Calling frequently linux function write() to log to file causes libmodbus timeout for modbus_read_registers()

Consider following code:

int ModbusDevice::modbusReceiveData( void )
{
    if( mb == nullptr )
        return -1;

    int regCnt = 0;
    for( ModbusResourceObjectList::iterator obj = mbMeasSyncDataSrc.resObjList.begin();
         obj != mbMeasSyncDataSrc.resObjList.end(); ++obj )
    {
        uint16_t reg[obj->regSize];
        int ret = modbus_read_registers( mb, obj->mbPtr, obj->regSize, reg );

        if( ret != obj->regSize || ret == -1 )
        {
            Kernel::Instance().message("<MB DEVICE: '%s'> Error reading measurement '%s' at register addres %d.\n\tError: %s\n",
                Kernel::MSG_TYPE::ERROR, Kernel::MSG_LEVEL::DEFAULT, mbDevName.c_str(), obj->ref.c_str(), obj->mbPtr, modbus_strerror(errno) );
            continue;
        }

        obj->mbVal = 0;
        for( int r = 0; r < obj->regSize; r++ )
            obj->mbVal |= reg[r]<<(r*16);

        Kernel::Instance().message("<MB DEVICE: '%s'> Successfully read measurement '%s' at register addres %d = %d.\n",
            Kernel::MSG_TYPE::DATA, Kernel::MSG_LEVEL::VERBOSE /*| Kernel::MSG_LEVEL::DONT_LOG_TO_FILE*/, mbDevName.c_str(),
            obj->ref.c_str(), obj->mbPtr, obj->mbVal );
        regCnt += obj->regSize;
    }

    return regCnt;
}

as you can see flag Kernel::MSG_LEVEL::DONT_LOG_TO_FILE is comment out in that example and everything is working fine. When we uncomment the flag method Kernel::message() logs everything to opened file using linux write() function, which causes function modbus_read_registers() to timeout always even if timeout are set to high values like 10 seconds.

Again commenting out write() or uncommenting Kernel::MSG_LEVEL::DONT_LOG_TO_FILE make libmodbus responsive again.

As I understand libmodbus uses write() to send data (??) so I assume that write() function does not finish writing to file descriptor and modbus_read_registers() is timedout?

stephane / libmodbus

Calling frequently linux function write() to log to file causes libmodbus timeout for modbus_read_registers() #272