libc: minimal: stdin: Add getc() implementation and unit tests #93062

seyoungjeong · 2025-07-12T21:01:27Z

Implement getc() in lib/libc/minimal/source/stdin/stdin_console.c.
Add unit tests for getc(), fgetc(), fgets(), and getchar() in tests/lib/sscanf using a mock stdin hook.
Update headers and CMakeLists for new input support.

Resolves basic input API coverage for issue #66943.

- Implement getc() in lib/libc/minimal/source/stdin/stdin_console.c. - Add unit tests for getc(), fgetc(), fgets(), and getchar() in tests/lib/sscanf using a mock stdin hook. - Update headers and CMakeLists for new input support. Resolves basic input API coverage for issue zephyrproject-rtos#66943. Signed-off-by: Seyoung Jeong <seyoungjeong@gmail.com>

sonarqubecloud · 2025-07-12T21:05:45Z

Quality Gate passed

Issues
6 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

cfriedt

Minor changes, but otherwise pretty good.

@stephanosio might have a better suggestion for location and name of the test suite and maybe the file names of the implementation.

cfriedt · 2025-07-13T12:24:24Z

tests/lib/sscanf/src/main.c

+	zassert_true(strcmp(buf, "World") == 0, "fgets did not read 'World'");
+}
+
+ZTEST_SUITE(sscanf, NULL, NULL, NULL, NULL, NULL);


sscanf seems like it isn't a particularly descriptive name for a testsuite. I would maybe consider calling this standard_input or something.

Additionally, pass in the "before" function to be called automatically by the test suite like so.

Suggested change

ZTEST_SUITE(sscanf, NULL, NULL, NULL, NULL, NULL);

ZTEST_SUITE(sscanf, NULL, NULL, before, NULL, NULL);

The one thing I would consider potentially is also creating a way to back up the previous stdin hook if one does not exist, and to restore it in a "after" callback.

cfriedt · 2025-07-13T12:30:06Z

tests/lib/sscanf/src/main.c

+	int c = getchar();
+	zassert_equal(c, 'H', "getchar() did not return 'H'");
+	c = getchar();
+	zassert_equal(c, 'e', "getchar() did not return 'e'");


Why not add a loop and check the entirety of the input?

cfriedt · 2025-07-13T12:39:18Z

tests/lib/sscanf/src/main.c

+
+static int mock_stdin_hook(void)
+{
+	if (test_input[input_pos] == '\0') {


With the suggested change, EOF is returned only after there is no input, which is maybe a bit more accurate.

It also allows you to test how the implementation reacts to processing \0 (which should be possible, afaik, since standard input or any other file can contain 0x00).

Maybe even put some additional text after the \0.

Suggested change

if (test_input[input_pos] == '\0') {

if (input_pos == ARRAY_SIZE(test_input)) {

cfriedt · 2025-07-13T12:40:40Z

tests/lib/sscanf/src/main.c

+void setup_stdin_hook(void)
+{


This can be a "before" callback provided to ZTEST_SUITE() and then it does not need to be called manually in each test.

Suggested change

void setup_stdin_hook(void)

{

static void before(void * arg)

{

ARG_UNUSED(arg);

cfriedt · 2025-07-13T12:41:25Z

tests/lib/sscanf/src/main.c

+	int c = fgetc(stdin);
+	zassert_equal(c, 'H', "fgetc(stdin) did not return 'H'");
+	c = fgetc(stdin);
+	zassert_equal(c, 'e', "fgetc(stdin) did not return 'e'");


Why not add a loop and check the entirety of the input?

cfriedt · 2025-07-13T12:41:36Z

tests/lib/sscanf/src/main.c

+	int c = getc(stdin);
+	zassert_equal(c, 'H', "getc(stdin) did not return 'H'");
+	c = getc(stdin);
+	zassert_equal(c, 'e', "getc(stdin) did not return 'e'");


Why not add a loop and check the entirety of the input?

cfriedt · 2025-07-13T12:43:00Z

lib/libc/minimal/source/stdin/stdin_console.c

+	return zephyr_fread(ptr, size, nitems, stream);
+}
+
+char *gets(char *s)


