Stupid Smart Pointers in C(blog.kevinalbs.com)
blog.kevinalbs.com
Stupid Smart Pointers in C
http://blog.kevinalbs.com/stupid_smart_pointers
173 comments
My C programs never consumed gigs of memory. So I (like many others I assume) made a memory manager and never freed anything. You'd ask it for memory and it kept a list of various sizes it allocated and returned what you needed to be re-used. Freeing and allocating is slow, and error prone, so just avoid it!
Well, it's basically an implementation of a memory allocator.
But how did you determine what you could re-use? That's the hard problem, one that's equivalent to calling free() at the right time.
But how did you determine what you could re-use? That's the hard problem, one that's equivalent to calling free() at the right time.
When something would normally be freed, it calls the memory manager's version of free(), which zeroes the memory and adds it back into the appropriate available list.
You're just describing every allocator in the world, except many (most?) skip the zeroing part.
libc already does that. What is it that yours is adding?
I'd say 25 years ago you could write your own naive allocator, make just a couple of assumptions for your use case, and beat libc. But no more.
One of the selling points of Java in the 90s was the compacting part. Because in the 90s fragmentation was a much bigger problem than it is today. Today the libc allocators have advanced by maybe tens of thousands of PhDs worth of theory and practice. Oh, and we have 64bit virtual address space, which helps with some (but not all) of the problems with memory fragmentation.
See this post from Ian Lance Taylor about why Go didn't even bother with a compacting GC: https://groups.google.com/g/golang-nuts/c/KJiyv2mV2pU?pli=1
libc already does that. What is it that yours is adding?
I'd say 25 years ago you could write your own naive allocator, make just a couple of assumptions for your use case, and beat libc. But no more.
One of the selling points of Java in the 90s was the compacting part. Because in the 90s fragmentation was a much bigger problem than it is today. Today the libc allocators have advanced by maybe tens of thousands of PhDs worth of theory and practice. Oh, and we have 64bit virtual address space, which helps with some (but not all) of the problems with memory fragmentation.
See this post from Ian Lance Taylor about why Go didn't even bother with a compacting GC: https://groups.google.com/g/golang-nuts/c/KJiyv2mV2pU?pli=1
> Today the libc allocators have advanced by maybe tens of thousands of PhDs worth of theory and practice.
Have they, though? Looking at the blame on glibc's malloc.c, the most substantial change in the last 10 years has been the addition of memory tagging. Apart from that, I mainly just see a bunch of small tweaks to the tcache, fastbins, etc., and the basic logic is broadly the same as it was 20 years ago. Similarly, the MallocInternals wiki page [0] appears mostly as it did in 2016, except for some more explanations of the tcache added in 2018.
I can easily believe that lots of work has been done on malloc/free-style allocators, but from what I've seen of most libc allocators, they hardly stand at the forefront of this work. (Except for the ones that just vendor some standalone allocator library and keep it up to date. But that describes neither Windows nor Linux.)
And of course, if you're writing your own allocator, you can relax some of the constraints, e.g., you can drop most of the multithreading support and not have to fool around with locks and shared arenas and whatnot.
[0] https://sourceware.org/glibc/wiki/MallocInternals
Have they, though? Looking at the blame on glibc's malloc.c, the most substantial change in the last 10 years has been the addition of memory tagging. Apart from that, I mainly just see a bunch of small tweaks to the tcache, fastbins, etc., and the basic logic is broadly the same as it was 20 years ago. Similarly, the MallocInternals wiki page [0] appears mostly as it did in 2016, except for some more explanations of the tcache added in 2018.
I can easily believe that lots of work has been done on malloc/free-style allocators, but from what I've seen of most libc allocators, they hardly stand at the forefront of this work. (Except for the ones that just vendor some standalone allocator library and keep it up to date. But that describes neither Windows nor Linux.)
And of course, if you're writing your own allocator, you can relax some of the constraints, e.g., you can drop most of the multithreading support and not have to fool around with locks and shared arenas and whatnot.
[0] https://sourceware.org/glibc/wiki/MallocInternals
> Today the libc allocators have advanced by maybe tens of thousands of PhDs worth of theory and practice.
For fun I wrote up a very basic memory manager and benchmarked it against free/malloc. On my Mac (M3) the manager was 3x faster. On a random kubernetes pod (alpine) it was 33x faster. Performance increases as memory size goes up.
For fun I wrote up a very basic memory manager and benchmarked it against free/malloc. On my Mac (M3) the manager was 3x faster. On a random kubernetes pod (alpine) it was 33x faster. Performance increases as memory size goes up.
If this is true, can some of your ideas be integrated into the standard libc allocator?
Well, not on MacOS, I imagine (you tell me which repository/mailing list they should send a PR to :p). Really Windows and MacOS seem like the places where a malloc replacement is generally quite useful (Windows is a particularly big offender in this regard, Windows malloc is abysmal - MacOS less so iirc but still pretty bad) - it's glibc that's somewhat of an outlier by being generally pretty good.
Isn't that just stacking your own allocator on top of the libc allocator, the same way the libc allocator is stacked on top of the OS's page mappings? It's often a sensible idea, of course, but I wouldn't describe it as "never freeing things", just substituting libc's malloc()/free() for your own. It's not like the libc allocator is doing something radically different from keeping lists of available and allocated memory.
libc allocator is considered "slow" for the extra overhead it has. I'll just leave it at that as you can GPT the rest. I did some of my own testing back in the days I wrote C, and mine was significantly faster than malloc/free.
It's also a way, I suppose, to ensure your program has the memory it needs, depending on how you design it. Mine basically allocates all the expected memory I need at the start, while still being able to grab more if it needs. This was more of an issue back when you shared a server with lots of others.
