Auto padding when partially declaring a char array

Forum 2005-2010 (read only) Software Syntax & Programs
1

I was wondering if the following would auto-pad with NULL characters up to the full 64 bytes?

char testArray[64] = "NotAFull64Bytes";

Thanks in advance for any help.

2

Easy to test?

Just Serial.println(testArray[20]), I'd say
I'd be interested in the result of that aswell.

3

No need to test. The array is auto-padded.

4

I would qualify that by saying that the array is zero-padded when it is global or static. I don't know that it is padded when it is automatic.

-Mike

5

Good point. I don't know either.

6

@Mike Murdock: I'm not sure I fully understand what you mean by that. Can you please elaborate?

Thanks heaps.

7

An automatic variable is a variable placed on the stack (as opposed to a variable placed in the data area). It is automatically created and destroyed as needed.

For example...

char StaticArray[10];  // <-- this is static because it is not inside a function

void setup( void )
{
  char AutomaticArray[12];  // <-- this is automatic because it is inside a function
}

void loop( void )
{
  static char AnotherStatic[4];  // <-- this one is static because we declared it static
}

Does that help?

8

@Coding Badly: Excellent. Thanks for that explanation.

So, going by that, the below example would remain with the null padding?

char testArray[64];

void setup() {
  testArray[64] = "NotAFull64Bytes";
}

What if we then did this:

char testArray[64];

void setup() {
  testArray[64] = "NotAFull64Bytes";
  testArray[64] = "EvenLess";
}

Would the value then be "EvenLess\0\0\0....", or would it be "EvenLess64Bytes\0\0\0..."?

9

Thanks for that explanation.

You are welcome.

This is filled with nulls before setup is called...

char testArray[64];

This assignment will not work the way you expect...

void setup() {
  testArray[64] = "NotAFull64Bytes";
}

testArray[64] references a single character past the end of your array. To save "NotAFull64Bytes" into your array of characters you have to do something like this...

  strcpy( testArray, "NotAFull64Bytes" );

After doing the strcpy, the extra elements of testArray passed the 's' will still have nulls.

In this example...

void setup() {
  strcpy( testArray, "NotAFull64Bytes" );
  strcpy( testArray, "EvenLess" );
}

After setup is called, testArray contains the following (where ~ marks the nulls)...

EvenLess~4Bytes~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It's important to remember that there is no native string type in Arduino. "Strings" are really an array of characters.