sys/stdio_uart: add stdio_clear_stdin

[HendrikVE]

Contribution description

This PR introduces a new pseudomodule. It is currently only implemented for stdio_uart.

The use case is to clear the input buffer so we can throw away the remaining input (like \n for example) after a call of scanf for the next call of scanf.

Testing procedure

Have an application with multiple calls of scanf and use stdio_flush_rx at some places and don't use it on others.

[kaspar030]

bikeshedding: you'd "flush" output, "drain" input?

[HendrikVE]

bikeshedding: you'd "flush" output, "drain" input?

I think I'd rather go with a simple "clear" then instead, maybe "stdio_clear_input"?

[MrKevinWeiss]

Should we get this in for the release?

[riot-ci]

Murdock results

✔️ PASSED

6f9edfe sys/stdio_uart: add stdio_clear_stdin

Success	Failures	Total	Runtime
10242	0	10246	15m:08s

Artifacts

• Documentation preview

[benpicco]

Needs a rebase, you can move this to sys/stdio/stdio.c now.

[fabian18]

What about ethos? stdio.c is not being compiled when using stdio_ethos.

stdio.inc.mk

STDIO_LEGACY_MODULES = \
  ethos_stdio \
  stdio_ethos \
  stdio_native  # requires #19002 \
  #

# select stdio_uart if no other stdio module is slected
ifeq (,$(filter $(STDIO_MODULES),$(USEMODULE)))
  USEMODULE += stdio_uart
endif

ifeq (,$(filter $(STDIO_LEGACY_MODULES),$(USEMODULE)))
  USEMODULE += stdio
endif

undefined reference to 'stdio_flush_rx' collect2: error: ld returned 1 exit status

[benpicco]

True, ethos would need a custom implementation.
Alternatively it could be ported to the new interface which would require something like #18066

[maribu]

bikeshedding: you'd "flush" output, "drain" input?

I think I'd rather go with a simple "clear" then instead, maybe "stdio_clear_input"?

@HendrikVE: Ping?

[HendrikVE]

It's called stdio_clear_rx now. I tested it with stdio_uart (works) and stdio_rtt (does not work):

Not using stdio_clear_rx

2024-11-22 18:55:23,799 # main(): This is RIOT! (Version: 2025.01-devel-177-g307c2-stdio_uart_add_flush_rx)
2024-11-22 18:55:23,801 # Welcome to RIOT!
2024-11-22 18:55:23,802 # Enter a character:
h
2024-11-22 18:55:28,159 # You entered: 'h'
2024-11-22 18:55:28,161 # Enter a character:
2024-11-22 18:55:28,162 # You entered: '
2024-11-22 18:55:28,163 # '
2024-11-22 18:55:28,164 # Enter a character:
help
2024-11-22 18:55:29,583 # You entered: 'h'
2024-11-22 18:55:29,585 # Enter a character:
2024-11-22 18:55:29,587 # You entered: 'e'
2024-11-22 18:55:29,588 # Enter a character:
2024-11-22 18:55:29,590 # You entered: 'l'
2024-11-22 18:55:29,591 # Enter a character:
2024-11-22 18:55:29,593 # You entered: 'p'
2024-11-22 18:55:29,595 # Enter a character:
2024-11-22 18:55:29,596 # You entered: '
2024-11-22 18:55:29,596 # '
2024-11-22 18:55:29,598 # Enter a character:

Using stdio_clear_rx

2024-11-22 18:56:31,336 # main(): This is RIOT! (Version: 2025.01-devel-177-g307c2-stdio_uart_add_flush_rx)
2024-11-22 18:56:31,338 # Welcome to RIOT!
2024-11-22 18:56:31,339 # Enter a character:
h
2024-11-22 18:56:32,593 # You entered: 'h'
2024-11-22 18:56:32,595 # Enter a character:
help
2024-11-22 18:56:33,969 # You entered: 'h'
2024-11-22 18:56:33,971 # Enter a character:

Minimal program that I used:

USEMODULE += stdio_clear_rx

#include <stdio.h>
#include "stdio_base.h"

int main(void)
{
    char input_line[2];
    char input;

    while (true) {
        puts("Enter a character:");
        stdio_clear_rx();
        if (fgets(input_line, sizeof(input_line), stdin) != NULL) {
            if (sscanf(input_line, "%c", &input) != 1) {
                puts("Error on parsing user input!");
            }
            else {
                printf("You entered: '%c'\n", input);
            }
        }
        else {
            puts("Error on reading user input!");
        }
    }

    return 0;
}

[HendrikVE]

A tiny edit makes sure that stdio_clear_rx is useful when declared by pulling in stdin:

ifneq (,$(filter stdio_clear_rx,$(USEMODULE)))
  USEMODULE += stdin
endif

[maribu]

The PR and commit title needs an update now

[HendrikVE]

The PR and commit title needs an update now

Did so and also renamed to stdio_clear_stdin