It's also a way, I suppose, to ensure your program has the memory it needs, depending on how you design it. Mine basically allocates all the expected memory I need at the start, while still being able to grab more if it needs. This was more of an issue back when you shared a server with lots of others.
Of course the libc allocator is slow, it's designed as a one-size-fits-all solution that's poorly tuned for most workloads and often doesn't get substantially updated for decades. By writing your own, you aren't avoiding an allocator, you're just avoiding the libc implementation of an allocator.
(In fact, most libcs let you just redefine the malloc(), free(), and realloc() functions to point to your own allocator instead of the default one, so you don't have to rewrite all the functions calling them. E.g., the mimalloc allocator [0] can be configured as a drop-in replacement for the libc allocator.)
[0] https://github.com/microsoft/mimalloc
(In fact, most libcs let you just redefine the malloc(), free(), and realloc() functions to point to your own allocator instead of the default one, so you don't have to rewrite all the functions calling them. E.g., the mimalloc allocator [0] can be configured as a drop-in replacement for the libc allocator.)
[0] https://github.com/microsoft/mimalloc
I guess I'm confused at this chain of responses, then. As I'm doing what you are already suggesting.
Re-reading your comment, about the "not really freeing anything" -- I beg to differ, as when you do a real free(), it frees up memory for any program to use. As I already mentioned one of the benefits is once you have control of who can use that memory and aren't risking it not being available - disregarding some program that constantly grows in memory.
Re-reading your comment, about the "not really freeing anything" -- I beg to differ, as when you do a real free(), it frees up memory for any program to use. As I already mentioned one of the benefits is once you have control of who can use that memory and aren't risking it not being available - disregarding some program that constantly grows in memory.
when you do a real free(), it frees up memory for any program to use
This is an extremely naive description of not only what the libc free() function does, but also of how memory management works in general.
This is an extremely naive description of not only what the libc free() function does, but also of how memory management works in general.
There are always details within details. The overall idea is easily understandable. You are being pedantic.
Your description reveals a complete lack of understanding, and pointing out someone's lack of understanding is not being pedantic. Your choice of wording—"real free()" and "any program"—misrepresents the inherent complexity of implementing an allocator and does a disservice to both those less experienced and to the discussion in this thread.
You're assuming a platform. There are not a great number of guarantees when it comes to free:
The free subroutine deallocates a block of memory previously allocated by the malloc subsystem. Undefined results occur if the Pointer parameter is not an address that has previously been allocated by the malloc subsystem, or if the Pointer parameter has already been deallocated. If the Pointer parameter is NULL, no action occurs.
All that is guaranteed, is a deallocation. Not how and when that memory will become available again.
Your "more detailed" understanding will break, and will cause headaches, on platforms ypu're not used to.
The free subroutine deallocates a block of memory previously allocated by the malloc subsystem. Undefined results occur if the Pointer parameter is not an address that has previously been allocated by the malloc subsystem, or if the Pointer parameter has already been deallocated. If the Pointer parameter is NULL, no action occurs.
All that is guaranteed, is a deallocation. Not how and when that memory will become available again.
Your "more detailed" understanding will break, and will cause headaches, on platforms ypu're not used to.
[deleted]
> you have control of who can use that memory and aren't risking it not being available
overcommit would like to have a word
overcommit would like to have a word
A venerable and completely reasonable approach for resource-constrained environments and/or programs with very small memory requirements (kilobytes).
What are your issues with the memory requirements being small? One of the programs was a MUD that consumed a couple hundred megabytes, and I never had issues with it.
I mentioned gigabytes because of how mine specifically worked. It allocated chunks in powers of 2, so there was some % of memory that wasn't being used. For instance, If you only need 20 bytes for a string, you got back a pointer for a chunk of 32 bytes. Being just a game, and side project, I never gave it much thought, so I'm curious to hear your input.
I mentioned gigabytes because of how mine specifically worked. It allocated chunks in powers of 2, so there was some % of memory that wasn't being used. For instance, If you only need 20 bytes for a string, you got back a pointer for a chunk of 32 bytes. Being just a game, and side project, I never gave it much thought, so I'm curious to hear your input.
What makes you think that this approach is only useful for resource-constrained circumstances?
Yeah it's reasonable. Unfortunately if you do it, and you run tools like Coverity, it'll produces reams of complaints about how you're leaking memory :-( There was one project which was genuinely a short-lived program that never needed to free memory, but in the end I gave in and added free() statements everywhere. Otherwise we could never have got it into RHEL.
Why not just put all the free()s at the end of main() behind an #ifdef DEBUG or something?
> we hope will be added to the C spec one day
defer seems to be making significant progress (having a passionate and motivated advocate in Meneide, and a full TS)
defer seems to be making significant progress (having a passionate and motivated advocate in Meneide, and a full TS)
Last time I looked this was (golang-like) function scoped, not { } scoped, which means it's a bad idea. My feedback was the committee should simply standardize the existing attribute / behaviour, as that is widely used already.
(EDIT: I'm wrong, see reply)
(EDIT: I'm wrong, see reply)
> Last time I looked this was (golang-like) function scoped, not { } scoped, which means it's a bad idea.
Might have been the previous attempt from years ago, because being block scoped (unlike go) literally has its own section in https://thephd.dev/c2y-the-defer-technical-specification-its...
Might have been the previous attempt from years ago, because being block scoped (unlike go) literally has its own section in https://thephd.dev/c2y-the-defer-technical-specification-its...
Was just reading about this one the other day.
Well-defined behavior and it actually solves a problem.
Conceptually, I can't say it is worse than variadic functions or setjmp.
defer is nice, but I really want the cleanup attribute since it could in theory by applied to the return type of a function. In other words you could have malloc return a pointer with the cleanup attribute that automatically frees it at end of scope if it's non-NULL. (And if you want to persist the pointer just assign to a different variable and zero out the one malloc gave you.)