This should probably be moved to a separate file.

cfriedt · 2025-07-13T12:43:32Z

lib/libc/minimal/source/stdin/stdin_console.c

+int fgetc(FILE *stream)
+{
+	return zephyr_fgetc(stream);
+}
+
+char *fgets(char *s, int size, FILE *stream)
+{
+	if (s == NULL || size <= 0) {
+		return NULL;
+	}
+
+	int i = 0;
+	int c;
+
+	while (i < size - 1) {
+		c = fgetc(stream);
+		if (c == EOF) {
+			if (i == 0) {
+				return NULL;
+			}
+			break;
+		}
+		s[i++] = (char)c;
+		if (c == '\n') {
+			break;
+		}
+	}
+	s[i] = '\0';
+	return s;
+}
+
+#undef getc
+int getc(FILE *stream)
+{
+	return zephyr_fgetc(stream);
+}
+
+#undef getchar
+int getchar(void)
+{
+	return zephyr_fgetc(stdin);
+}


These should probably be moved to separate files

andyross

Not a -1, but just because someone needs to be That Guy who asks the question:

Why do we want this, exactly? Minimal libc is there for embedded code that wants bare-minimum functionality with no extra dependencies. Does studio really qualify? What's the use case for an app that wants to port code using fgetc() but can't just enable picolibc?

seyoungjeong · 2025-07-13T17:12:55Z

Not a -1, but just because someone needs to be That Guy who asks the question:

Why do we want this, exactly? Minimal libc is there for embedded code that wants bare-minimum functionality with no extra dependencies. Does studio really qualify? What's the use case for an app that wants to port code using fgetc() but can't just enable picolibc?

Guess one common use case would be to get input from UART console from boot loader or diagnostics firmware? Can it be done by enabling picolibc? Bear with me - I'm new to Zephyr.

aescolar · 2025-07-14T07:19:53Z

@seyoungjeong picolibc is the default libc and has a smaller footprint in general than this minimal C library. Minimal libc is "minimal" in the sense of "minimal functionality".

stephanosio

From the "Minimal libc" documentation:

The minimal libc implements the minimal subset of the ISO/IEC 9899:2011 standard C library functions required to meet the needs of the Zephyr kernel, as defined by the Coding Guidelines Rule A.4.

None of the functions implemented in this PR are required by the Zephyr kernel and it is not foreseeable that they will be required in the future.

As others have already pointed out, the "minimal libc" is supposed to be minimal and Picolibc is intended to serve as the default and fully featured C library for non-"kernel" Zephyr components and applications (see the Coding Guidelines Rule A.4 for the definition of "kernel").

I am blocking this PR on the grounds that:

Picolibc already supports the C library functions implemented in this PR.
Zephyr kernel, as defined by the Rule A.4, does not and should not require these functions.

seyoungjeong marked this pull request as ready for review July 13, 2025 04:37

github-actions bot added area: C Library C Standard Library area: Base OS Base OS Library (lib/os) labels Jul 13, 2025

github-actions bot requested review from aescolar, andyross, cfriedt, dcpleung, keith-packard, nashif, peter-mitsis and stephanosio July 13, 2025 04:38

github-actions bot assigned stephanosio Jul 13, 2025

cfriedt requested changes Jul 13, 2025

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andyross reviewed Jul 13, 2025

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stephanosio requested changes Jul 15, 2025

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stephanosio mentioned this pull request Jul 15, 2025

libc: implement getc() #66943

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libc: minimal: stdin: Add getc() implementation and unit tests #93062

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	ZTEST_SUITE(sscanf, NULL, NULL, NULL, NULL, NULL);
	ZTEST_SUITE(sscanf, NULL, NULL, before, NULL, NULL);

	if (test_input[input_pos] == '\0') {
	if (input_pos == ARRAY_SIZE(test_input)) {

-void setup_stdin_hook(void)
-{
+static void before(void * arg)
+{
+    ARG_UNUSED(arg);

libc: minimal: stdin: Add getc() implementation and unit tests #93062

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