diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index 9b63891..d73cb86 100644
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -99,7 +99,7 @@
     va_list   args;
 
     va_start(args, fmt);
-    p = ngx_vsnprintf(buf, /* STUB */ 65536, fmt, args);
+    p = ngx_vslprintf(buf, (void *) -1, fmt, args);
     va_end(args);
 
     return p;
@@ -113,7 +113,21 @@
     va_list   args;
 
     va_start(args, fmt);
-    p = ngx_vsnprintf(buf, max, fmt, args);
+    p = ngx_vslprintf(buf, buf + max, fmt, args);
+    va_end(args);
+
+    return p;
+}
+
+
+u_char * ngx_cdecl
+ngx_slprintf(u_char *buf, u_char *last, const char *fmt, ...)
+{
+    u_char   *p;
+    va_list   args;
+
+    va_start(args, fmt);
+    p = ngx_vslprintf(buf, last, fmt, args);
     va_end(args);
 
     return p;
@@ -121,9 +135,9 @@
 
 
 u_char *
-ngx_vsnprintf(u_char *buf, size_t max, const char *fmt, va_list args)
+ngx_vslprintf(u_char *buf, u_char *last, const char *fmt, va_list args)
 {
-    u_char                *p, zero, *last;
+    u_char                *p, zero;
     int                    d;
     float                  f, scale;
     size_t                 len, slen;
@@ -134,12 +148,6 @@
     ngx_str_t             *v;
     ngx_variable_value_t  *vv;
 
-    if (max == 0) {
-        return buf;
-    }
-
-    last = buf + max;
-
     while (*fmt && buf < last) {
 
         /*
diff --git a/src/core/ngx_string.h b/src/core/ngx_string.h
index 0560bd8..81e1ea1 100644
--- a/src/core/ngx_string.h
+++ b/src/core/ngx_string.h
@@ -140,7 +140,11 @@
 u_char *ngx_pstrdup(ngx_pool_t *pool, ngx_str_t *src);
 u_char * ngx_cdecl ngx_sprintf(u_char *buf, const char *fmt, ...);
 u_char * ngx_cdecl ngx_snprintf(u_char *buf, size_t max, const char *fmt, ...);
-u_char *ngx_vsnprintf(u_char *buf, size_t max, const char *fmt, va_list args);
+u_char * ngx_cdecl ngx_slprintf(u_char *buf, u_char *last, const char *fmt,
+    ...);
+u_char *ngx_vslprintf(u_char *buf, u_char *last, const char *fmt, va_list args);
+#define ngx_vsnprintf(buf, max, fmt, args)                                   \
+    ngx_vslprintf(buf, buf + (max), fmt, args)
 
 ngx_int_t ngx_strcasecmp(u_char *s1, u_char *s2);
 ngx_int_t ngx_strncasecmp(u_char *s1, u_char *s2, size_t n);