195276]

I would agree that strict-aliasing is a hurt point for a lot of C devs, which is unfortunate. I'd only suggest that in general, if the strict-aliasing rule is coming into play you're probably already doing some really shady to begin with. Like in this example, casting a `long ` to an `int ` is likely a bad way to go about things even without worrying about strict-aliasing. In a lot of ways, I'd say that problems with the strict-aliasing rule are a symptom of a larger problem. If you can convince them that what they're doing is just bad coding practice to begin with, you might have a better time making them write correct code in the long run.

Now if you're working more directly with hardware (Which is of course possible/likely with C) then it might just be easier to disable strict-aliasing all together if you can, since identifying all the spots where it might be a problem tends to be an issue.

[caf]

In the really simple cases (like accessing a float as a long, or similar), you're right.

The problem is the interpretation that's been applied to aggregate types:

  struct a {
    int variant;
  };

  struct b {
    int variant;
    long  data;
  };

I have a pointer to a struct b - can I cast it to a pointer to struct a and still dereference 'variant'? It has the correct type and is guaranteed to live at the start of the struct in both cases. The prevailing opinion seems to be "no" (see https://trust-in-soft.com/wp-content/uploads/2017/01/vmcai.p... eg. example st1).

The BSD socket API was built on exactly this kind of type punning.

[DSMan195276]

> I have a pointer to a struct b - can I cast it to a pointer to struct a and still dereference 'variant'?

I think it is a bit of a gray area, but personally I've always held the opinion/understanding of no, that is invalid. The C standard does make one point fairly clear on strict-aliasing, which is the idea that strict-aliasing revolves on the idea that an object can only be considered to be one type of data (Or a `char` array, the only exception). Your example would be invalid for the reason that you can't treat an object of `struct b` as though it is a `struct a` - the fact that they share the same preamble doesn't change that. To be clear with what I'm saying: `struct b` can alias an `int`. `struct a` can also alias an `int`. But `struct b` can't alias a `struct a`, and because of this an int accessed through a `struct a` can't be accessed through a `struct b`.

That said, in general I find this to usually be a fixable problem, which also has (IMO) a cleaner implementation:

    struct head {
        int variant;
    };

    struct a {
        struct head h;
    };

    struct b {
        struct head h;
        long data;
    };

Now you can take a pointer to a `struct b` object and treat it like a pointer to a `struct head` object (Because it is a `struct head` object). You could do the same thing with objects of type `struct a`. So now you can cast both of them to `struct head` and examine the `variant` of both. Then later you could cast it back to the proper type.

This approach to aggregate types is heavily used in the Linux Kernel and other places (Including most of my own code). The `container_of` macro makes it even nicer to use (Though the legality of the `container_of` macro is an interesting debate...).

> The BSD socket API was built on exactly this kind of type punning.

Kinda. It's actually surprising how close it comes to skirting this issue (And it does skirt it), but I believe it's actually possible to use BSD sockets without ever violating the strict-aliasing rule (Though of course, there are ways of using it which would arguably violate the rule). In most cases for BSD sockets, strict-aliasing is more likely going to be broken in the kernel, not your code.

To note though, the strict-aliasing rule only applies to dereferencing pointers. You can cast pointers back and forth all day, you just have to ensure that when you're done you're treating it the object as you originally declared it. Thus, if you pass a `struct sockaddr_in` to `bind` and cast it to a `struct sockaddr`, the strict-aliasing rule isn't violated because you never dereferenced the casted pointer.

Going along with that, as long as you correctly declare your `struct sockaddr`s from the beginning you won't have any strict-aliasing woes. The only situation where this could technically be a problem is `accept` and `recvfrom`, since they are the only functions that gives a `struct sockaddr` back. But assuming you already know what address-family the socket is using, you can declare the correct `struct sockaddr` for that family from the start, cast it and pass it to `accept` or `recvfrom`, and then use it as your originally declared type without breaking strict-aliasing.

Of course, it's also worth keeping in mind that the BSD sockets interface came before C89. You definitely wouldn't design it the same way if you were to do it today.

[caf]

Kinda. It's actually surprising how close it comes to skirting this issue (And it does skirt it), but I believe it's actually possible to use BSD sockets without ever violating the strict-aliasing rule (Though of course, there are ways of using it which would arguably violate the rule). In most cases for BSD sockets, strict-aliasing is more likely going to be broken in the kernel, not your code.

Well, firstly it's pretty unsatisfying to hear that yes, this API contravenes strict aliasing restrictions, but only on the library side! - essentially that it is impossible to implement the sockets C API in C.

That aside, this still excludes long-standing examples like embedding a pointer to struct sockaddr in your client struct, which points to either a sockaddr_in, sockaddr_in6 or sockaddr_un depending on where that client connected from (well, you can still do it, but now you can't examine the sockaddr's sa_family member to see what type the address really is - you need to have a redundant, duplicate copy of that field in the client struct itself).

The situation is similar with sockaddr_storage. The whole point of that type is to allow you to stash either AF_INET or AF_INET6 addresses in the same object and then examine the ss_family field to see what it really is - the text in POSIX says:

  The <sys/socket.h> header shall define the sockaddr_storage structure, which shall be:

    Large enough to accommodate all supported protocol-specific address structures

    Aligned at an appropriate boundary so that pointers to it can be cast as pointers to protocol-specific address structures and used to access the fields of those structures without alignment problems

( http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys... )

Of course, it's also worth keeping in mind that the BSD sockets interface came before C89. You definitely wouldn't design it the same way if you were to do it today.

Well, the aforementioned sockaddr_storage came about after C89.

And wasn't C89 supposed to be about codifying existing practice, anyway?

