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Sending a file via MQTT C [message #1326478]

Thu, 01 May 2014 19:30

Brandon Palm

Messages: 1
Registered: May 2014

Junior Member

Hello everyone,

I am trying to send a file (rather than a string; see the example) via MQTT to my broker, however I'm not sure if I'm going about this in the right way:

long GetFileSize(std::string absoluteFile)
{
FILE * myFile;
long size = 0;

myFile = std::fopen(absoluteFile.c_str(), "rb");
if(myFile == NULL)
{
std::cout << "Error opening file/file not found." << std::endl;
}
else
{
std::fseek(myFile, 0, SEEK_END);
size = ftell(myFile);
fclose(myFile);
}

return size;
}

// Add file to the payload.
FILE * pFile = fopen(myFile.c_str(), "rb");
pubmsg.payload = pFile;
pubmsg.payloadlen = GetFileSize(myFile); // Assuming I shouldn't be using sizeof(pFile) here.
pubmsg.qos = g_QOS;
pubmsg.retained = 0;

Is this an acceptable implementation of sending a file using synchronous MQTT?

The broker receives a byte array of the file in unreadable ASCII characters, so I will have to figure out how to interpret this data on the other side as well. Any tips on how to reconstruct this file on the other end for other subscribers to the topic?

Thanks,
Brandon

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Re: Sending a file via MQTT C [message #1327605 is a reply to message #1326478]

Fri, 02 May 2014 08:39

Ian Craggs

Messages: 83
Registered: March 2013

Member

Hi Brandon,

pFile is a file handle, not the contents of a file. You need to read the contents of the file into a buffer (with fread or similar http://pubs.opengroup.org/onlinepubs/009695399/functions/fread.html) and set that buffer to be the payload.

Ian

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