Concurrent access

Hi, I'm running postgres in a Linux box, kernel 2.2.11.
I have a C program that forks and with one child it works perfectly.
With 2 or more childs the select queries return different number of fields
for each child.
For example,

select a, b, c, d, e from table1

the child 1 returns n tuples with 5 fields each one.
The child 2 returns n tuples with 2 fields each one.

What could be the problem?
Where can I find some info about it?

Thanks.

                    Pedro Aguilar Iniestra
                    paguilar@isite.ciceana.org.mx