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