diff --git a/src/os/unix/ngx_files.h b/src/os/unix/ngx_files.h
index e05cb4409..c4eccf1fe 100644
--- a/src/os/unix/ngx_files.h
+++ b/src/os/unix/ngx_files.h
@@ -22,6 +22,7 @@
 #define ngx_open_file_n          "open()"
 
 #define NGX_FILE_RDONLY          O_RDONLY
+#define NGX_FILE_WRONLY          O_WRONLY
 #define NGX_FILE_RDWR            O_RDWR
 #define NGX_FILE_CREATE_OR_OPEN  O_CREAT
 #define NGX_FILE_OPEN            0
diff --git a/src/os/win32/ngx_files.h b/src/os/win32/ngx_files.h
index 7db5c8100..f5a97f8b4 100644
--- a/src/os/win32/ngx_files.h
+++ b/src/os/win32/ngx_files.h
@@ -38,6 +38,7 @@
 #define ngx_open_file_n             "CreateFile()"
 
 #define NGX_FILE_RDONLY             GENERIC_READ
+#define NGX_FILE_WRONLY             GENERIC_WRITE
 #define NGX_FILE_RDWR               GENERIC_READ|GENERIC_WRITE
 #define NGX_FILE_CREATE_OR_OPEN     OPEN_ALWAYS
 #define NGX_FILE_OPEN               OPEN_EXISTING