Cursor and Fetch Problem of INFORMIX?

I use INFORMIX DYNAMIC SERVER 9.21UC5 on AIX4.3.3 with ML08, in one program, I do this:

…
$ delcare c_cur cursor for select * from s_table;
$ open c_cur;
$ fetch c_cur;
while sqlca.sqlcode==0
{
do something;
$ fetch c_cur;}
put it simple, I have omitted the error dection syntax. the problem is:
ON theory, It let me fetch every record of table s_table, but occasionally It does not : some rows are not fetched or one row is fetched more than one time , can this be possible?