Question

Please bear with me, I’m new at C and I’m trying to program an Arduino. I want to write a program that spits out a data frame of specific length with byte values ranging from 0-255. The minimum code to reproduce the error is in the code block below. When compiling I receive the following error:

sketch_apr09b.cpp: In function ‘char assembleFrame()’:
sketch_apr09b.cpp:9:10: error: invalid conversion from ‘char*’ to ‘char’

Now my impression is that I’m mistreating the ‘return frame’, but I just can’t figure out what’s wrong.

char assembleFrame() {
  char frame[] = { 
    0x61 , 0x62 , 0x63
  };
  return frame;
}

void setup() {
  Serial.begin( 115200 );
};

void loop() {
  char frame = assembleFrame();
  Serial.print( frame );
}

When I run a hexdump on the receiving PC, I want to see:

00000000  61 62 63                                          |abc|
00000003

I’ve found a lot of similar questions, wasn’t able to figure out what I’m doing wrong.

EDIT:
This is what I came up with so far, but receiving wrong data. I think I’m sending the pointer to the actual data with this.

byte *assembleFrame() {
  byte frame[] = { 4 , 'a' , 'b' , 'c' };
  return frame;
}

void setup() {
  Serial.begin( 115200 );
};

void loop() {
  byte *frame = assembleFrame();
  Serial.write( frame , frame[ 0 ] );
}

Answer 1

The char type is used to store a single char (a byte).

Your function definition char assembleFrame specifies the function will only be returning a single char, so when you try to return a char[] / char * (char array/pointer), it fails.

Looks like Serial.print() can handle a char *, which will potentially need to be null-terminated (because it asks for no length specifier).

char *assembleFrame() {
  char frame[] = {
    0x61, 0x62, 0x63, 0x00 // null byte to signify end of string
  };
  return frame;
}

void setup() {
  Serial.begin( 115200 );
};

void loop() {
  char *frame = assembleFrame();
  Serial.print( frame );
}