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it's all in the title ; I'm developping a library and use *TwoWire pointers in this library ; but I am using an external library that calls Wire.begin(). I've seen something about TWCR register in arduino but I don't know what it is exaclty.

Thanks in advance :)

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What you are interested in is this bit of the Wire library:

TWCR = _BV(TWEN) | _BV(TWIE) | _BV(TWEA);

The important bit is the TWEN bit - TwoWire ENable.

If that bit is set in the TWCR register than Wire.begin() has been called already and you don't need to call it again:

if (TWCR & _BV(TWEN) == 0) {
    Wire.begin();
}

However, it really doesn't matter if you call it multiple times.

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    other thing is if you do setFrequency(400000L) and then some other library calls Wire.begin()
    – Juraj ♦
    Commented Jun 16, 2018 at 13:49
  • Just to precise, it was to avoid such problems github.com/esp8266/Arduino/issues/2607 I don't know if it concerns me but I'll see with the prototype
    – Storca
    Commented Jun 20, 2018 at 10:02
  • @Storca Note that that's for a completely different architecture, so the "fix" I detail above has no relevance to it. The proper place for it to be fixed would be in the Wire library. We do it properly chipKIT: github.com/chipKIT32/chipKIT-core/blob/master/pic32/libraries/… - only allow begin to run the first time it's called.
    – Majenko
    Commented Jun 20, 2018 at 10:06

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