Teensy 4.0 Builder Error: size_t is undefined

Encountered a Teensy 4.0 build error in file


where size_t is undefined.

The problem is caused by a missing new.h file which is missing in folder teensy4 and the include is commented out in Printable.h. I don’t know if it was intentional to remove new.h from teensy4 but that file in teensy3 included stdlib.h which sets the needed typedef.

To correct the problem, either bring back new.h and include in Printable.h if appropriate or add

#include <stdlib.h> to Printable.h .

Build environment was

platform = teensy
board = teensy40
framework = arduino

I also used a variant of the Arduino WiFiNINA library, which allows the ability to change the pins available at https://github.com/adafruit/WiFiNINA/archive/master.zip.

What main.cpp are you using so that we can reproduce the problem?

Not familiar with this interface. How do I provide the code or specify the code in a reply? Thanks!

You can use Markdown style, meaning three backticks, then the code, then 3 backticks. See here.



 This example connects to an unencrypted Wifi network.
 Then it prints the  MAC address of the Wifi module,
 the IP address obtained, and other network details.

 created 13 July 2010
 by dlf (Metodo2 srl)
 modified 31 May 2012
 by Tom Igoe
#include <Arduino.h>
#include <SPI.h>
#include <WiFiNINA.h>

#include "arduino_secrets.h" 
///////please enter your sensitive data in the Secret tab/arduino_secrets.h
char ssid[] = SECRET_SSID;        // your network SSID (name)
char pass[] = SECRET_PASS;    // your network password (use for WPA, or use as key for WEP)
int status = WL_IDLE_STATUS;     // the Wifi radio's status

#define SPIWIFI       SPI  // The SPI port
#define SPIWIFI_SS     5   // Chip select pin
#define ESP32_RESETN   6   // Reset pin
#define SPIWIFI_ACK    9   // a.k.a BUSY or READY pin
#define ESP32_GPIO0   -1
#define LED_PIN       14

void printMacAddress(byte mac[]) 
    for (int i = 5; i >= 0; i--) 
        if (mac[i] < 16) 
        Serial.print(mac[i], HEX);
        if (i > 0) 

void printWifiData() 
    // print your board's IP address:
    IPAddress ip = WiFi.localIP();
    Serial.print("IP Address: ");

    // print your MAC address:
    byte mac[6];
    Serial.print("MAC address: ");

void printCurrentNet() 
    digitalWrite(LED_PIN, HIGH);

    // print the SSID of the network you're attached to:
    Serial.print("SSID: ");

    // print the MAC address of the router you're attached to:
    byte bssid[6];
    Serial.print("BSSID: ");

    // print the received signal strength:
    long rssi = WiFi.RSSI();
    Serial.print("signal strength (RSSI):");

    // print the encryption type:
    byte encryption = WiFi.encryptionType();
    Serial.print("Encryption Type:");
    Serial.println(encryption, HEX);

    digitalWrite(LED_PIN, LOW);

void setup() 
    //Initialize serial and wait for port to open:
    while (!Serial) 
        ; // wait for serial port to connect. Needed for native USB port only

    pinMode(LED_PIN, OUTPUT);

    // Set up the pins!

    // check for the WiFi module:
    if (WiFi.status() == WL_NO_MODULE) 
        Serial.println("Communication with WiFi module failed!");
        // don't continue
        //while (true);

    String fv = WiFi.firmwareVersion();
    if (fv < "1.0.0") 
        Serial.println("Please upgrade the firmware");

    // attempt to connect to Wifi network:
    while (status != WL_CONNECTED) 
        Serial.print("Attempting to connect to WPA SSID: ");
        // Connect to WPA/WPA2 network:
        status = WiFi.begin(ssid, pass);

        // wait 10 seconds for connection:

    // you're connected now, so print out the data:
    Serial.print("You're connected to the network");


void loop() 
    // check the network connection once every 10 seconds:

The source code of the framework is just still wrong:

Still has new.h commented out, so the type size_t is unknown. The core issue must be fixed at Issues · PaulStoffregen/cores · GitHub.

For the moment, you can workaround it by adding #include <stddef.h> (since only size_t is needed) to the Printable.h header (C\User\<user>\.platformio\packages\framework-arduinoteensy\cores\teensy4\Printable.h)

1 Like

Yes, that is the workaround I used.

Opened issue Missing size_t inclusion in Printable.h · Issue #393 · PaulStoffregen/cores · GitHub. When this is fixed and a new core is released, it will be included in PIO.

You can also go the other route and fix the caller, that is before this block

You #include <stddef.h>. Then you can put this fixed WiFiNINA library in the lib/ folder (and just write lib_deps= WiFiNINA in the platformio.ini.

1 Like