misc: Implement grub_strlcpy()

grub_strlcpy() acts the same way as strlcpy() does on most *NIX,
returning the length of src and ensuring dest is always NUL
terminated except when size is 0.

Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/misc.h

@@ -64,6 +64,45 @@ grub_stpcpy (char *dest, const char *src)
   return d - 1;
 }
 
+static inline grub_size_t
+grub_strlcpy (char *dest, const char *src, grub_size_t size)
+{
+  char *d = dest;
+  grub_size_t res = 0;
+  /*
+   * We do not subtract one from size here to avoid dealing with underflowing
+   * the value, which is why to_copy is always checked to be greater than one
+   * throughout this function.
+   */
+  grub_size_t to_copy = size;
+
+  /* Copy size - 1 bytes to dest. */
+  if (to_copy > 1)
+    while ((*d++ = *src++) != '\0' && ++res && --to_copy > 1)
+      ;
+
+  /*
+   * NUL terminate if size != 0. The previous step may have copied a NUL byte
+   * if it reached the end of the string, but we know dest[size - 1] must always
+   * be a NUL byte.
+   */
+  if (size != 0)
+    dest[size - 1] = '\0';
+
+  /* If there is still space in dest, but are here, we reached the end of src. */
+  if (to_copy > 1)
+    return res;
+
+  /*
+   * If we haven't reached the end of the string, iterate through to determine
+   * the strings total length.
+   */
+  while (*src++ != '\0' && ++res)
+   ;
+
+  return res;
+}
+
 /* XXX: If grub_memmove is too slow, we must implement grub_memcpy.  */
 static inline void *
 grub_memcpy (void *dest, const void *src, grub_size_t n)