dynstr_prepend(), fixed typo in comment

dynstr.c

@@ -59,6 +59,51 @@ enum dynstr_err dynstr_append(struct dynstr *const d, const char *const format,
     return DYNSTR_OK;
 }
 
+enum dynstr_err dynstr_prepend(struct dynstr *const d, const char *const format, ...)
+{
+    va_list ap;
+
+    va_start(ap, format);
+
+    {
+        const size_t src_len = vsnprintf(NULL, 0, format, ap);
+        const size_t new_len = d->len + src_len + 1;
+        va_end(ap);
+
+        d->str = realloc(d->str, new_len * sizeof *d->str);
+
+        if (d->str && d->len)
+        {
+            /* Keep byte that will be removed by later call to vsprintf. */
+            const char c = *d->str;
+
+            for (size_t i = new_len - 1, j = d->len; j <= d->len; i--, j--)
+            {
+                d->str[i] = d->str[j];
+            }
+
+            va_start(ap, format);
+            vsprintf(d->str, format, ap);
+            va_end(ap);
+            d->str[src_len] = c;
+        }
+        else if (!d->len)
+        {
+            va_start(ap, format);
+            vsprintf(d->str + d->len, format, ap);
+            va_end(ap);
+        }
+        else
+        {
+            return DYNSTR_ERR_ALLOC;
+        }
+
+        d->len += src_len;
+    }
+
+    return DYNSTR_OK;
+}
+
 enum dynstr_err dynstr_dup(struct dynstr *const dst, const struct dynstr *const src)
 {
     if (!dst->str && !dst->len)

include/dynstr.h

@@ -48,7 +48,7 @@
 #define dynstr_append_or_ret_zero(...) if (dynstr_append(__VA_ARGS__) != DYNSTR_OK) return 0
 
 /**
- * Convenience macro that calls dynstr_append and returns zero if failed.
+ * Convenience macro that calls dynstr_append and returns one if failed.
  */
 #define dynstr_append_or_ret_nonzero(...) if (dynstr_append(__VA_ARGS__) != DYNSTR_OK) return 1
 
@@ -95,6 +95,18 @@ void dynstr_init(struct dynstr *d);
  */
 enum dynstr_err dynstr_append(struct dynstr *d, const char *format, ...);
 
+/**
+ * This function takes a string literal in printf format and a variable
+ * number of arguments, calculates its size and prepends it into the
+ * beginning of the dynamic string.
+ * @param d Dynamic string where new string will be prepended.
+ * @param format String literal in printf format.
+ * @return Returns one of the following error codes:
+ *    # DYNSTR_OK if successful.
+ *    # DYNSTR_ERR_ALLOC if no more memory is available.
+ */
+enum dynstr_err dynstr_prepend(struct dynstr *d, const char *format, ...);
+
 /**
  * This function duplicates a dynamic string to another instance.
  * @attention Destination instance must be initialized before calling