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update README a bit

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Ariadne Conill 2021-03-09 00:42:26 -07:00
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@ -71,15 +71,26 @@ $ make DESTDIR=out install_docs
```
## real-world use cases
`libucontext` is used on almost all musl distributions to provide the legacy `ucontext.h` API.
Additionally, it is used by:
* [UTM](https://getutm.app) -- friendly qemu distribution for macOS and iOS devices. UTM uses libucontext
as qemu's coroutine backend.
* [Lwan](https://lwan.ws) -- a high-performance embeddable asynchronous web server. Lwan uses libucontext
to provide green threads when building on non-x86 architectures.
## caveats
`libucontext`, while largely functionally equivalent does have some differences over traditional POSIX
ucontext functions:
* Saving and restoring the signal mask is not implemented. This is largely a non-issue because most
uses of these functions did not modify the signal mask anyway, but saving/restoring the signal mask
(even though it is unmodified in basically all cases in practice) induces a significant performance
penalty due to having to make kernel syscalls.
* Saving and restoring the signal mask is not implemented by default in order to avoid kernel syscall
overhead. Use `-lucontext_posix` if you actually need this functionality, which provides a POSIX
compliant implementation at the cost of performance.
* Only basic GPR registers are saved and restored when context swapping. The glibc implementation uses
hardware capability detection to save/restore other register groups, such as the FPU registers or