> In other words you could have malloc return a pointer with the cleanup attribute that automatically frees it at end of scope if it's non-NULL.
That is not, as far as I know, how __attribute__((cleanup)) works. It just invokes the callback when the value goes out of scope. So you can't have malloc return an implicitly cleanup'd pointer unless malloc is a macro, in which case you can do the same with a defer block.
That is not, as far as I know, how __attribute__((cleanup)) works. It just invokes the callback when the value goes out of scope. So you can't have malloc return an implicitly cleanup'd pointer unless malloc is a macro, in which case you can do the same with a defer block.
Yeah that's why I said "in theory". You can't do that with the attribute today, but it'd be possible to add that behavior to a standardized variant of it.
I desperately hope it is not added, Meneide throws a tantrum like he did about his Rust conference talk, and he leaves the C committee forever. He is a malign influence that is on record as saying he dislikes C and wants it to be replaced by Rust.
https://github.com/acbits/reftrack-plugin
Hey, I wrote this GCC plugin to automate reference counting in C. Any bug reports welcome.
Hey, I wrote this GCC plugin to automate reference counting in C. Any bug reports welcome.
> __attribute__((cleanup))
Interesting. I'm not very proficient in C, this looks like some sort of finalizers for local variables?
Interesting. I'm not very proficient in C, this looks like some sort of finalizers for local variables?
Correct. You can use it in a simple way to free memory, but we've also used it to create scoped locks[1].
This being C, it's not without its problems. You cannot use it for values that you want to return from the function (as you don't want those to be freed), so any such variables cannot be automatically cleaned up on error paths either. Also there's no automated checking (it's not Rust!)
Note it's {...} scoped, not function scoped, which makes it more useful than Golang's defer.
[1] https://gitlab.com/nbdkit/nbdkit/-/blob/8b36e5a2ea331eed2a73...
This being C, it's not without its problems. You cannot use it for values that you want to return from the function (as you don't want those to be freed), so any such variables cannot be automatically cleaned up on error paths either. Also there's no automated checking (it's not Rust!)
Note it's {...} scoped, not function scoped, which makes it more useful than Golang's defer.
[1] https://gitlab.com/nbdkit/nbdkit/-/blob/8b36e5a2ea331eed2a73...
> You cannot use it for values that you want to return from the function
I would say this is only half true. With some macro magic you can actually also return the values :)
https://github.com/systemd/systemd/blob/0201114bb7f347015ed4...
To be fair though, you probably meant without any such shenanigans.
I would say this is only half true. With some macro magic you can actually also return the values :)
https://github.com/systemd/systemd/blob/0201114bb7f347015ed4...
To be fair though, you probably meant without any such shenanigans.
While Go rules effectively prevents usage of defer in loops, it is useful occasionally to write:
if complex_nested_condition {
defer cleanup()
}Even with scope-based defer, you can accomplish conditional defers easily enough. In a sane language, where conditions are expressions, you could just do:
I feel like the number of times I've needed conditional defers is almost zero, while the number of times I've had to make a new function to ensure scoping is correct is huge.
Of especial note, 'mu.Lock(), defer mu.Unlock()' not being scope-based is the largest source of deadlocks in code. People don't use 'defer' because the scoping rules are wrong, code panics before the manual unlock call, and then the program is deadlocked forever.
defer if complex_nested_condition { cleanup() } else { noop() }
defer func(run bool) {
if !run { return }
}(condition)
I feel like the number of times I've needed conditional defers is almost zero, while the number of times I've had to make a new function to ensure scoping is correct is huge.
Of especial note, 'mu.Lock(), defer mu.Unlock()' not being scope-based is the largest source of deadlocks in code. People don't use 'defer' because the scoping rules are wrong, code panics before the manual unlock call, and then the program is deadlocked forever.
You can just zero out the pointer variable with __attribute__((cleanup)) before returning its value (from a temporary copy of course).
There's a complete implementation available at https://github.com/Snaipe/libcsptr
> It's available in GCC and Clang, and we hope will be added to the C spec one day. This is widely used by open source software, eg. in systemd.
It’s odd that the suggestion for a feature lacking in C is to use a non standard but well used supported path. c’s main selling point (IMO) is that it _is_ a standard, and relying on compiler vendor extensions kind of defeats the purpose of that.
It’s odd that the suggestion for a feature lacking in C is to use a non standard but well used supported path. c’s main selling point (IMO) is that it _is_ a standard, and relying on compiler vendor extensions kind of defeats the purpose of that.
It's so widely used by OS software that you're likely using already, that it's unlikely to be removed and much more likely to be standardized. This is in fact how standardization ought to work - standardize the proven best practices.
I agree. But if we follow that logic then any compiler specific feature of either or clang is fair game, even if it’s not standard. MSVC doesn’t support it when compiling in C mode, for example.
> But if we follow that logic then any compiler specific feature of either or clang is fair game, even if it’s not standard.
Well, yeah...
How do you think Annex K got in?
Well, yeah...
How do you think Annex K got in?
> relying on compiler vendor extensions kind of defeats the purpose of that.
Let's be honest, how many compilers are available, and how many of those would you actually use?
The answer isn't more than 4 and the 2 compilers you are most likely to use among those already support this and probably won't stop supporting without a good alternative.
I like standardisation, but you have to be realistic when it helps you without a large real cost other than fighting your ideals for getting this into the standard first.
Let's be honest, how many compilers are available, and how many of those would you actually use?
The answer isn't more than 4 and the 2 compilers you are most likely to use among those already support this and probably won't stop supporting without a good alternative.
