I'm working on a home automation project using Arduino and MQTT.

I've found a problem and my limited programming knowledge find a way to solve it.

I can send text trough MQTT normally, using

client.Publish("some/topic","Some text here");

But when I try to send a variable (int, char, etc), the compiler says "invalid conversion from 'char' to 'const char*' [-fpermissive]"

How can I store DHT11 sensor reading in const char* to send it?

BTW, I've found this code where

char *tempC;
tempC = dtostrf(((((analogRead(tempPinIn) * 5.0) / 1024) - 0.5) * 100), 5, 2, message_buffer);

is used. But the extra sensor calibration makes it even harder for me to understand whats going on.

1 Answer 1


The function dtostrf() is prototyped as:

char *dtostrf (double          val,
               signed char     width,
               unsigned char   prec,
               char           *sout);

   val      Your float variable;
   width    Length of the string that will be created INCLUDING decimal point;
   prec     Number of digits after the deimal point to print;
   sout     Destination of output buffer;

An example of usage is as follows:


    char msgBuffer[20];           // make sure this is big enough to hold your string
    char *pointer_to_created_string;

    float testFloat = 123.45;

    pointer_to_created_string = dtostrf(testFloat, 6, 2, msgBuffer);
    client.Publish("...topic...", pointer_to_created_string);


    char msgBuffer[20];           // make sure this is big enough to hold your string

    float testFloat = 123.45;

    client.Publish("...topic...", dtostrf(testFloat, 6, 2, msgBuffer));

Don't be put off by the code in the link you included in your question. The first argument sent to dtostrf (everything before the first comma) is nothing more than a some calculations bundled into one single line.

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