diff -r cf19b7820ff0 -r 94360453bce4 src/string.c
--- a/src/string.c	Thu Dec 25 12:07:37 2025 +0100
+++ b/src/string.c	Sun Dec 28 14:10:14 2025 +0100
@@ -99,13 +99,15 @@
     return size;
 }
 
-int cx_strcat_a(
+cxmutstr cx_strcat_a(
         const CxAllocator *alloc,
-        cxmutstr *str,
+        cxmutstr str,
         size_t count,
         ...
 ) {
-    if (count == 0) return 0;
+    if (count == 0) {
+        return cx_strdup_a(alloc, str);
+    }
     va_list ap;
     va_start(ap, count);
     va_list ap2;
@@ -113,7 +115,7 @@
 
     // compute overall length
     bool overflow = false;
-    size_t slen = str->length;
+    size_t slen = str.length;
     for (size_t i = 0; i < count; i++) {
         cxstring s = va_arg(ap, cxstring);
         if (slen > SIZE_MAX - s.length) overflow = true;
@@ -125,31 +127,31 @@
     if (overflow) {
         va_end(ap2);
         errno = EOVERFLOW;
-        return -1;
+        return CX_NULLSTR;
     }
 
-    // reallocate or create new string
-    if (cxReallocate(alloc, &str->ptr, slen + 1)) {
+    // reallocate or create a new string
+    if (cxReallocate(alloc, &str.ptr, slen + 1)) {
         // LCOV_EXCL_START
         va_end(ap2);
-        return -1;
+        return CX_NULLSTR;
         // LCOV_EXCL_STOP
     }
 
     // concatenate strings
-    size_t pos = str->length;
-    str->length = slen;
+    size_t pos = str.length;
+    str.length = slen;
     for (size_t i = 0; i < count; i++) {
         cxstring s = va_arg(ap2, cxstring);
-        memcpy(str->ptr + pos, s.ptr, s.length);
+        memcpy(str.ptr + pos, s.ptr, s.length);
         pos += s.length;
     }
     va_end(ap2);
 
     // terminate string
-    str->ptr[str->length] = '\0';
+    str.ptr[str.length] = '\0';
 
-    return 0;
+    return str;
 }
 
 cxstring cx_strsubs(