I like standardisation, but you have to be realistic when it helps you without a large real cost other than fighting your ideals for getting this into the standard first.
For me, the point of writing something in C is portability. There were C compilers 30 years ago, there are C compilers now, and there will almost certainly be C compilers 30 years from now. If I want to write a good, portable library that's going to be useful for a long time, I'll do it in C. This is, at least, the standard in many gamedev circles (see: libsdl, libfreetype, the stb_* libraries). Under that expectation, I write to a standard, not a compiler.
The bad news is that C23 broke a lot of existing code[1]. We had to do a lot of work in Fedora to fix the resulting mess. [1] https://gcc.gnu.org/gcc-15/porting_to.html#c23
For the cases in linked doc, does adding -std=gnu17 to packages not suffice?
I would consider the union initializer change (require adding -fzero-init-padding-bits=unions for old behavior) much more hidden and dangerous, which is not directly related to ISO C23 standard.
I would consider the union initializer change (require adding -fzero-init-padding-bits=unions for old behavior) much more hidden and dangerous, which is not directly related to ISO C23 standard.
It's true that it does, yes. However that would still require changes to the build system. In any case for the vast majority of the packages we decided to fix (if you think this is a fix!) the code.
>if you think this is a fix
I would count it as doing maintenance work for the upstream, kudos for doing this!
I would count it as doing maintenance work for the upstream, kudos for doing this!
> Note that the bool type is not the same as int at ABI level,
Huh. Wonder what the benefits of that are. Bools being ints never struck me as a serious problem. I wonder if this catches people who accidentally assign rather than compare tho....
Huh. Wonder what the benefits of that are. Bools being ints never struck me as a serious problem. I wonder if this catches people who accidentally assign rather than compare tho....
For accessing any post-1970s operating system feature (e.g. async IO or virtual memory) you already cannot use standard C anymore (and POSIX is not the C stdlib).
The libraries you listed are all full of platform-specific code, and also have plenty of compiler-specific code behind ifdefs (for instance the stb headers have MSVC specific declspec declarations in them).
E.g. there is hardly any real-world C code out there that is 'pure standard C', if the code compiles on different compilers and for different target platforms then that's because the code specifically supports those compilers and target platforms.
The libraries you listed are all full of platform-specific code, and also have plenty of compiler-specific code behind ifdefs (for instance the stb headers have MSVC specific declspec declarations in them).
E.g. there is hardly any real-world C code out there that is 'pure standard C', if the code compiles on different compilers and for different target platforms then that's because the code specifically supports those compilers and target platforms.
My argument is that using these non-standard extensions to do important things like memory management in a C library is malpractice—it effectively locks down the library to specific C compilers. I'm sure that's fine if you're writing to clang specifically, but at that point, you can just write C++. libfreetype & stb_* are used and continue to be used because they can be relied on to be portable, and using compiler-specific extensions (without ifdefs) defeats that. If I relied on a clang-specific `defer`, I'm preventing my library from possibly being compiled via a future C compiler, let alone the compilers that exist now. To me, that's the point of writing C instead of C++ for a library (unless you're just a fan of the simplicity, which is more of an ideological, opinion-based reason).
If I touch C it has to have control over allocations, memory layout, and wrapping low level code into functions I can call from other languages.
I'd target the latest C standard and won't even care to know how many old, niche compilers I'm leaving out. These are vastly different uses for C and obviously your gaols drastically change your standard or compiler targeted.
I'd target the latest C standard and won't even care to know how many old, niche compilers I'm leaving out. These are vastly different uses for C and obviously your gaols drastically change your standard or compiler targeted.
There are quite a lot of embedded code that relies on obscure C compilers created and maintained by the CPU manufacturer.
But then again you are probably not doing a whole lot of heap management in embedded code.
But then again you are probably not doing a whole lot of heap management in embedded code.
That sounds like the kind of code that you want to be done with and never touch again. You can only dream of it not being buggy or catching up with the newest standard.
No it is more like the 10000 lines of code running in your washing machine, you will probably be updating it in the next year revision of the product.
It is quite common for this code to have all variables be global and just not have any heap allocations at all. Sometimes you don't even have variables in the stack either (besides the globals).
It is quite common for this code to have all variables be global and just not have any heap allocations at all. Sometimes you don't even have variables in the stack either (besides the globals).
I much prefer the purely-mechanical washing machines for this reason.. Way less to go wrong..
It is not that bad, the code is easier than your average web server and it is usually only one CPU for consumer devices. It gets bad if:
1) You add embedded linux
2) You have multiple CPUs that need to communicate with each other.
3) Both of the above (then the complexity skyrockets)
(this happens in modern cars)
So basically if it has a full LCD display (instead of 7-segment with a few extra toggle lights) I don't buy it.
It is actually quite refreshing code to read and write, yes the quality is often bad, but it often feels like school projects where they are small enough you can hold the complete system in your head. And it usually doesn't integrate with anything else besides the eletronics in the board
1) You add embedded linux
2) You have multiple CPUs that need to communicate with each other.
3) Both of the above (then the complexity skyrockets)
(this happens in modern cars)
So basically if it has a full LCD display (instead of 7-segment with a few extra toggle lights) I don't buy it.
It is actually quite refreshing code to read and write, yes the quality is often bad, but it often feels like school projects where they are small enough you can hold the complete system in your head. And it usually doesn't integrate with anything else besides the eletronics in the board
static allocation is the recommended mechanism in embedded code for reliability reasons, you have thought about you worst-case allocation and accounted for it... right?
Also fragmentation and dynamic behaviour are bad things when your code has to run "forever".
If this is the argument then the actual standardisation is useless. I primarily use windows so I’m affected by one of the major compilers that doesn’t support this feature. This is no different to saying “chrome supports feature X, and realistically has y% market share so don’t let the fact that other browsers exist get in the way”.
