Segmentation Fault - Basic Test of RRDTool 1.2.19

Yes, I am a beginner on this - trying to work out how to use it. I can create an RRD file, can update values to it and then fetch those values. But when I come to do a graph I get a segmentation fault.

//Create graph

sprintf(argstr, "temperature.rrd --start %ld --step 60 DS:temp:GAUGE:120:-273:5000 RRA:AVERAGE:0.5:1:1200 RRA:MIN:0.5:60:400 RRA:MAX:0.5:60:400 RRA:AVERAGE:0.5:60:400", start_time);  // evey min for 20 hours : every 10 minutes for 400 hours (max, min, ave)

ret = wrap_rrd_create( argstr);

//Add in many values using this
sprintf(str, "temperature.rrd %ld:%f", (long)next_time, value);
ret = wrap_rrd_update( str);

//When I do a fetch I can see that the values have loaded correctly, so now using the same start and end time I want to produce a temp.png file

//Create Graph - but this returns a segmentation fault (core dumped)
sprintf(argstr, "temp.png --start %lu --end %lu DEF:mytemp=temperature.rrd:temp:AVERAGE", (long)start_time, (long)end_time);

ret = wrap_rrd_graph( argstr);

//And now I get a segmentation fault. What am I doing wrong please?

TIA
Mark