--- a/src/os/unix/ngx_linux_sendfile_chain.c
+++ b/src/os/unix/ngx_linux_sendfile_chain.c
@@ -4,6 +4,17 @@
 #include <ngx_event.h>
 
 
+/*
+ * On Linux up to 2.4.21 sendfile() (syscall #187) works with 32-bit
+ * offsets only and the including <sys/sendfile.h> breaks building
+ * if off_t is 64 bit wide.  So we use own sendfile() definition where
+ * offset paramter is int32_t.  It allows to use sendfile() with
+ * the file parts below 2G.
+ *
+ * Linux 2.4.21 has a new sendfile64() syscall #239.
+ */
+
+
 ngx_chain_t *ngx_linux_sendfile_chain(ngx_connection_t *c, ngx_chain_t *in)
 {
     int              rc;