Call it a posix extension, fair enough. But if your reason for writing C is that it’s portable, don’t go relying on non portable vendor specific extensions.
Call it a posix extension, fair enough. But if your reason for writing C is that it’s portable, don’t go relying on non portable vendor specific extensions.
It's not, and even if there's something in the standard, your compiler of choice might not support it yet.
It's the same thing with the web and browser vendors, there's a constant mismatch, browsers propose and implement things and they may get standardized, and the standard dictates new requirements which might get implemented by all vendors.
The point of standardisation is defining behaviour for the things that are implemented as exploratory improvements and should be implemented on the more conservative compilers.
It's your choice whether to target the standard or a few selected compilers, there's a cost for both options between being late to improvements vs the possibility of needing to revisit your code around each of the "extensions" you decided to depend on.
If in certain projects portability is somehow of upmost importance, then any discussion around looking through the standard's black box to reach out for new stuff is kind of useless.
It's the same thing with the web and browser vendors, there's a constant mismatch, browsers propose and implement things and they may get standardized, and the standard dictates new requirements which might get implemented by all vendors.
The point of standardisation is defining behaviour for the things that are implemented as exploratory improvements and should be implemented on the more conservative compilers.
It's your choice whether to target the standard or a few selected compilers, there's a cost for both options between being late to improvements vs the possibility of needing to revisit your code around each of the "extensions" you decided to depend on.
If in certain projects portability is somehow of upmost importance, then any discussion around looking through the standard's black box to reach out for new stuff is kind of useless.
[deleted]
Defer was almost part of the C23 standard, but didn't quite make it. They've slimmed and changed a few things, and it looks like it very much might be part of the next.
[0] https://thephd.dev/c2y-the-defer-technical-specification-its...
[1] https://thephd.dev/_vendor/future_cxx/technical%20specificat...
[0] https://thephd.dev/c2y-the-defer-technical-specification-its...
[1] https://thephd.dev/_vendor/future_cxx/technical%20specificat...
Standard C is only the least common denominator that compiler vendors agreed on, and the C standard committee works 'reactively' by mostly standardizing features that have been in common use as non-standard extensions - sometimes for decades before standardization happens (this is probably the main difference to the C++ committee).
The *actual* power and flexibility of C lies in the non-standard, vendor-specific language extensions.
The *actual* power and flexibility of C lies in the non-standard, vendor-specific language extensions.
C's main selling point is not standardisation. It was widely used before standardisation and only standardised because it was useful.
> Use __attribute__((cleanup)). It's available in GCC and Clang
Really, don’t do this, it’s a portability nightmare. If you’re going to write C stick to code that easy to run under MSVC.
Really, don’t do this, it’s a portability nightmare. If you’re going to write C stick to code that easy to run under MSVC.
Running under MSVC is overrated and more trouble than its worth. Clang and mingw-w64 GCC work just fine for targeting Windows.
Disagree. It’s fine and trivially easy. It’s only hard for Linux people who build software for Linux first and only. Then have a shocked pikachu face when they need to run in a different environment.
mingw is radically more problematic than MSVC. Don’t use mingw.
mingw is radically more problematic than MSVC. Don’t use mingw.
Running under MSVC implies I'm building on Windows. In reality I'm cross compiling with Clang.
It’s a shame that glibc is so badly designed that cross-compiling for Linux is only vaguely possible because Andrew Kelley moved mountains via Zig.
[deleted]
> ... plus [reference counting] interacts badly with how modern CPUs work.
Can you expand on this?
Can you expand on this?
I'm not sure what the reference to "modern CPUs" is, but a common complaint is that most reasonable reference counting implementations suffer very badly under contention. Specifically, if an object's reference count is contended (an object is being accessed/its pointer copied by many threads), it's possible that incrementing and then decrementing the reference count can take several hundred cycles due to either cache lines ping-ponging between cores or relatively expensive remote atomics (some Arm CPUs, I believe, allow the memory system to execute atomic operations in caches themselves to try and cope with contention/avoid moving cache lines back and forth by simply leaving them in a shared cache).
In reality, your milage will heavily vary. If you don't have contention (you don't commonly share objects across multiple threads concurrently), it's likely that reference counting will perform very well. Whether this is the common case really depends on the kinds of software you write.
In reality, your milage will heavily vary. If you don't have contention (you don't commonly share objects across multiple threads concurrently), it's likely that reference counting will perform very well. Whether this is the common case really depends on the kinds of software you write.
Same object doesn't have to be contended if the reference count is sharing cache line with another reference count.
I've seen this happen in cases arrays of objects are allocated, ie one per thread, and then handed to a thread pool to work on.
Even if heap allocated, if the object is just a reference counter and a few pointers, the memory allocator can fit several of them next to each other causing them to share cache lines, which causes the performance issues with atomic operations.
Depends on implementation of things of course, but can be a pitfall.
I've seen this happen in cases arrays of objects are allocated, ie one per thread, and then handed to a thread pool to work on.
Even if heap allocated, if the object is just a reference counter and a few pointers, the memory allocator can fit several of them next to each other causing them to share cache lines, which causes the performance issues with atomic operations.
Depends on implementation of things of course, but can be a pitfall.
Sure, but TFA is a very clever hack -- disgusting, yes, impractical and non-portable, true, subject to sad limitations -of course-, but genius and entertaining.
The "tl;dr" summary is that `free_on_exit()` replaces the caller's return address with a trampoline that calls a `do_exit()` function that frees all the memory marked to be freed "on exit", and it uses its own stack of cleanup handler closures so-to-speak. When returning all the allocations are freed and then the original return address is returned to.
