Legacy Platform Help Articles
Debug

#define BLYNK_PRINT

#define BLYNK_DEBUG

To enable debug prints on the default Serial port add on the top of your sketch IMPORTANT: This should be the first line in your code:
#define BLYNK_PRINT Serial // Defines the object that is used for printing
#define BLYNK_DEBUG // Optional, this enables more detailed prints
Then enable Serial Output in setup():
Serial.begin(9600);
Open Serial Monitor and you'll see the debug prints.
You can also use spare Hardware serial ports or SoftwareSerial for debug output (you will need an adapter to connect to it with your PC).
WARNING: Enabling BLYNK_DEBUG will slowdown your hardware processing speed up to 10 times!

BLYNK_LOG()

When BLYNK_PRINT is defined, you can use BLYNK_LOG to print your logs. The usage is similar to printf:
BLYNK_LOG("This is my value: %d", 10);
On some platforms (like Arduino 101) the BLYNK_LOG may be unavailable, or may just use too much resources. In this case you can use a set of simpler log functions:
BLYNK_LOG1("Hello World"); // Print a string
BLYNK_LOG1(10); // Print a number
BLYNK_LOG2("This is my value: ", 10); // Print 2 values
BLYNK_LOG4("Temperature: ", 24, " Humidity: ", 55); // Print 4 values
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#define BLYNK_PRINT
#define BLYNK_DEBUG
BLYNK_LOG()