libpq debug

Does anyone know how to set the debug TTY in libpq client library ?
I did not find any example what to put in property pgtty:

PGconn *PQsetdbLogin(const char *pghost,
const char *pgport,
const char *pgoptions,
const char *pgtty,
const char *dbName,
const char *login,
const char *pwd)

I tried with :
/dev/ptsxx
/dev/pts/xx
ptsxx
pts/xx
but no result.

I've also tried with -o pgoption to set debug log file, but no output was written to that file.
I'm trying to debug some functions and without those two methods, it is very hard work !!!

Darko