char *dbname = valueOfParam("dbname");
char *dbuser = valueOfParam("dbuser");
char *dbpass = valueOfParam("dbpass");
char *conninfo;
// Then  I try compile new variable like this:
printf("PQconnectdb("dbname=%s host=localhost user=%s password=%s");", dbname, dbuser, dbpass);
// But I need create the variable
sprintf(conninfo, "PQconnectdb("dbname=%s host=localhost user=%s password=%s");", dbname, dbuser, dbpass);

int len = snprintf(NULL,0,......);
char * buf = malloc(len+1);