[DSMan195276]

> Well, firstly it's pretty unsatisfying to hear that yes, this API contravenes strict aliasing restrictions, but only on the library side! - essentially that it is impossible to implement the sockets C API in C.

Then it'd also be pretty unsatisfying to hear that it's impossible to write an OS kernel in standard C too - it requires compiler extensions for lots of various details. It's impossible to write a standard C library in nothing but standard C as well. I would absolutely agree, however, that the fact that you have to use an extension to get past strict-aliasing is unfortunate (It'd be nice if in a future C they add something like the `may_alias` extension into the standard). But you can do it, and OS code is definitely one where extensions are going to be rampant anyway. For example, the Linux Kernel disables strict-aliasing completely.

> That aside, this still excludes long-standing examples like embedding a pointer to struct sockaddr in your client struct, which points to either a sockaddr_in, sockaddr_in6 or sockaddr_un depending on where that client connected from (well, you can still do it, but now you can't examine the sockaddr's sa_family member to see what type the address really is - you need to have a redundant, duplicate copy of that field in the client struct itself).

Strictly speaking, that's not true. You can examine it directly, but it requires casting the `struct sockaddr * ` to a `sa_family_t * ` instead. This is valid because regardless of what type of object it really is, it must start with a `sa_family_t` entry, so you are treating it as the correct type (And by that notion, `sa_family_t` is allowed to alias any of the `struct sockaddr` types). Besides that, you could also use a `union` to combine all the possible `sockaddr`s that you're going to handle together with a `sockaddr_storage`. Then you can do things like normal and simply access the `sockaddr` through the correct union member.

> Well, the aforementioned sockaddr_storage came about after C89.

That it did. However, that was more about making the current API work with larger addresses (namely ipv6) not to fix the API. There really wasn't any other way to do it.

> And wasn't C89 supposed to be about codifying existing practice, anyway?

Some compilers implemented the strict-aliasing optimization (And did lots of other strange things), some did not. The C standards committee chose to go with strict-aliasing since it provides a lot of optimization opportunities.

All that said, I'm not saying things are perfect by any means, as our conversation here shows. I don't think things are nearly as bad as people tend to think, however. Generally speaking, unless you're doing something a little bit shady it's possible to avoid any strict-aliasing issues, and if it's really not possible there's generally a way to simply "turn it off", albeit that may result in a little less portable code - though if you're breaking strict-aliasing the portability of your code is already a bit suspect to begin with.

[caf]

Strictly speaking, that's not true. You can examine it directly, but it requires casting the `struct sockaddr * ` to a `sa_family_t * ` instead. This is valid because regardless of what type of object it really is, it must start with a `sa_family_t` entry,...

No, that isn't guaranteed by POSIX - it has to have an sa_family_t member, but it doesn't have to be the first one.

I also think it's problematic that the use explicitly contemplated by POSIX is considered ill-formed C.

Besides that, you could also use a `union` to combine all the possible `sockaddr`s that you're going to handle together with a `sockaddr_storage`. Then you can do things like normal and simply access the `sockaddr` through the correct union member.

I do not think this is that easy when you include sockaddr_un into the mix, because of the way that sockaddr doesn't include the full size of its path member. This is, in fact, the point that I throw up my hands and use -fno-strict-aliasing because the fact that pointer provenance, rather than just value and type, is important together with the fact that it's not actually clear whether you've correctly laundered the pointer through a union or not, makes it all too... grey.

Some compilers implemented the strict-aliasing optimization (And did lots of other strange things), some did not.

C compilers existed in 1989 that would assume different structure types with common initial sequences couldn't alias? With the "common initial sequence" carve-out in §3.3.2.3? I am sceptical...

[TorKlingberg]

A common class assignment or interview question is to write your own memcpy. Towards the end you usually start optimizing it by copying multiple bytes at once. That is undefined behavior. You cannot just cast a pointer to uint32_t* and start using it, unless the underlying object is actually uint32_t. In practice it works fine, so people don't care. We'll see what future compilers will do, especially when the homemade memcpy is inlined somewhere.

An other one is custom malloc backed by a static char array. You're allowed to access any object as char*, but not the other way around. A static char array is always a char array, and accessing it through a pointer to anything else is a strict aliasing violation. Only the built-in malloc and siblings can create untyped memory.

[rwj]

You can cast from char* to uint32_t* and start using it. I forget the exact standardese, but there is an execption for char*.

[TorKlingberg]

There is an exception for accessing any object through a character type pointer, but not the other way around. uint32_t is not a character type, and it doesn't matter if it was casted to a char* first.

Also, apparently uint8_t may not be a character type.

[DSMan195276]

> Also, apparently uint8_t may not be a character type.

I think that goes hand-in-hand with the fact that `char` is not guaranteed to be 8-bits wide in C, so by that fact `uint8_t` may not be a `char`. In practice I highly doubt this distinction really matters: Platforms without an 8-bit `char` are basically guaranteed to not support (a standards compliant) `uint8_t` anyway, and it is reasonable to target 8-bit `char` systems in which case it's safe to assume `uint8_t` and `char` are the same thing. (Well, `unsigned char`)

[TorKlingberg]

Yes, a system with char >8 bits wouldn't have a uint8_t type at all. But, even when char and (u)int8_t are the same size, it may not be the same type. Compilers could go and apply strict aliasing for pointers to uint8_t. I'm at the limit of my C types knowledge here, so correct me if I'm just wrong.

[yorwba]

Only if the pointer is correctly aligned for the uint32_t data type. Otherwise you might get problems with unaligned memory acesses. (Like when you get some data over the wire that is clearly just a memory dump of a C struct, so you just do a pointer cast. Boom, unaligned read.)