posix: c_lib_ext: implement getsubopt()

The getsubopt() function is required as part of the
POSIX_C_LIB_EXT option group.

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

Author: cfriedt

lib/posix/c_lib_ext/CMakeLists.txt

@@ -12,6 +12,7 @@ zephyr_library_sources(
   getentropy.c
   getopt/getopt.c
   getopt/getopt_common.c
+  getopt/getsubopt.c
 )
 
 zephyr_library_sources_ifdef(CONFIG_GETOPT_LONG getopt/getopt_long.c)

lib/posix/c_lib_ext/getopt/getsubopt.c

@@ -0,0 +1,87 @@
+/*
+ * Copyright (c) 2025 Tenstorrent AI ULC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <zephyr/logging/log.h>
+
+LOG_MODULE_DECLARE(getopt, CONFIG_GETOPT_LOG_LEVEL);
+
+int getsubopt(char **optionp, char *const *keylist, char **valuep)
+{
+	size_t i, len;
+	char *begin, *end, *eq, *key, *val;
+
+	if (optionp == NULL || keylist == NULL) {
+		LOG_DBG("optionp or keylist is NULL");
+		return -1;
+	}
+
+	if (valuep != NULL) {
+		*valuep = NULL;
+	}
+
+	for (i = 0, begin = *optionp, end = begin, eq = NULL, len = strlen(*optionp); i <= len;
+	     ++i, ++end) {
+
+		char c = *end;
+
+		if ((c == '=') && (eq == NULL)) {
+			eq = end;
+		} else if (c == ',') {
+			*end = '\0';
+			*optionp = end + 1;
+			break;
+		} else if (c == '\0') {
+			*optionp = end;
+			break;
+		}
+	}
+
+	key = begin;
+	if (eq == NULL) {
+		len = end - begin;
+		if (valuep != NULL) {
+			*valuep = NULL;
+		}
+		LOG_DBG("key: %.*s", (int)len, key);
+	} else {
+		len = eq - begin;
+		val = eq + 1;
+		if (valuep != NULL) {
+			*valuep = val;
+		}
+		LOG_DBG("key: %.*s val: %.*s", (int)len, key, (int)(end - val), val);
+	}
+
+	if (len == 0) {
+		LOG_DBG("zero-length key");
+		return -1;
+	}
+
+	for (i = 0; true; ++i) {
+		if (keylist[i] == NULL) {
+			LOG_DBG("end of keylist of length %zu reached", i);
+			break;
+		}
+
+		if (strncmp(key, keylist[i], len) != 0) {
+			continue;
+		}
+
+		if (keylist[i][len] != '\0') {
+			LOG_DBG("key %.*s does not match keylist[%zu]: %s", (int)len, key, i,
+				keylist[i]);
+			continue;
+		}
+
+		return (int)i;
+	}
+
+	return -1;
+}