The "tl;dr" summary is that `free_on_exit()` replaces the caller's return address with a trampoline that calls a `do_exit()` function that frees all the memory marked to be freed "on exit", and it uses its own stack of cleanup handler closures so-to-speak. When returning all the allocations are freed and then the original return address is returned to.
IMHO trying to emulate smart pointers in C is fixing a problem that shouldn't exist in the first place, and is also a problem in C++ code that uses smart pointers for memory management of individual objects.
Objects often come in batches of the same type and similar maximum lifetime, so let's make use of that.
Instead of tracking the individual lifetimes of thousands of objects it is often possible to group thousands of objects into just a handful of lifetime buckets.
Then use one arena allocator per lifetime bucket, and at the end of the 'bucket lifetime' discard the entire arena with all items in it (which of course assumes that there are no destructors to be called).
And suddenly you reduced a tricky problem (manually keeping track of thousands of lifetimes) to a trivial problem (manually keeping track of only a handful lifetimes).
And for the doubters: Zig demonstrates quite nicely that this approach works well also for big code bases, at least when the stdlib is built around that idea.
Objects often come in batches of the same type and similar maximum lifetime, so let's make use of that.
Instead of tracking the individual lifetimes of thousands of objects it is often possible to group thousands of objects into just a handful of lifetime buckets.
Then use one arena allocator per lifetime bucket, and at the end of the 'bucket lifetime' discard the entire arena with all items in it (which of course assumes that there are no destructors to be called).
And suddenly you reduced a tricky problem (manually keeping track of thousands of lifetimes) to a trivial problem (manually keeping track of only a handful lifetimes).
And for the doubters: Zig demonstrates quite nicely that this approach works well also for big code bases, at least when the stdlib is built around that idea.
This is the way
It makes the code so much simpler. And quite probably faster as there's less malloc/free churn.
A lot of problems break down to:
* we need this effectively forever (i.e. until config reload)
* we need this very briefly when processing a task or request
Sometimes you need a cache that has intermediate lifetimes, but that is a much smaller problem to deal with, and you can often cope with manual memory management for that
Hook any file handles and other resource cleanup functions into the same pools and you have a pretty easy life.
A lot of problems break down to:
* we need this effectively forever (i.e. until config reload)
* we need this very briefly when processing a task or request
Sometimes you need a cache that has intermediate lifetimes, but that is a much smaller problem to deal with, and you can often cope with manual memory management for that
Hook any file handles and other resource cleanup functions into the same pools and you have a pretty easy life.
Around 1994, when I was a nerdy, homeschooled 8th grader teaching myself coding I came up with something I was inordinately proud of. I had gotten a book on PowerPC assembly so using Metrowerks CodeWarrior on my brand-new PowerMac 7100 I wrote a C function with inlined assembly that I called debugf. As I recall, it had the same signature as printf, called sprintf and then passed that resulting string to DebugStr. But the part I was proud of was that it erased itself from the stack so when the debugger popped up it was pointing to the line where you called debugf. I'm still proud of it :-).
Oh well, maybe we'll soon have `defer`? [0]
[0] https://thephd.dev/c2y-the-defer-technical-specification-its...
[0] https://thephd.dev/c2y-the-defer-technical-specification-its...
That's sad. Having migrated from C++ to golang a few years ago, I find defer vastly inferior to C++ destructors. Rust did it right with its drop trait, I think it's a much better approach
The proposed C defer is scope-based unlike Go. So in the spirit of OP article, you can basically hand roll not only C++ destructor as defer {obj.dtor()} but also Rust Drop as defer {obj.notmoved() ? Drop()}
What do you mean defer isn't scope based in Go?
(not super experienced Go developer)
(not super experienced Go developer)
In Go, defers are function scoped not block scoped.
I mean, you just write all your scopes as `(func() { })()` in go, and it works out fine.
Adding `func() {}()` scopes won't break existing code usually, though if you use 'break' or 'continue' you might have to make some changes to make it compile, like so:
https://go.dev/play/p/_Gq4QYtyMmp
see, no other issues, works exactly like you'd expect
Adding `func() {}()` scopes won't break existing code usually, though if you use 'break' or 'continue' you might have to make some changes to make it compile, like so:
https://go.dev/play/p/_Gq4QYtyMmp
see, no other issues, works exactly like you'd expect
Right, that makes more sense.
mentioned in the article at [1]
[1] https://thephd.dev/c2y-the-defer-technical-specification-its...
[1] https://thephd.dev/c2y-the-defer-technical-specification-its...
Although that is true, the author has expounded at lengths on the unsuitability of RAII to the C programming langage, and as a big fan of RAII the explanations were convincing.
Even Zig which is extremely against "Hidden control flow" (so no operator overloading, etc.) added in the "defer" feature.
Destructors/drop have issues though:
* cannot return errors/throw exceptions * cannot take additional parameters (and thus do not play well with "access token" concepts like pyo3's `Python` token that proves the GIL was acquired -- requiring the drop implementation to re-acquire the lock just in case)
I think `defer` would be a better language construct than destructors, if it's combined with some kind of linear types that produce compiler errors if there is some code path that does not move/destroy the object.
* cannot return errors/throw exceptions * cannot take additional parameters (and thus do not play well with "access token" concepts like pyo3's `Python` token that proves the GIL was acquired -- requiring the drop implementation to re-acquire the lock just in case)
I think `defer` would be a better language construct than destructors, if it's combined with some kind of linear types that produce compiler errors if there is some code path that does not move/destroy the object.
What's "drop trait" for C? There are no any traits in C.
Parent post said Rust.
edit: TIL: Looks like the [drop trait][1] is an opt-in destructor that the Rust compiler understands.
