diff --git a/include/sexpr/cstring.h b/include/sexpr/cstring.h
index 79e7458..3a921b1 100644
--- a/include/sexpr/cstring.h
+++ b/include/sexpr/cstring.h
@@ -78,14 +78,14 @@ extern "C" {
    * Set the growth size.  Values less than one are ignored.
    */
   void sgrowsize(size_t s);
-  
+
   /**
    * Allocate a new CSTRING of the given size.
    * A NULL return value indicates that something went wrong and that
    * sexp_errno should be checked for the cause.
    */
   CSTRING *snew(size_t s);
-  
+
   /**
    * Concatenate the second argument to the CSTRING passed in the first.
    * The second argument must be a pointer to a null terminated string.
@@ -96,7 +96,18 @@ extern "C" {
    * leak if an error condition occurs.
    */
   CSTRING *sadd(CSTRING *s, char *a);
-  
+
+  /**
+   * Concatenate the len bytes of the second argument to the CSTRING
+   * passed in the first.
+   * A NULL return value indicates that something went wrong and that
+   * sexp_errno should be checked for the cause.  The contents of s are
+   * left alone.  As such, the caller should check the pointer returned
+   * before overwriting the value of s, as this may result in a memory
+   * leak if an error condition occurs.
+   */
+  CSTRING *snadd(CSTRING *s, char *a, size_t len);
+
   /**
    * Append a character to the end of the CSTRING.
    * A NULL return value indicates that something went wrong and that
@@ -106,7 +117,7 @@ extern "C" {
    * leak if an error condition occurs.
    */
   CSTRING *saddch(CSTRING *s, char a);
-  
+
   /**
    * Trim the allocated memory to precisely the string length plus one char
    * to hold the null terminator
diff --git a/lib/cstring.c b/lib/cstring.c
index 62a72be..18a4995 100644
--- a/lib/cstring.c
+++ b/lib/cstring.c
@@ -86,7 +86,18 @@ CSTRING *snew(size_t s) {
   return cs;
 }
 
-CSTRING *sadd(CSTRING *s, char *a) {
+CSTRING *sadd(CSTRING *s, char *a)
+{
+  size_t len = strlen(a);
+
+  if(!s) return NULL;
+  if(!a || !len) return s;
+
+  return snadd(s, a, len);
+}
+
+CSTRING *snadd(CSTRING *s, char *a, size_t len)
+{
   size_t alen;
   char *newbase;
 
@@ -100,7 +111,7 @@ CSTRING *sadd(CSTRING *s, char *a) {
     return s;
   }
 
-  alen = strlen(a);
+  alen = len;
 
   if (s->curlen + alen >= s->len) {
 #ifdef __cplusplus
@@ -112,12 +123,12 @@ CSTRING *sadd(CSTRING *s, char *a) {
                            s->len+cstring_growsize+alen,
                            s->len);
 #endif
-    
+
     /* do NOT destroy s anymore.  if realloc fails, the original data is
        still valid, so just report the error to sexp_errno and return NULL.
     */
     if (newbase == NULL) {
-      sexp_errno = SEXP_ERR_MEMORY;      
+      sexp_errno = SEXP_ERR_MEMORY;
       return NULL;
     }
 
@@ -125,9 +136,10 @@ CSTRING *sadd(CSTRING *s, char *a) {
     s->base = newbase;
   }
 
-  memcpy(&s->base[s->curlen],a,alen);
+  memcpy(&s->base[s->curlen], a, alen);
   s->curlen += alen;
   s->base[s->curlen] = 0;
+
   return s;
 }