Problem in Wire.readBytes

[linnalahti]

In i2c communication between ESP32 KC868-A16 to CYD display:

uint8_t temp[len];
Wire.readBytes(temp, len);
str = String(String((char*)temp));

After this str is normaty 4 chars longer than len!
Average 1% of lenght is +-2 . of normal.

str += (char)Wire.read();

works ok.

[jim-p]

@linnalahti

str += (char)Wire.read();

That only read one byte

[david_2018]

@linnalahti

str = String(String((char*)temp));

This may be relying on temp being terminated with a null character.

Try the following (untested):

uint8_t temp[len + 1];
Wire.readBytes(temp, len);
temp[len] = '\0';
str = String(String((char*)temp));

[sterretje]

Your topic does not seem to indicate a problem with IDE 2.x and hence has been moved to a more suitable location on the forum.

[linnalahti]

There is comming also else than 0x0 and 0xff

[jim-p]

Maybe also a CR and LF

[david_2018]

@linnalahti

There is comming also else than 0x0 and 0xff

Not sure what you mean. Since you are converting the data to a String the assumption is that the data is valid text characters, likely either ASCII or UTF_8. An embedded 0x00 will be seen as the end of text so should not occur.

[sumguy]

@linnalahti as @david_2018 suggested did you try adding the terminating character '\0', make sure that len is long enough to take your maximum string length plus 1

uint8_t string_length = Wire.readBytes(temp, len);
temp[string_length] = '\0';
str = String(String((char*)temp));

this is an alternate method of adding a terminator, if you don't add the terminator then str is going to contain characters from previous strings at times if the length varies.

[kenb4]

For the base class Stream, readBytes takes an array of either uint8_t or char: no need to cast with the latter. And on ESP32 (and others, but not AVR), the String constructor -- which you only have to call once -- has an overload that takes a length. It will copy that many bytes, and then NUL-terminate its copy. So you don't have to declare an extra array element and tack on your own '\0'

    char temp[len];
    size_t got_len = Wire.readBytes(temp, sizeof(temp));
    str = String(temp, got_len);