[1]: https://doc.rust-lang.org/nomicon/dropck.html
edit: TIL: Looks like the [drop trait][1] is an opt-in destructor that the Rust compiler understands.
[1]: https://doc.rust-lang.org/nomicon/dropck.html
I maintain a combined error and resource state per thread.
It is first argument to all functions.
If no errors, function proceeds - if error, function instead simply immediately returns.
When allocating resource, resource is recorded in a btree in the state.
When in a function an error occurs, error is recorded in state; after this point no code executes, because all code runs only if no errors.
At end of function is boilerplate error, which is added to error state if an error has occurred. So for example if we try to open file and out of disk, we first get error "fopen failed no disk", then second error "opening file failed", and then all parent functions in the current call stack will submit their errors, and you get a call stack.
Program then proceeds to exit(), and immediately before exit frees all resources (and in correct order) as recorded in btree, and prints error stack.
It is first argument to all functions.
If no errors, function proceeds - if error, function instead simply immediately returns.
When allocating resource, resource is recorded in a btree in the state.
When in a function an error occurs, error is recorded in state; after this point no code executes, because all code runs only if no errors.
At end of function is boilerplate error, which is added to error state if an error has occurred. So for example if we try to open file and out of disk, we first get error "fopen failed no disk", then second error "opening file failed", and then all parent functions in the current call stack will submit their errors, and you get a call stack.
Program then proceeds to exit(), and immediately before exit frees all resources (and in correct order) as recorded in btree, and prints error stack.
Why bother freeing before exit?
Assuming that this would include properly closing sockets, etc. which the OS doesn't do the same way you might want to
Smart pointers in C often feel like trying to force a square peg into a round hole. They’re powerful, but without native language support like C++, they can lead to more complexity than they solve.
I've heard enough "C is superior to C++" arguments from game developers who then go and use header structs for inheritance, or X macros, enums, and switch statements for virtual functions, to know that more complexity isn't an issue as long as people feel clever and validated.
X macros can do tons of stuff C++ can't. And if you stick with C, you avoid mountains of C++ problems, even with all the stuff you're talking about.
I love using x macros with c++ to create static types and hooks to disambiguate from basic types. This is more applicable to final executables than libraries - I would never provide anyone with an API based on the mess it creates, but it allows application code to be strongly checked and makes it really easy to add whole classes of assertions to debug builds.
I never said X macros are bad on their own. With C++ you can code exactly as you would in C, but you don't have to manually implement C++ features when there's a need. C++ doesn't have any more problems than C, it's programmers who abuse language features that creates problems.
I don't think they do those things for validation. They do those things for control. Even c++ game developers create there own entire standard library replacements.
Highjacking the return address can only be done if you know you actually have a return address, and a reliable way to get to that return address. Function inlining can change that, adding local variables could change that, omitting frame pointer, etc.
It would also need to be a function that will truly be implemented as one following the ABI, which usually happens when the function is exported. Often times, internal functions won't follow the platform ABI exactly.
Just changing the compiler version is probably enough to break anything like this.
Save the return address highjacking stuff for assembly code.
---
Meanwhile, I personally have written C code that does mess with the stack pointer. It's GBA homebrew, so the program won't quit or finish execution, and resetting the stack pointer has the effect of giving you a little more stack memory.
It would also need to be a function that will truly be implemented as one following the ABI, which usually happens when the function is exported. Often times, internal functions won't follow the platform ABI exactly.
Just changing the compiler version is probably enough to break anything like this.
Save the return address highjacking stuff for assembly code.
---
Meanwhile, I personally have written C code that does mess with the stack pointer. It's GBA homebrew, so the program won't quit or finish execution, and resetting the stack pointer has the effect of giving you a little more stack memory.
Hacky and not really fit for production for more reasons than one, but clever and nice nonetheless. Good stuff.
C is the LS engine of programming languages. People love to drop it in and mod it until it blows up.
I see undefined behaviours.
they walk
they talk
they [0?W0OF??0?r??reeBSD
they don't know they're undefined.Note that this will probably cause branch prediction misses, just like thread switching does - modern CPUs have a return address predictor which is just a simple stack. I don’t think you can avoid this without compiler support.
Not to mention that any future CPU microcode update released in order to mitigate some serious CVE might break the entire product you've been shipping, just because it relied on some stack manipulation wizardry.
> Managing memory in C is difficult and error prone. C++ solves this with smart pointers like std::unique_ptr and std::shared_ptr.
No, it does not. Smart pointers are useful to help model lifetimes and ownership, but the real killer feature is RAII. Add that to C (standardized) and you can make smart pointers, and any other memory management primitive you need. Smart pointers are not a solution, they are one of many tools enabled by RAII.
No, it does not. Smart pointers are useful to help model lifetimes and ownership, but the real killer feature is RAII. Add that to C (standardized) and you can make smart pointers, and any other memory management primitive you need. Smart pointers are not a solution, they are one of many tools enabled by RAII.
1) 2018
2) I recently discovered the implementation of free_on_exit won't work if called directly from main if gcc aligns the stack. In this case, main adds padding between the saved eip and the saved ebp, (example). I think this can be fixed some tweaking, and will update this article when it is fixed.
I do not believe the article was updated, suggesting that the "tweaking" was far more complex than the author expected...
...which doesn't surprise me, because the overall tone is one of a clever but far-less-experienced-than-they-think programmer having what they think is a flash of insight and realizing thereby they can solve simply a problem that has plagued the industry and community for decades.
2) I recently discovered the implementation of free_on_exit won't work if called directly from main if gcc aligns the stack. In this case, main adds padding between the saved eip and the saved ebp, (example). I think this can be fixed some tweaking, and will update this article when it is fixed.
I do not believe the article was updated, suggesting that the "tweaking" was far more complex than the author expected...
