I am trying to find the select() source code (Linux, i386 arch) in the glibc source code, but I cannot find anything that is related to the said architecture.
Could anybody point me to the select() source code?
select() is not a function of the libc, but a kernel function, so you need to take a look into the kernel source.
You can tell this by looking into the man page: If it is in section 2, it's a kernel function, if it's in section 3, it's a function of the standard C library, in your case the glibc.
Edit: Like some other people remarked correctly (thank you!), a function described in section 2 is officially called a system call and it is actually a call to a library that wraps the operating system's actual call interface.