toCharArray not copying all characters - loses last char

Troubleshooting
1

Hi I’m new to this, so could be me being dense, but String.toCharArray seems to only copy length -1 characters to a char buffer or in other words it chops the last character. I’ll just post up the interesting bits of the code:

char readEquip[20] = "";

  Particle.variable("readEquip", readEquip);

//------------------------------------------------------------
int readSerial()
{
    String msg = "Hello";
    msg.toCharArray(readEquip, msg.length());
    return msg.length();
}

Obviously the readSerial() function is a dummy one, but it returns the correct length of the string (5) but when I read the char array readEquip, it returns “Hell” which is fairly apt :smile: .

Am I being a dummy? I’ve only had 2 cups of coffee today.

Cheers

Steve

2

The docs for String.toCharArray() state that the len parameter represents the size of the target buffer and not the string length.

You must know that a C string has to be terminated with a zero character ('\0') which needs to fit into the buffer too.

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3

Hi @picitup

I see that yet again @ScruffR beat me to it!

With a char array like your readEquip, you always need to “leave room” for the terminating zero character at the end of the string. That is just how C-style strings work.

As to copying using the String.toCharArray method, it takes a maximum length so the correct thing to plug in there is the length of the readEquip array - 1.

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4

@bko, actually you could put it like this msg.toCharArray(readEquip, sizeof(readEquip)); since toCharArray() already makes sure to fit the zero-terminator in too.
It’s not the max string length but the buffer size you pass as len parameter.

1 Like
5

Building on @ScruffR, If you want to return the length of the character portion of the C string (not the size of the buffer) you can try sort of like this:

char readEquip[20] = "";

  Particle.variable("readEquip", readEquip);

//------------------------------------------------------------
int readSerial()
{
    String msg = "Hello";
    msg.toCharArray(readEquip, sizeof(readEquip));
    int i = 0;
    while(readEquip[i])
    {
      i++;
    }
    return i;
}
6

Or just use strlen(readEquip) for the size.

2 Likes
7

Hi All thanks for your comments and code suggestions which were very helpful. I had mis-read the docs and assumed the len was the length of the string to copy, not the max buff size.

Before I post up again I’ll have that extra coffee!

Kind Regards

Steve

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