tests: posix: common: add a test for confstr()

Add a test for confstr(), which is required by the
POSIX_SINGLE_PROCESS Option Group, as per IEEE
1003.1-2017.

Signed-off-by: Christopher Friedt <[email protected]>

Author: cfriedt

tests/posix/common/CMakeLists.txt

@@ -8,3 +8,5 @@ FILE(GLOB app_sources src/*.c)
 zephyr_include_directories(${ZEPHYR_BASE}/lib/posix)
 
 target_sources(app PRIVATE ${app_sources})
+
+target_compile_options(app PRIVATE -U_POSIX_C_SOURCE -D_POSIX_C_SOURCE=200809L)

tests/posix/common/src/confstr.c

@@ -0,0 +1,67 @@
+/*
+ * Copyright (c) 2024, Meta
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <errno.h>
+#include <unistd.h>
+
+#include <zephyr/ztest.h>
+#include <zephyr/sys/util.h>
+
+ZTEST(confstr, test_confstr)
+{
+	char buf[1];
+
+	/* degenerate cases */
+	{
+		struct arg {
+			int name;
+			char *buf;
+			size_t len;
+		};
+
+		const struct arg arg1s[] = {
+			{-1, NULL, 0},
+			{-1, NULL, sizeof(buf)},
+			{-1, buf, 0},
+			{-1, buf, sizeof(buf)},
+		};
+
+		const struct arg arg2s[] = {
+			{_CS_PATH, NULL, 0},
+			{_CS_PATH, buf, 0},
+		};
+
+		const struct arg arg3s[] = {
+			{_CS_PATH, NULL, sizeof(buf)},
+		};
+
+		ARRAY_FOR_EACH_PTR(arg1s, arg) {
+			errno = 0;
+			zassert_equal(0, confstr(arg->name, arg->buf, arg->len));
+			zassert_equal(errno, EINVAL);
+		}
+
+		ARRAY_FOR_EACH_PTR(arg2s, arg) {
+			errno = 0;
+			buf[0] = 0xff;
+			zassert_true(confstr(arg->name, arg->buf, arg->len) > 0);
+			zassert_equal(errno, 0);
+			zassert_equal((uint8_t)buf[0], 0xff);
+		}
+
+		ARRAY_FOR_EACH_PTR(arg3s, arg) {
+			errno = 0;
+			zassert_true(confstr(arg->name, arg->buf, arg->len) > 0);
+			zassert_equal(errno, 0);
+		}
+	}
+
+	buf[0] = 0xff;
+	zassert_true(confstr(_CS_PATH, buf, sizeof(buf) > 0));
+	zassert_equal(buf[0], '\0');
+}
+
+ZTEST_SUITE(confstr, NULL, NULL, NULL, NULL, NULL);