Arduino Data Types

(Update 2013-01-23: this article is about the Arduino Uno and similar boards that use the eight-bit Atmel AVR ATmega microcontrollers. If you're looking for the Arduino Due that uses the thirty-two-bit ARM Cortex-M3 microcontroller, see Arduino Due Data Types.)

I think for every system I've ever worked on that had a C compiler, going all the way back to my PDP-11 days, I eventually wrote this program or its equivalent. I have had some surprises. Here it is for Arduino, as part of my Amigo project.

#include <stdint.h>
#include <avr/pgmspace.h>

void setup() {
Serial.begin(9600);
}

void loop() {
Serial.print("sizeof(byte)="); Serial.println(sizeof(byte));
Serial.print("sizeof(char)="); Serial.println(sizeof(char));
Serial.print("sizeof(short)="); Serial.println(sizeof(short));
Serial.print("sizeof(int)="); Serial.println(sizeof(int));
Serial.print("sizeof(long)="); Serial.println(sizeof(long));
Serial.print("sizeof(long long)="); Serial.println(sizeof(long long));
Serial.print("sizeof(bool)="); Serial.println(sizeof(bool));
Serial.print("sizeof(boolean)="); Serial.println(sizeof(boolean));
Serial.print("sizeof(float)="); Serial.println(sizeof(float));
Serial.print("sizeof(double)="); Serial.println(sizeof(double));
Serial.print("sizeof(int8_t)="); Serial.println(sizeof(int8_t));
Serial.print("sizeof(int16_t)="); Serial.println(sizeof(int16_t));
Serial.print("sizeof(int32_t)="); Serial.println(sizeof(int32_t));
Serial.print("sizeof(int64_t)="); Serial.println(sizeof(int64_t));
Serial.print("sizeof(uint8_t)="); Serial.println(sizeof(uint8_t));
Serial.print("sizeof(uint16_t)="); Serial.println(sizeof(uint16_t));
Serial.print("sizeof(uint32_t)="); Serial.println(sizeof(uint32_t));
Serial.print("sizeof(uint64_t)="); Serial.println(sizeof(uint64_t));
Serial.print("sizeof(char*)="); Serial.println(sizeof(char*));
Serial.print("sizeof(int*)="); Serial.println(sizeof(int*));
Serial.print("sizeof(long*)="); Serial.println(sizeof(long*));
Serial.print("sizeof(float*)="); Serial.println(sizeof(float*));
Serial.print("sizeof(double*)="); Serial.println(sizeof(double*));
Serial.print("sizeof(void*)="); Serial.println(sizeof(void*));
Serial.print("sizeof(prog_char)="); Serial.println(sizeof(prog_char));
Serial.print("sizeof(prog_char*)="); Serial.println(sizeof(prog_char*));
Serial.println();
delay(5000);
}

And here's it's output for one of my Arduino Uno boards with the ATmega328p microcontroller.

sizeof(byte)=1

sizeof(char)=1

sizeof(short)=2

sizeof(int)=2

sizeof(long)=4

sizeof(long long)=8

sizeof(bool)=1

sizeof(boolean)=1

sizeof(float)=4

sizeof(double)=4

sizeof(int8_t)=1

sizeof(int16_t)=2

sizeof(int32_t)=4

sizeof(int64_t)=8

sizeof(uint8_t)=1

sizeof(uint16_t)=2

sizeof(uint32_t)=4

sizeof(uint64_t)=8

sizeof(char*)=2

sizeof(int*)=2

sizeof(long*)=2

sizeof(float*)=2

sizeof(double*)=2

sizeof(void*)=2

sizeof(prog_char)=1

sizeof(prog_char*)=2

Pointers are sixteen bits. Just like, in fact, on my beloved PDP-11s. Sixteen bits can address at most sixty-four kilobytes of memory, addresses 0x0000 through 0xffff. This now seems outrageously large compared to the two kilobytes of SRAM available on the AVR microcontroller on my Arduino Uno board.

Sometimes I miss Java.

Update 2012-02-06

The GCC tool chain for the AVR family has a command line switch -mint8 that can make the int data type eight bits wide; Arduino doesn't use it, but you might encounter it elsewhere in other AVR-based projects.

I strongly recommend using the types defined in stdint.h that have their widths explicitly defined, e.g. int8_t, uint32_t, etc., when such things matter (as they so often do in the embedded realm).

Update 2012-02-20

Here's the same for an Arduino Mega ADK board that uses the ATmega2560 microcontroller. Short answer: it's the same as the Uno.

sizeof(byte)=1

sizeof(char)=1

sizeof(short)=2

sizeof(int)=2

sizeof(long)=4

sizeof(long long)=8

sizeof(bool)=1

sizeof(boolean)=1

sizeof(float)=4

sizeof(double)=4

sizeof(int8_t)=1

sizeof(int16_t)=2

sizeof(int32_t)=4

sizeof(int64_t)=8

sizeof(uint8_t)=1

sizeof(uint16_t)=2

sizeof(uint32_t)=4

sizeof(uint64_t)=8

sizeof(char*)=2

sizeof(int*)=2

sizeof(long*)=2

sizeof(float*)=2

sizeof(double*)=2

sizeof(void*)=2

sizeof(prog_char)=1

sizeof(prog_char*)=2

The ATmega2560 has more input and output pins and more on-board flash (256 kilobytes instead of 32) and more SRAM (8 kilobytes instead of 2) than the ATmega328 used on the Uno. The flash and SRAM increases make the Mega a particularly interesting target for a multitasking real-time operating system (RTOS) where each task needs its own stack and context. Because of its larger program memory, the ATmega2560 requires three bytes instead of two for its program counter (PC), the hardware register that keeps track of the current instruction address.