An improved chkstk function on Windows

If you’ve spent much time developing with Mingw-w64 you’ve likely seen the symbol ___chkstk_ms, perhaps in an error message. It’s a little piece of runtime provided by GCC via libgcc which ensures enough of the stack is committed for the caller’s stack frame. The “function” uses a custom ABI and is implemented in assembly. So is the subject of this article, a slightly improved implementation soon to be included in w64devkit as libchkstk (-lchkstk).

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Two handy GDB breakpoint tricks

Over the past couple months I’ve discovered a couple of handy tricks for working with GDB breakpoints. I figured these out on my own, and I’ve not seen either discussed elsewhere, so I really ought to share them.

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So you want custom allocator support in your C library

This article was discussed on Hacker News and on reddit.

Users of mature C libraries conventionally get to choose how memory is allocated — that is, when it cannot be avoided entirely. The C standard never laid down a convention — perhaps for the better — so each library re-invents an allocator interface. Not all are created equal, and most repeat a few fundamental mistakes. Often the interface is merely a token effort, to check off that it’s “supported” without actual consideration to its use. This article describes the critical features of a practical allocator interface, and demonstrates why they’re important.

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My personal C coding style as of late 2023

This article was discussed on Hacker News and on reddit.

This has been a ground-breaking year for my C skills, and paradigm shifts in my technique has provoked me to reconsider my habits and coding style. It’s been my largest personal style change in years, so I’ve decided to take a snapshot of its current state and my reasoning. These changes have produced significant productive and organizational benefits, so while most is certainly subjective, it likely includes a few objective improvements. I’m not saying everyone should write C this way, and when I contribute code to a project I follow their local style. This is about what works well for me.

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A simple, arena-backed, generic dynamic array for C

Previously I presented an arena-friendly hash map applicable to any programming language where one might use arena allocation. In this third article I present a generic, arena-backed dynamic array. The details are specific to C, as the most appropriate mechanism depends on the language (e.g. templates, generics). Just as in the previous two articles, the goal is to demonstrate an idea so simple that a full implementation fits on one terminal pager screen — a concept rather than a library.

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An easy-to-implement, arena-friendly hash map

My last article had tips for for arena allocation. This next article demonstrates a technique for building bespoke hash maps that compose nicely with arena allocation. In addition, they’re fast, simple, and automatically scale to any problem that could reasonably be solved with an in-memory hash map. To avoid resizing — both to better support arenas and to simplify implementation — they have slightly above average memory requirements. The design, which we’re calling a hash-trie, is the result of fruitful collaboration with NRK, whose sibling article includes benchmarks. It’s my new favorite data structure, and has proven incredibly useful. With a couple well-placed acquire/release atomics, we can even turn it into a lock-free concurrent hash map.

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Arena allocator tips and tricks

This article was discussed on Hacker News.

Over the past year I’ve refined my approach to arena allocation. With practice, it’s effective, simple, and fast; typically as easy to use as garbage collection but without the costs. Depending on need, an allocator can weigh just 7–25 lines of code — perfect when lacking a runtime. With the core details of my own technique settled, now is a good time to document and share lessons learned. This is certainly not the only way to approach arena allocation, but these are practices I’ve worked out to simplify programs and reduce mistakes.

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How to link identical function names from different DLLs

For the typical DLL function call you declare the function prototype (via header file), you inform the link editor (ld, link) that the DLL exports a symbol with that name (import library), it matches the declared name with this export, and it becomes an import in your program’s import table. What happens when two different DLLs export the same symbol? The link editor will pick the first found. But what if you want to use both exports? If they have the same name, how could program or link editor distinguish them? In this article I’ll demonstrate a technique to resolve this by creating a program which links with and directly uses two different C runtimes (CRTs) simultaneously.

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Everything you never wanted to know about Win32 environment blocks

In an effort to avoid programming by superstition, I did a deep dive into the Win32 “environment block,” the data structure holding a process’s environment variables, in order to better understand it. Along the way I discovered implied and undocumented behaviors. (The environment block must not to be confused with the Process Environment Block (PEB) which is different.) Because I cannot possibly retain all the quirky details in my head for long, I’m writing them down for future reference. I ran my tests on different Windows versions as far back as Windows XP SP3 in order to fill in gaps where documentation is ambiguous, incomplete, or wrong. Overall conclusion: Correct, direct manipulation of an environment block is impossible in the general case due to under-specified and incorrect documentation. This has important consequences mainly for programming language runtimes.

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"Once" one-time concurrent initialization with an integer

We’ve previously discussed integer barriers, integer queues, and integer wait groups as tiny concurrency utilities. Next let’s tackle “once” initialization, i.e. pthread_once, using an integer. We’ll need only three basic atomic operations — store, load, and increment — and futex wait/wake. It will be zero-initialized and the entire source small enough to fit on an old-fashioned terminal display. The interface will also get an overhaul, more to my own tastes.

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null program

Chris Wellons

wellons@nullprogram.com (PGP)
~skeeto/public-inbox@lists.sr.ht (view)