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Input/Output Streams

lbuild module: modm:io

The modm::IOStream class contains efficient formatting that supports both C++ std::basic_ostream-like formatting via operator << as well as implementing printf via the modm:printf module.

modm::IOStream stream(device);
stream << "format number 8: " << uint8_t(8) << " or as signed -100: " << int8_t(-100);
stream << modm::endl;

stream.printf("format number 8: %u or as signed -100: %d", 8, -100);

AVR supported features

All expensive features including printf are disabled by default to reduce firmware size! Please check the options.

modm::endl does NOT implicitly flush!

Flushing is extremely expensive on embedded systems, therefore modm::endl does not implicitly flush the stream. Please call modm::flush explicitly.

Redirecting IOStreams

The modm::IODeviceWrapper transforms any peripheral device that provides static write() and read() functions into an IODevice.

You have to decide what happens when the device buffer is full and you cannot write to it at the moment. There are two options:

  1. busy wait until the buffer is free, or
  2. discard the bytes that cannot be written.

Option 1 has the advantage, that none of your data will be lost, however, busy-waiting can take a long time and can mess up your program timings. There is also a high risk of deadlock, when writing to a IODevice inside of an interrupt and then busy-waiting forever because the IODevice requires interrupts itself to send out the data.

It is therefore highly recommended to use option 2, where surplus data will be discarded. You should increase the IODevice buffer size, if you experience missing data from your connection. This behavior is also deadlock safe when called from inside another interrupt, and your program timing is minimally affected (essentially only coping data into the buffer).

There is no default template argument, so that you hopefully make a conscious decision and be aware of this behavior.


// configure a UART
using Uart = Uart0;

// wrap it into an IODevice
modm::IODeviceWrapper<Uart, modm::IOBuffer::DiscardIfFull> device;

// use this device to print a message

// or create a IOStream and use the stream to print something
modm::IOStream stream(device);
stream << " World!";



Support for floating point formatting

On AVRs floating point values can be printed, however, the formatting cannot be specified and all values are printed as scientific-notation exponential floating point

Default: no avr
Default: yes hosted, rp, sam, stm32
Inputs: [yes, no]


Support for 64-bit integer formatting

Default: no avr
Default: yes hosted, rp, sam, stm32
Inputs: [yes, no]
Input Dependency: yes -> modm:printf


Support for printf-style formatting

Default: no avr
Default: yes hosted, rp, sam, stm32
Inputs: [yes, no]
Input Dependency: yes -> modm:printf


modm:io modm_io modm: io modm_architecture_accessor modm: architecture: accessor modm_io->modm_architecture_accessor modm_math_utils modm: math: utils modm_io->modm_math_utils modm_printf modm: printf modm_io->modm_printf