...which doesn't surprise me, because the overall tone is one of a clever but far-less-experienced-than-they-think programmer having what they think is a flash of insight and realizing thereby they can solve simply a problem that has plagued the industry and community for decades.
Right, and then there's interactions with StackGuard-like features.
Like many C hacks, this is a fun one, but hacks like this should absolutely be avoided for any serious C code–the magic is simply not worth the potential bugs!
PuTTY is written in a Duff's Device co-routine hack on steroids that I'm sure someone at one point said "should absolutely be avoided for any serious C code–the magic is simply not worth the potential bugs!"! And PuTTY is no small program, and it's very successful.
One person's hack-that-should-be-avoided-at-all-costs can be someone else's secret sauce.
One person's hack-that-should-be-avoided-at-all-costs can be someone else's secret sauce.
This kind of approach of allocating objects to a context which is freed all at once is implemented in GNU obstacks.
this is way overkill
the way i do this in C looks like
the way i do this in C looks like
initialize all resource pointers to NULL;
attempt all allocations;
if all pointers are non-NULL, do the thing (typically calling another routine)
free all non-NULL pointers
if you must have a `free_on_exit()`
(for example, if you allocate a variable number of pointers in a loop)
then build your own defer stack registering pointers using memory that you allocate
might as well free the NULL pointers as well - this is totally valid C and can simplify the code
I suppose the implication is that, checking for null allows your cleanup logic to be more complex than simply calling free()
for example, the object could be managing an open file, or an open socket
for example, the object could be managing an open file, or an open socket
simply checking for NULL doesn't allow that - for example for something that might have been opened with fopen() needs to be closed with fclose() rather than free(), and you can't tell that from the pointer.
I think the implication was that they're pointers to objects that own resources (like containing FILE handles) and need to be "freed" with a custom function, not just "free".
void my_thing_free(MyThing *thing) {
fclose(thing->file);
free(thing);
}
Or rather, given that every relevant C compiler is also a c++ compiler, just compile as c++ and use std::unique_ptr? I love C but I just can't understand the mental gymnastics of people that prefer this kind of hacks compared to just using C++
Unfortunately, that's not true.
C is not 100% compatible with C++.
There's a whole heap of incompatibilities that you can hit, that will prevent a lot of non-trivial C programs from compiling under C++. Things like character literals being a char in C++ and an int in C. Or C allowing designated initialisers for arrays, but C++ not.
C is not 100% compatible with C++.
There's a whole heap of incompatibilities that you can hit, that will prevent a lot of non-trivial C programs from compiling under C++. Things like character literals being a char in C++ and an int in C. Or C allowing designated initialisers for arrays, but C++ not.
I know, but it's not incredibly hard to work with them to be fair. People have been mixing C and C++ for decades now, we know where the sharp edges are pretty well
There's a lot of either "I like to pretend C is simple and simple is good" or "C++ has things I don't like, so I will refuse to also use the things I do like out of spite". You see it all over the place here whenever C or C++ comes up.
Using C++ while keeping people on a common subset of C++ features is very hard in practice. Google for example do a pretty extreme amount of restrictive code standard, training, linting and reinventing libraries to that end.
A lot of the motivation behind inventing golang seems to be to remove C++ toys, because it's too hard to get people to not use them if they're there.
A lot of the motivation behind inventing golang seems to be to remove C++ toys, because it's too hard to get people to not use them if they're there.
This article should of had the conclusion of this is why you should use arena allocator.
It's a clever use of assembler, but in production code, it's much better to use a bounded model checker, like CBMC, to verify memory safety across all possible execution paths.
See also the "arena allocator", which has been discussed here before: https://nullprogram.com/blog/2023/09/27/
I haven't used it personally yet, but it addresses the same issue with a different approach, also related to stack-like lifetimes.
I've used simple reference counting before, also somewhat relevant in this context, and which skeeto also has a nice post about: https://nullprogram.com/blog/2015/02/17/
I haven't used it personally yet, but it addresses the same issue with a different approach, also related to stack-like lifetimes.
I've used simple reference counting before, also somewhat relevant in this context, and which skeeto also has a nice post about: https://nullprogram.com/blog/2015/02/17/
I wonder how that affects compiler optimization
Let's just say there's a reason the author is compiling everything with -O0.
and stack protection cookies
The naive assumption is that shared_ptr is always better than manual tracking. It's not. Tracking and cleaning up resources individually is a burden at scale.
[deleted]
That's completely asinine since it can't be made to work properly with inlining (including LTO), architectures that use a shadow stack or don't use a frame pointer, and also ridiculously inefficient and requiring assembly code for all architectures.
Use C++ or __attribute__((cleanup)) instead.
Use C++ or __attribute__((cleanup)) instead.
Maybe it is.
Or maybe the asinine "thing", is some folks lack of text comprehension skills that can't distinguish an experiment from a best practice recommendation despite a title and content that clearly does not invite that.
Or maybe the asinine "thing", is some folks lack of text comprehension skills that can't distinguish an experiment from a best practice recommendation despite a title and content that clearly does not invite that.
bsenftner(9)
Wow, an actual, purposeful, and quite general return-pointer-smashing gadget, built right into the program itself. Just what any program written in C needs.
C programmers are better off with either of these two techniques:
* Use __attribute__((cleanup)). It's available in GCC and Clang, and we hope will be added to the C spec one day. This is widely used by open source software, eg. in systemd.
* Use a pool allocator like Samba's talloc (https://talloc.samba.org/talloc/doc/html/libtalloc__tutorial...) or Apache's APR.
(I didn't include using reference counting, since although that is also widely used, I've seen it cause so many bugs, plus it interacts badly with how modern CPUs work.)