sending data with COPY SQL command, libpq and structure of SELECT resulting buffer

 From the docs i understood, that with COPY SQL command we should use 
for synchr. sending:
PQexec
PQResultstatus returns PGRES_COPY_OUT
PQputCopyData
PQputCopyEnd
PQResultstatus returns PGRES_COMMAND_OK

How should look the structure of buffer in
int PQputCopyData(PGconn *conn,const char *buffer,int nbytes);
id for me clear (it should be the same like for example in text dumps of 
tables done with pg_dump).

The question is:
Can I in function:
int PQputCopyData(PGconn *conn,const char *buffer,int nbytes);
use as a result buffer directly given by using PQexec for SELECT SQL 
command ?
In other way:
Is the structure given as result of PQexec with SELECT SQL command 
directly usable as buffer for PQputCopyData ?
Brgds
Adam