Minimal example

Serial serial(USBRX, USBTX, 19200);
I2C i2c(I2C_SDA, I2C_SCL);

const uint8_t slaveAddr = 0x42;

void read(void const *args) {
    I2C* i2c = (I2C*)args;
    char buf[128];
    Thread::wait(100);

    const char I2C_ADDR = slaveAddr << 1; // Address of I2C slave to read
    const char COUNT_REG[] = { 0xFD };   // Count register address - how many bytes are available to read
    const char DATA_REG[] = { 0xFF };    // Data register address - the data stream

    // Set read register to COUNT_REG to open I2C stream
    bool success = !i2c->write(I2C_ADDR, COUNT_REG, sizeof(COUNT_REG));

    while (success) {
        success = !i2c->write(I2C_ADDR, COUNT_REG, sizeof(COUNT_REG), true) && 
              !i2c->read(I2C_ADDR, buf, 2);

        if (!success) {
            serial.printf("Write/read failed...\r\n");
			Thread::wait(500);
			NVIC_SystemReset();
        }

        int bytesAvailable = (unsigned short)(buf[0] << 8 | buf[1]);

        if (bytesAvailable) {
            serial.printf("\r\nbytesAvailable  %i\r\n", bytesAvailable);

            while (bytesAvailable && success) {
                int bytesToRead = (bytesAvailable < (unsigned) sizeof(buf)) ? bytesAvailable : sizeof(buf);

                success = !i2c->write(I2C_ADDR, DATA_REG, sizeof(DATA_REG), true) && 
                        !i2c->read(I2C_ADDR, buf, bytesToRead);

                if (success) {
                    for (int i = 0; i < bytesToRead; i++) {
                        serial.printf("%02X", buf[i]);
                    }

                    bytesAvailable -= bytesToRead;
                } else {
                    serial.printf("Couldn't read even though bytes available = %d\r\n", bytesAvailable);
                }
            }

            serial.printf("\r\n");
        }
        Thread::wait(100);
    }
}

int main() {
	Thread readThread;
    readThread.start(callback(read, (void *)&i2c));

    while (1)
    {
        led = !led;
        Thread::wait(500);
    }

    return 0;
}