Creation of RRD files with C++ on Raspberry behaves strangely

Question

I want to use my Raspberry Pi to record temperature from a series of sensors. For this purpose I am writing a C++ program which uses librrd.

For every connected sensor I want to create a rrd with 12 rra. The following call should create my wanted rrd:

rrd_create(mNumberOfCreateParams, mCreateParams);

mNumberOfCreateParams is 17 and the content of mCreateParams is the following:

rrdcreate
28-000005fd934f.rrd
--step=300
--no-overwrite
DS:temperature:GAUGE:600:-55:125
RRA:AVERAGE:0.5:1:288
RRA:AVERAGE:0.5:3:672
RRA:AVERAGE:0.5:12:744
RRA:AVERAGE:0.5:72:1464
RRA:MAX:0.5:1:288
RRA:MAX:0.5:3:672
RRA:MAX:0.5:12:744
RRA:MAX:0.5:72:1464
RRA:MIN:0.5:1:288
RRA:MIN:0.5:3:672
RRA:MIN:0.5:12:744
RRA:MIN:0.5:72:1464

The second line changes each time corresponding to the id of the sensor.

Now the problem: Some time the call to rrd_create works as intended but at some point it stops working and just creates errors on further calls. This is even true if I want to recreate an rrd which was successfully created previously.

By changing mNumberOfCreateParams I can alter the number of parsed arguments. If the parameter is in range of 13 to 17 the error returned by rrd_get_error() is "can't parse argument ' ' " (added space between ' for readability). If I let the function parse 10 to 12 parameters it will "work" the first time and return "opening '@': No such file or directory" the second time because the first time the following file was created:

image of created file in file browser

If the number of parsed parameters is below 10 it is working as intended.

There isn't any difference if I change the order of the RRA lines.

If I call rrdtool create [...same parameters as above] from terminal everything works fine indifferent how many parameters are parsed.

In hopes of rrd_create again working I restarted the Raspberry serveral times and it even worked once for a short amount of time (one run of my application).

Are there any suggestions what I am doing wrong or how I can move rrd_create into a more stable state?

Edit: I'm using version 1.4.7 of RRDtool (rrdtool version in shell). Here is the code I'm using for creation of rrd files:

// mCreateParams & mNumberOfCreateParams will be set here
setupRrdCreateParamsDS18B20(lStepSize);
char lCurrentPath[255];
getcwd(lCurrentPath, sizeof(lCurrentPath));
// since I wasn't able to create rrd files outside current working directory I
// change working directory to where I want all files
chdir(DS18B20_PATH.c_str());
// the dump of mCreateParams postet above was created here
int lStatus = rrd_create(mNumberOfCreateParams, mCreateParams);

Since I dumped mCreateParams just before calling rrd_create(...) I think they shouldn't be corrupted.

My current workaround uses popen() and mCreateParams are used to create an shell command calling rrdtool.

stringstream ss;
// create shell command from create params
ss << "rrdtool create ";
for (int i = 1; i < mNumberOfCreateParams - 1; i++) {
    ss << mCreateParams[i] << " ";
}
ss << mCreateParams[mNumberOfCreateParams - 1];
// needed for capturing output from executed command
FILE * in;
char buff[512];
if(!(in = popen(ss.str().c_str(), "r"))){
    return false;
}
ss.str("");
ss.clear();
// get output
while(fgets(buff, sizeof(buff), in)!=NULL){
    ss << buff;
}
lRrdError = ss.str();
ss.str("");
ss.clear();
int lTemp = pclose(in);
// get exit code from rrdtool create
lStatus = WEXITSTATUS(lTemp);

I am thankful for every advise.

Which version of the RRDtool library are you using? Can you post the actual code you are using (or a part of it)? It is possible that your calling parameters are being somehow corrupted prior to the call? Maybe, for testing purposes, print out the passed argv list just prior to the call to verify. Also, make sure you're not using threads or similar when calling this - many of the calls are not thread-safe. Finally, your attached file was not done correctly - I just get a binary file icon :) — Steve Shipway, Aug 31 '14 at 22:29
Yeah, right. It just created an binary file. But as far as I can see rrdtool ignores the provided filename. — Nazrim, Sep 01 '14 at 01:08

score 2 · Accepted Answer · answered Sep 02 '14 at 02:11

2

When you call rrd_create(int argc, char**argv), you need to pass the parameters in an ARGV list, very similar to the way a normal C main() function take its parameters.

In particular, you do not need to pass the create function name (this is implicit) and, of course, the argcparameter must match the number of elements in the argv array.

So, in short: your parameter list to rrd_create should not include the rrdcreate parameter, and your argc MUST match the number of argv parameters passed.

If you still get errors returned from the rrd_create function call, then print out the error message.

answered Sep 02 '14 at 02:11

Steve Shipway

3,754
3
22
39

I removed `rrdcreate` from my parameter list and set the number of parameters to 16. The rest of the parameters is untouched beside the heartbeat of `RRA:temperature`. Not I get the error "Unrecognized consolidation function". The confusing fact for me is that the same parameters are working with `popen()`. – Nazrim Sep 04 '14 at 00:07
"Unrecognized consolidation function" should be immediately followed by the invalid function name. If you get nothing, then it looks as if you have not set up your mCreateParams correctly. This needs to be a char** with an array of null-terminated char arrays. I notice that you're using C++, so be sure you're not using some class that is not compatible with the C library. – Steve Shipway Sep 04 '14 at 03:28
Yeah, nothing follows the "Unrecognized consolidation function". mCreateParams is declared as `char* mCreateParams[mNumberOfCreateParams]` but refering to http://stackoverflow.com/questions/5192068/c-char-argv-vs-char-argv this should be equal to `char**`. Maybe the conversation from string to char* (`mCreateParams[i] = (char*) lConstructedString`) does not add a null-termination? I will take a look at this. – Nazrim Sep 12 '14 at 08:25
I strongly suspect the problem lies in the way C++ is passing the string parameters to the C function library, resulting in RRDTool being unable to parse what it receives. – Steve Shipway Sep 16 '14 at 22:36

Creation of RRD files with C++ on Raspberry behaves strangely

1 Answers1