How do you convert a formatted print statement into a string variable?

Question

I'm using the following routine to print the current time on the com port. I would like to capture this as a string so I can display it using ePaper.

 void printLocalTime()
 {
 time_t rawtime;
 struct tm timeinfo;
 if(!getLocalTime(&timeinfo))
 {
 Serial.println("Failed to obtain time");
 return;
 }
 Serial.println(&timeinfo, "%A, %B %d %Y %H:%M:%S");
 }

produces -

Saturday, May 12 2018 12:18:27

• Majenko: That's clever. How are you getting Serial.println to respond to that format?
• Juraj: try CStringBuilder from StreamLib
• Juraj: @Majenko, it is ESP32 Arduino core package
• Majenko: Oh, so nothing to do with Arduino then. It's non-standard functionality.
• Majenko: Personally I'd use sprintf() to place things into a char array - though you'd have to do the textual formatting (day names, etc) yourself. Or the standard C library functions may work for you (or they may not).

Answer 1

To achieve what you want you probably want to use the "string format time" function strftime (docs). You would write the result in a character buffer, which you can also print directly without having to convert it to String object.

So, the following code should work:

void printLocalTime()
{
 time_t rawtime;
 struct tm timeinfo;
 if(!getLocalTime(&timeinfo))
 {
 Serial.println("Failed to obtain time");
 return;
 }
 char timeStringBuff[50]; //50 chars should be enough
 strftime(timeStringBuff, sizeof(timeStringBuff), "%A, %B %d %Y %H:%M:%S", &timeinfo);
 //print like "const char*"
 Serial.println(timeStringBuff);
 //Optional: Construct String object 
 String asString(timeStringBuff);
}

The overload for Serial.print also does this exact same thing:

size_t Print::print(struct tm * timeinfo, const char * format)
{
 const char * f = format;
 if(!f){
 f = "%c";
 }
 char buf[64];
 size_t written = strftime(buf, 64, f, timeinfo);
 print(buf);
 return written;
}

• Simon Markham: Perfect, except '&time_info);' should be &timeinfo);, Just what I was aiming for and missing.