diff --git a/io.h b/io.h
index 8ab8432..c151c66 100644
--- a/io.h
+++ b/io.h
@@ -197,6 +197,7 @@ int io_queueforread(int64 d);
 int io_queueforwrite(int64 d);
 
 typedef int64 (*io_write_callback)(int64 s,const void* buf,uint64 n);
+typedef int64 (*io_sendfile_callback)(int64 out,int64 in,uint64 off,uint64 bytes,io_write_callback writecb);
 
 /* used internally, but hey, who knows */
 /* sandboxing: write(2), mmap(2), munmap(2), lseek(2), pread(2), read(2), linux: epoll_ctl(2), bsd: kevent(2), solaris: write(2), oldlinux: poll(2) */
diff --git a/io/iob_write.c b/io/iob_write.c
index 11019db..dfd4974 100644
--- a/io/iob_write.c
+++ b/io/iob_write.c
@@ -1,6 +1,6 @@
 #include <iob_internal.h>
 
-int64 iob_write(int64 s,io_batch* b,io_write_callback cb) {
+int64 iob_write2(int64 s,io_batch* b,io_write_callback cb,io_sendfile_callback sfcb) {
   iob_entry* e,* last;
   uint64 total;
   int64 sent;
@@ -15,7 +15,7 @@ int64 iob_write(int64 s,io_batch* b,io_write_callback cb) {
     int thatsit;
     if (!e[i].n) continue;
     if (e[i].type==FROMFILE)
-      sent=io_mmapwritefile(s,e[i].fd,e[i].offset,e[i].n,cb);
+      sent=sfcb(s,e[i].fd,e[i].offset,e[i].n,cb);
     else
       sent=cb(s,e[i].buf+e[i].offset,e[i].n);
     if (sent>0 && (uint64)sent>e[i].n) sent=e[i].n; /* can't happen */
@@ -32,3 +32,7 @@ int64 iob_write(int64 s,io_batch* b,io_write_callback cb) {
     iob_reset(b);
   return total;
 }
+
+int64 iob_write(int64 s,io_batch* b,io_write_callback cb) {
+  return iob_write2(s,b,cb,io_mmapwritefile);
+}
diff --git a/iob.h b/iob.h
index e04d579..c5b9231 100644
--- a/iob.h
+++ b/iob.h
@@ -47,6 +47,7 @@ int iob_addfile(io_batch* b,int64 fd,uint64 off,uint64 n);
 int iob_addfile_close(io_batch* b,int64 fd,uint64 off,uint64 n);
 int64 iob_send(int64 s,io_batch* b);
 int64 iob_write(int64 s,io_batch* b,io_write_callback cb);
+int64 iob_write2(int64 s,io_batch* b,io_write_callback cb,io_sendfile_callback sfcb);
 void iob_reset(io_batch* b);
 void iob_free(io_batch* b);
 void iob_prefetch(io_batch* b,uint64 bytes);