106 lines
5.4 KiB
Markdown
106 lines
5.4 KiB
Markdown
# `libucontext`
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`libucontext` is a library which provides the `ucontext.h` C API. Unlike other implementations,
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it faithfully follows the kernel process ABI when doing context swaps.
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Notably, when combined with `gcompat`, it provides a fully compatible implementation of the ucontext
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functions that are ABI compatible with glibc.
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Since version 0.13, for some architectures, you can deploy to bare metal using newlib via the
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`FREESTANDING=yes` make option. Systems which use a syscall cannot work this way. The table
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below shows which architecture ports have been adapted to build with `FREESTANDING=yes`.
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Adding support for new architectures is easy, but you need to know assembly language for the
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target to do it.
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## supported features
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| Architecture | Works on musl | Syscall | Supports FREESTANDING | Common trampoline |
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|--------------|---------------|---------|-----------------------|-------------------|
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| aarch64 | ✓ | | ✓ | ✓ |
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| arm | ✓ | | ✓ | ✓ |
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| loongarch64 | ✓ | | ✓ | |
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| m68k | ✓ | | ✓ | ✓ |
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| mips | ✓ | | ✓ | |
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| mips64 | ✓ | | ✓ | |
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| or1k | ✓ | | ✓ | ✓ |
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| ppc | ✓ | ✓ | | |
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| ppc64 | ✓ | ✓ | | |
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| riscv32 | ✓ | | ✓ | ✓ |
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| riscv64 | ✓ | | ✓ | ✓ |
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| s390x | ✓ | | ✓ | |
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| sh | ✓ | | ✓ | ✓ |
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| x86 | ✓ | | ✓ | |
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| x86_64 | ✓ | | ✓ | ✓ |
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## building
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`libucontext` uses a simple makefile build system. You should define `ARCH=` at build time, otherwise
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the build system will attempt to guess using `uname -m`.
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```
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$ make ARCH=x86_64
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$ make ARCH=x86_64 check
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$ make ARCH=x86_64 DESTDIR=out install
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```
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There are a few options:
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* `ARCH`: The architecture libucontext is being built for. Must be set to one of the architectures
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listed in the feature support table. If unset, the build system will attempt to guess based on what
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architecture the host is running. Setting this option explicitly is highly recommended.
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* `FREESTANDING`: If this is set to `yes`, the system ucontext.h headers will not be used. Instead,
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the headers in `arch/${ARCH}/freestanding` will be used for definitions where appropriate.
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Default is `no`.
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* `EXPORT_UNPREFIXED`: If this is set to `yes`, the POSIX 2004 names `getcontext`, `setcontext`,
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`swapcontext` and `makecontext` will be provided as weak symbols aliased against their `libucontext_`
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namespaced equivalents. This is necessary for libucontext to provide these functions on musl
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systems, but you may wish to disable this when using `FREESTANDING` mode to avoid conflicts with
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the target's libc. Default is `yes`.
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* `DESTDIR`: If this variable is set, the installed files will be installed to the specified path instead
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of the system root.
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If you have `scdoc` installed, you can build manpages and install them:
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```
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$ make docs
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$ make DESTDIR=out install_docs
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```
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## real-world use cases
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`libucontext` is used on almost all musl distributions to provide the legacy `ucontext.h` API.
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Additionally, it is used by:
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* [UTM](https://getutm.app) -- friendly qemu distribution for macOS and iOS devices. UTM uses libucontext
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as qemu's coroutine backend.
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* [Lwan](https://lwan.ws) -- a high-performance embeddable asynchronous web server. Lwan uses libucontext
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to provide green threads when building on non-x86 architectures.
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## caveats
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`libucontext`, while largely functionally equivalent does have some differences over traditional POSIX
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ucontext functions:
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* Saving and restoring the signal mask is not implemented by default in order to avoid kernel syscall
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overhead. Use `-lucontext_posix` if you actually need this functionality, which provides a POSIX
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compliant implementation at the cost of performance.
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* Only basic GPR registers are saved and restored when context swapping. The glibc implementation uses
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hardware capability detection to save/restore other register groups, such as the FPU registers or
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vector processing (AltiVec/AVX/NEON) registers. Adding this capability detection would significantly
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increase the complexity of the project and thus is not implemented. Support for compiling in code to
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save/restore FPU registers or vector registers may be added in a later release as a build-time
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setting -- for now, we assume a soft-float ABI with no optional processor features. In practice, this
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does not really matter, code using these functions are unlikely to be impacted by this design
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assumption. This is a work in progress, as newer compilers will spill even non-floating-point state
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through floating point registers when allowed to do so.
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