A printf function for serial communication from Arduino boards

ardprintf.c

/* 
    This code should be pasted within the files where this function is needed.
    This function will not create any code conflicts.
    
    The function call is similar to printf: ardprintf("Test %d %s", 25, "string");

    To print the '%' character, use '%%'

    This code was first posted on http://arduino.stackexchange.com/a/201
*/

#ifndef ARDPRINTF
#define ARDPRINTF
#define ARDBUFFER 16 //Buffer for storing intermediate strings. Performance may vary depending on size.
#include <stdarg.h>
#include <Arduino.h> //To allow function to run from any file in a project

int ardprintf(char *str, ...) //Variadic Function
{
  int i, count=0, j=0, flag=0;
  char temp[ARDBUFFER+1];
  for(i=0; str[i]!='\0';i++)  if(str[i]=='%')  count++; //Evaluate number of arguments required to be printed
  
  va_list argv;
  va_start(argv, count);
  for(i=0,j=0; str[i]!='\0';i++) //Iterate over formatting string
  {
    if(str[i]=='%')
    {
      //Clear buffer
      temp[j] = '\0'; 
      Serial.print(temp);
      j=0;
      temp[0] = '\0';
      
      //Process argument
      switch(str[++i])
      {
        case 'd': Serial.print(va_arg(argv, int));
                  break;
        case 'l': Serial.print(va_arg(argv, long));
                  break;
        case 'f': Serial.print(va_arg(argv, double));
                  break;
        case 'c': Serial.print((char)va_arg(argv, int));
                  break;
        case 's': Serial.print(va_arg(argv, char *));
                  break;
        default:  ;
      };
    }
    else 
    {
      //Add to buffer
      temp[j] = str[i];
      j = (j+1)%ARDBUFFER;
      if(j==0)  //If buffer is full, empty buffer.
      {
        temp[ARDBUFFER] = '\0';
        Serial.print(temp);
        temp[0]='\0';
      }
    }
  };
  
  Serial.println(); //Print trailing newline
  return count + 1; //Return number of arguments detected
}

#undef ARDBUFFER
#endif

Hi, this really a handy function, I need to print float with 4 decimal points, this does two, any way how it can be achieved?

Howdy ... some problems with this code as it is today (2015-08-15)....

1. The temp buffer is not flushed at the end so there may be lost data
2. The va_start should be: va_start(argv, count)

Hi,

I've some troubles implementing this function on my sketch.

I get this problem:

'va_start' used in function with fixed args

I just paste this code on my sketch and adapted the prototype to my needs:

    int ardprintf(char *str, int sensorValue, unsigned long time) //Variadic Function

Then, on the loop I do:

     ardprintf("%d %l", sensorValue,time);

Being:

     unsigned long time;
     int sensorValue = analogRead(A10);

What can be happening?

Thanks!

Why not using something like this?

void _log(const char *format, ...)
{
#if DEBUG_MODE
    char buffer[256];
    va_list args;
    va_start(args, format);
    vsprintf(buffer, format, args);
    va_end(args);
    Serial.print(buffer);
#endif
}

@Lucianovici Your solution works perfectly for me and actually makes a lot more sense to my brain.

@angelorz It's been many years since your post, but I believe the problem that you were experiencing was that the function should have worked as-is without change. The , ... tells the compiler that you are willing to accept a variable/infinite amount of arguments, no matter the type, as long as char *str is the first argument.

Just a note that the 'clear buffer' on line 57 does not work!

Hi, It is very useful function!
I have one remark. When I invoked it like this:
ardprintf("Test %d %s other text", 25, "string");
"other text" was not shown"
Adding this code in line 65 resolve it:

  temp[j] = '\0';
  Serial.print(temp);