[PostgreSQL-Hackers] What the function name to get the contents table/tuple ?

Using this code below (particularly heap_open) somehow i can fetch all
table column name, and put in on list of string. Before raw_parsetree is
processed by analyze part.

------start code ----------------
---------------------------------

RangeVar *relation = makeNode(RangeVar);
relation->schemaname = NULL;
relation->catalogname = NULL;
relation->relname = "pg_class"; //table name
relation->inhOpt = INH_DEFAULT;
relation->istemp = false ;
relation->alias = NULL;

Relation Rel;
LOCKMODE lockmode = AccessShareLock;

Rel = heap_open(TableSpaceRelationId,lockmode);
Value *attrname ;
TupleDesc td;
td = Rel->rd_att;

List *colnames;
colnames = NIL;
int z;
int maxatribut = td->natts;
Form_pg_attribute fpa;

for (z=0;z<maxatribut;z++)
{
fpa = td->attrs[z];
attrname = makeString(pstrdup(NameStr(fpa->attname)));
colnames = lappend(colnames,attrname); //this where list cols name
colected
}

------end code ----------------
---------------------------------

My question, is there same kind function that i can use to get table
content (tuple)?

Thank You.