#include <pgsql/postgres.h>
#include <string.h>

char *fernando(char *texto)
{
	char *resultp = palloc(strlen(texto)+5);
	*resultp = *texto;
	strcat(resultp," mais");
	return resultp;
}

gcc -shared fernando.c -o fernando.so

CREATE FUNCTION fernando (bpchar) RETURNS bpchar
  AS '/u/src/tef/fernando.so' LANGUAGE 'c';
CREATE
SELECT fernando ('Teste');
ERROR:  Memory exhausted in AllocSetAlloc(287341377)