sys/string_utils: add strscpy()

sys/Kconfig

@@ -37,6 +37,7 @@ rsource "fs/Kconfig"
 rsource "hashes/Kconfig"
 rsource "iolist/Kconfig"
 rsource "isrpipe/Kconfig"
+rsource "libc/Kconfig"
 
 menu "Libc"
 

sys/include/string_utils.h

@@ -22,11 +22,13 @@
  * @author      Marian Buschsieweke <[email protected]>
  */
 
+#include <errno.h>
+#include <stdint.h>
 /* if explicit_bzero() is provided by standard C lib, it may be defined in
  * either `string.h` or `strings.h`, so just include both here */
-#include <stdint.h>
 #include <string.h>
 #include <strings.h>
+#include <sys/types.h>
 
 #include "kernel_defines.h"
 
@@ -70,6 +72,26 @@ static inline void explicit_bzero(void *dest, size_t n_bytes)
 }
 #endif
 
+/**
+ * @brief Copy the string, or as much of it as fits, into the dest buffer.
+ *
+ * Preferred to `strncpy` since it always returns a valid string, and doesn't
+ * unnecessarily force the tail of the destination buffer to be zeroed.
+ * If the zeroing is desired, it's likely cleaner to use `strscpy` with an
+ * overflow test, then just memset the tail of the dest buffer.
+ *
+ * @param[out] dest     Where to copy the string to
+ * @param[in]   src     Where to copy the string from
+ * @param[in] count     Size of destination buffer
+ *
+ * @pre       The destination buffer is at least one byte large, as
+ *            otherwise the terminating zero byte won't fit
+ *
+ * @return the number of characters copied (not including the trailing zero)
+ * @retval  -E2BIG      the destination buffer wasn't big enough
+ */
+ssize_t strscpy(char *dest, const char *src, size_t count);
+
 #ifdef __cplusplus
 }
 #endif

sys/libc/Kconfig

@@ -0,0 +1,11 @@
+# Copyright (c) 2021 HAW Hamburg
+#
+# This file is subject to the terms and conditions of the GNU Lesser
+# General Public License v2.1. See the file LICENSE in the top level
+# directory for more details.
+#
+
+config MODULE_LIBC
+    bool "C Library helper functions"
+    default y
+    depends on TEST_KCONFIG

sys/libc/Makefile

@@ -0,0 +1 @@
+include $(RIOTBASE)/Makefile.base

sys/libc/string.c

@@ -0,0 +1,40 @@
+/*
+ * Copyright (C) 2022 ML!PA Consulting GmbH
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @{
+ *
+ * @file
+ *
+ * @author      Benjamin Valentin <[email protected]>
+ */
+
+#include <errno.h>
+#include "string_utils.h"
+
+ssize_t strscpy(char *dest, const char *src, size_t count)
+{
+    const char *start = dest;
+
+    if (!count) {
+        return -E2BIG;
+    }
+
+    while (--count && *src) {
+        *dest++ = *src++;
+    }
+
+    *dest = 0;
+
+    if (*src == 0) {
+        return dest - start;
+    } else {
+        return -E2BIG;
+    }
+}
+/** @} */