While compiling socat-2.0.0-b5 on cygwin (Windows) I got a few errors and here’s how I fixed it:
xioopts.c: In function 'applyopts_single':
xioopts.c:3998: error: 'struct single' has no member named 'fd1'
xioopts.c:4000: error: 'struct single' has no member named 'fd1'
make[1]: *** [xioopts.o] Error 1
Edit the file xioopts.c in your favorite editor and replace ‘fd1’ by ‘rfd’ in both lines (3998 & 4000). That fixed this error but then I got my next error.

xio-ip.c:480: error: structure has no member named `ipi_spec_dst'
Edit xio-ip.c and comment out the entire snprintf statement in xio-ip.c line 480.

Continue compilation and it should now work fine.