linker error: undefined reference to `select'



I'm using Platform IO with Arduino project type for teensy 4.1. I wanted to embed a third party library which makes use of sockets. I am now able to compile all references but at the end the linker fails with undefined reference to `select'.
Is there a way to use socket's 'select' within teensy 4.1?

You could always modify one of the Ethernet libraries to enable the ‘sockets’ API. Note: I say this just as a guess without having done it or tested it myself.

For example, in the QNEthernet library, you can modify the appropriate option(s) in src/lwipopts.h. See the src/lwip/opt.h file for guidance.
I don't think there is a simple answer to this as it would depend on how select is being used by the library. Sometimes it is used purely as a delay (by specifying no sockets with a timeout value) and other times it's used as a non-blocking wait for stdin input. There is no way to know without more context.