diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index 8aaa1d2ed..3b392f7b2 100644
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -1827,7 +1827,7 @@ ngx_sort(void *base, size_t n, size_t size,
 #if (NGX_MEMCPY_LIMIT)
 
 void *
-ngx_memcpy(void *dst, void *src, size_t n)
+ngx_memcpy(void *dst, const void *src, size_t n)
 {
     if (n > NGX_MEMCPY_LIMIT) {
         ngx_log_error(NGX_LOG_ALERT, ngx_cycle->log, 0, "memcpy %uz bytes", n);
diff --git a/src/core/ngx_string.h b/src/core/ngx_string.h
index fa386ca72..92d246ebc 100644
--- a/src/core/ngx_string.h
+++ b/src/core/ngx_string.h
@@ -89,7 +89,7 @@ ngx_strlchr(u_char *p, u_char *last, u_char c)
 
 #if (NGX_MEMCPY_LIMIT)
 
-void *ngx_memcpy(void *dst, void *src, size_t n);
+void *ngx_memcpy(void *dst, const void *src, size_t n);
 #define ngx_cpymem(dst, src, n)   (((u_char *) ngx_memcpy(dst, src, n)) + (n))
 
 #else