CMSIS-Compiler splits low-level I/O retargeting into standard I/O stream related components and file stream related components.

The low-level I/O retarget decision tree is below:

Low-Level I/O Retarget Decision Tree

Low-Level I/O retargeting implementation checks whether the incoming stream is a standard stream (i.e. stderr, stdin, stdout) or a file system related stream and redirects incoming request to the appropriate software component. Chapter I/O Components explains these software components in detail.

I/O Components

CMSIS-Compiler:I/O software component can retarget the input/output functions to the following channels:

Component	Description
File	Channel for all file stream related operations.
STDERR	Standard error stream of the application to output diagnostic messages.
STDIN	Standard input stream going into the application (`scanf` etc.).
STDOUT	Standard output stream of the application (`printf` etc.).
TTY	Teletypewriter, which is the last resort for error output.

Note: TTY is only available for Arm Compiler toolchain.

Most retargeting channels have various I/O Retarget Variants.

I/O Retarget Variants

The variant selection allows you to change the target hardware interface of the I/O channel.

I/O:File Variants

I/O:File component variants

The following variants are available:

Variant	Description
File Interface	Use the File Interface API for file related operations.
Breakpoint	When the I/O channel is used, the application stops with BKPT instruction. No additional code is required.

I/O:STDERR/STDIN/STDOUT Variants

I/O:STDERR/STDIN/STDOUT component variants

The following variants are available:

Variant	Description
Breakpoint	When the I/O channel is used, the application stops with BKPT instruction. No additional code is required.
ITM	Use Instrumentation Trace Macrocell (ITM) for I/O communication via the debugger (only available for Cortex-M3/M4/M7/M33/M55/M85 processors). Usually, data is shown in a dedicated window. No additional code is required to output or input data through the ITM channel. However, you have to configure the ITM channel for tracing.
User	Retarget I/O functions to a user defined interface (such as USART or application specific). See I/O Retarget Variant "User" for further details.
EVR	STDOUT can also be redirected using the Event Recorder (especially interesting for targets without ITM (such as Cortex-M0/M0+/M23)).

Note

Depending on the selected variant, certain #defines are set in the header file RTE_Components.h that enable the appropriate code sections in the retarget interface implementation file.
Retargeting STDOUT using the EVR variant is available for all Cortex-M based devices.
The microlib of Arm Compiler C run-time library interfaces to the hardware via low-level functions. It implements a reduced set of high-level functions and therefore does not implement system I/O functions. Thus, in case of using the microlib, you cannot redefine the system I/O functions. Using any of the features of the Arm Compiler component provides the assert facility for microlib.

I/O Retarget Variant "User"

The CMSIS-Compiler:I/O component variant User provides code templates that help you to implement the functionality.

In µVision, you can import the templates into a project using the context menu Add New Item to Group from the Project window. Depending on your selection, you get templates that support retargeting via display, USART, or keyboard. Also, a template for a custom implementation is available.

Retargeting Code Templates

Input/Output via USART

You may use a CMSIS-Driver USART or write your own output routines. The following user code templates are available:

Component	Name	File Name
I/O:STDOUT	STDOUT via USART	stdout_USART.c
I/O:STDIN	STDIN via USART	stdin_USART.c
I/O:STERR	STDERR via USART	stderr_USART.c

Note: To have the full flexibility for retargeting different channels to different target hardware, each USART user code template contains a function to initialize the USART. When using the same hardware for multiple channels, the initialization functions need to be merged in one single function. Call this function during the initialization of the device's peripherals.

Input/Output via Template

Use these templates for custom interfaces that are not mentioned above. The following user code templates are available:

Component	Name	File Name
I/O:STDOUT	STDOUT User template	stdout_user.c
I/O:STDIN	STDIN User template	stdin_user.c
I/O:STERR	STDERR User template	stderr_user.c
I/O:TTY	TTY User template	tty_user.c

File Interface Components

CMSIS-Compiler:File Interface software component provides generic shim layer interface between the C library and an arbitrary file system implementation.

Standard C library functions interact with files in a same manner as with standard I/O streams and offer the same retargeting interface. The existing interface is split to enable component interchangeability and File Interface API is used to enable quick file system retargeting.

Default component is as follows:

Component	Description
Custom	Placeholder for custom File Interface implementation

Custom component is available to enable access to File Interface API header file when application provides an custom implementation. Custom implementation is typically needed when file system components do not provide its own implementation. To help you implement the functionality the File Interface:Custom component also provides the code template.

Note: File Interface component is enabled when I/O:File variant File Interface is used.