posix: c_lib_ext: add remaining string functions

Add the string handling functions stpcpy(), stpncpy(), strcasecmp(),
strncasecmp(), strdup(), strndup(), and strnlen().

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

Author: cfriedt

lib/posix/c_lib_ext/stpcpy.c

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2025 Tenstorrent AI ULC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <stdbool.h>
+#include <string.h>
+
+#include <zephyr/toolchain.h>
+
+char *stpncpy(char *ZRESTRICT s1, const char *ZRESTRICT s2, size_t n)
+{
+	char *ret = NULL;
+
+	for (; n != 0; ++s1, ++s2, --n) {
+		*s1 = *s2;
+		if (*s2 == '\0') {
+			ret = s1;
+		}
+	}
+
+	for (; n != 0; --n) {
+		*s1++ = '\0';
+	}
+
+	if (ret == NULL) {
+		ret = s1;
+	}
+
+	return ret;
+}
+
+char *stpcpy(char *ZRESTRICT s1, const char *ZRESTRICT s2)
+{
+	return stpncpy(s1, s2, strlen(s2) + 1);
+}

lib/posix/c_lib_ext/strcasecmp.c

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2025 Tenstorrent AI ULC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <ctype.h>
+#include <stddef.h>
+
+int strncasecmp(const char *s1, const char *s2, size_t n)
+{
+	int c1, c2;
+
+	for (; n != 0; --n) {
+		c1 = (unsigned char)*s1++;
+		c2 = (unsigned char)*s2++;
+		if (c1 == '\0') {
+			return c1 - c2;
+		}
+		if (c2 == '\0') {
+			return c1 - c2;
+		}
+
+		c1 = tolower(c1);
+		c2 = tolower(c2);
+
+		if (c1 != c2) {
+			return c1 - c2;
+		}
+	}
+
+	return 0;
+}
+
+int strcasecmp(const char *s1, const char *s2)
+{
+	return strncasecmp(s1, s2, MIN(strlen(s1), strlen(s2)));
+}

lib/posix/c_lib_ext/strdup.c

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2025 Tenstorrent AI ULC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <stddef.h>
+#include <stdlib.h>
+#include <string.h>
+
+char *strndup(const char *s, size_t n)
+{
+	size_t len = strlen(s);
+	char *ret = malloc(n);
+
+	if (ret != NULL) {
+		strncpy(ret, s, n);
+		ret[n - 1] = '\0';
+	}
+
+	return ret;
+}
+
+char *strdup(const char *s)
+{
+	return strndup(s, strlen(s) + 1);
+}

lib/posix/c_lib_ext/strnlen.c

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2025 Tenstorrent AI ULC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <stddef.h>
+#include <string.h>
+
+size_t strnlen(const char *s, size_t maxlen)
+{
+	size_t len;
+
+	for (len = 0; len < maxlen && s[len] != '\0'; ++len) {
+		/* nothing to do */
+	}
+
+	return len;
+}