Send a 2d PROGMEM array over SoftwareSerial

Question

I have a 2d array:

const byte messages_for_measurement[2][8] PROGMEM = 
{
  { 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 },
  { 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F, 0x10 },
};

I want to send one of these sub-array over software serial.

Initially, this produced junk data:

swSerial.write( messages_for_measurement[0], sizeof(messages_for_energy_meter[0]) );

Then I learned about pgm_read_byte_near() and pgm_read_word(). I tried every combination but I did not get this to work:

swSerial.write(pgm_read_byte_near(pgm_read_word(messages_for_measurement[0]))), sizeof(pgm_read_byte_near(pgm_read_word(messages_for_measurement[0])));

and

swSerial.write(pgm_read_byte_near(&pgm_read_word(messages_for_measurement[0]))), sizeof(pgm_read_byte_near(pgm_read_word(&messages_for_measurement[0])));

and

swSerial.write(char(pgm_read_byte_near(pgm_read_word(messages_for_measurement[0])))), sizeof(char(pgm_read_byte_near(pgm_read_word(messages_for_measurement[0]))));

and

swSerial.write(char(pgm_read_byte_near(pgm_read_word(&messages_for_measurement[0])))), sizeof(char(pgm_read_byte_near(pgm_read_word(&messages_for_measurement[0]))));

How can I send the sub-array over serial, when the 2d array is in PROGMEM?

Answer 1

The write() method from SoftwareSerial (which is inherited from the abstract Print class) does not support printing PROGMEM-based binary buffers. The print() method does have some PROGMEM support, but it is limited to printing NUL-terminated character strings, and is meant to be used with the F() macro. You could coerce this method into printing your data, but this would require you to add a null byte at the end of each inner array, and the trick would break if you ever have to send a null byte.

Also, the pgm_read_stuff() macros cannot read an array at a time. You could do achieve this with memcpy_P(), but that would require allocating an array in RAM. I think the simplest solution here is to get the bytes from the flash one by one with pgm_read_byte():

const byte *message = messages_for_measurement[0];
size_t size = sizeof messages_for_measurement[0];
for (size_t i = 0; i < size; i++) {
    swSerial.write(pgm_read_byte(&message[i]));
}

Note that the first line does not attempt to access the array data: it only copies into message the address of the first byte: this is the decay-to-pointer semantics.