View source for Termux

Termux is an application on Android phones that lets you run applications in a "linux" like environment.
FEX-Emu runs under this environment and manages the termux package as well.
There are some major caveats towards running anything under this environment although.

== Problems ==

=== Seccomp ===
The latest versions of Android use a feature called seccomp to restrict the syscalls that applications can use. This restricts a /significant/ number of syscalls from FEX.

* Termux gets around this problem by significantly modifying system libraries and applications to avoid restricted syscalls
* FEX-Emu is not a syscall emulator. It relies heavily on the kernel to give a sane syscall environment
* FEX-Emu uses the kernel syscall interface directly so it is highly likely to hit a SIGSYS fault.

=== Completely unsupported syscalls ===
Android outright disables a bunch of syscalls which applications expect to have working.

* SystemV IPC syscalls
** shmctl, msgctl, semctl
* Namespaces
* Way more

Termux gets around this by providing some emulation libraries that applications use. 
* [https://github.com/termux/libandroid-shmem libandroid-shmem] provides the shm emulation
* msgctl and semctl don't have equivalents?

Termux also modifies system libraries to avoid syscalls that Android doesn't have.
* X11 things are heavily modified to make it a wayland service with [https://github.com/termux/termux-x11 termux-x11]
* If you're not using the modified libraries then things break catastrophically
** FEX-Emu's x86 rootfs doesn't have the modifications that termux has, so these library replacements don't work


== Applications that don't work ==
=== Wine ===
Wine doesn't work in Termux+FEX. It immediately hits problems with seccomp.

* Disabling seccomp will likely get you slightly further
* Wine's multiprocess communication immediately breaks down though

=== Chroot ===
Chrooting in to FEX's rootfs on Termux has issues

* First you need root support on your Android device, which isn't common.
* Second Android doesn't seem to support devtmpfs? So mounting /dev/ and /dev/pts inside the chroot doesn't seem to work.
* Proot might work around the chrooting problems.