was not declared in this scope

[victorbnl]

I have many errors like
bluetooth.cpp:291:17: error: ‘close’ was not declared in this scope close(sk); ^~~~~
:/
On Ubuntu 18.04 LTS

[mstr3336]

I get the same problems- I started editing the original code to #include <unistd.h>, but I get the feeling that downgrading your g++ compiler to <4.7 may be the trick, https://gcc.gnu.org/gcc-4.7/porting_to.html
https://stackoverflow.com/a/37734860

Alternatively -
Just keep bashing your head against adding that '#include <unistd.h>' line to any and all .cpp files that throw errors at you until eventually you stop getting errors.
Worked for me, after changing maybe 4-6 files.