diff --git a/CHANGES b/CHANGES
index 14ed007..94766a5 100644
--- a/CHANGES
+++ b/CHANGES
@@ -4,6 +4,8 @@
   #include <unistd.h> in io/io_sendfile.c (broken OpenBSD, thx Rob)
   add scan_urlencoded2 (like scan_urlencoded but will not convert '+' to
     ' '; needed for web servers, so they can serve libstdc++.tar.gz)
+  fix iob_write to handle failure properly
+  document that the iob_write callback should limit itself
 
 0.21:
   errno cleanup and man page updates (Rolf Eike Beer)
diff --git a/io/iob_write.3 b/io/iob_write.3
index 90e4e6d..62a3639 100644
--- a/io/iob_write.3
+++ b/io/iob_write.3
@@ -25,5 +25,13 @@ writable, and then calling iob_write until it returns 0, -1 or -3.
 
 If it returns 0, terminate the loop (everything was written OK).  If it
 returns -1, call io_wait again.  If it returned -3, signal an error.
+
+.SH NOTE
+iob_write will continue to call your callback until it returns an error.
+So if you are in a state machine, for example a web server using this
+for SSL support, make sure to write at most n bytes at a time (e.g. 64k)
+and the next time you are called return -1.  Otherwise iob_write might
+not return until the whole file is served.
+
 .SH "SEE ALSO"
 iob_send(3)
diff --git a/io/iob_write.c b/io/iob_write.c
index 025762d..d4cba7d 100644
--- a/io/iob_write.c
+++ b/io/iob_write.c
@@ -18,16 +18,14 @@ int64 iob_write(int64 s,io_batch* b,io_write_callback cb) {
       sent=io_mmapwritefile(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 > e[i].n) sent=e[i].n; /* can't happen */
+    if (sent>0 && sent>e[i].n) sent=e[i].n; /* can't happen */
     thatsit=(sent != e[i].n);
-    if (sent==-1 && errno!=EAGAIN)
-      sent=-3;
-    if (sent>0) {
-      e[i].offset+=sent;
-      e[i].n-=sent;
-      total+=sent;
-    } else
+    if (sent<=0)
       return total?total:sent;
+    e[i].offset+=sent;
+    e[i].n-=sent;
+    total+=sent;
+    b->bytesleft-=sent;
     if (thatsit) break;
   }
   if (total == b->bytesleft)
diff --git a/t.c b/t.c
index 90398d2..8714ce2 100644
--- a/t.c
+++ b/t.c
@@ -19,22 +19,43 @@
 #include <errno.h>
 #include <string.h>
 #include <assert.h>
-#include <errmsg.h>
+#include "errmsg.h"
+#include "iob.h"
 
 #define rdtscl(low) \
      __asm__ __volatile__ ("rdtsc" : "=a" (low) : : "edx")
 
+int64 writecb(int64 fd,const void* buf,uint64 n) {
+#if 0
+  int r;
+  int todo=n>=65536?65536:n;
+  r=write(fd,buf,todo);
+  if (r==-1 && r!=EAGAIN) r=-3;
+  return r;
+#endif
+  return -1;
+}
+
 int main(int argc,char* argv[]) {
+  static io_batch b;
+  int64 fd=open("t.c",0);
+  iob_addbuf(&b,"fnord",5);
+  iob_addfile_close(&b,fd,0,7365);
+  iob_write(1,&b,writecb);
+#if 0
   char dest[1024];
   unsigned long len;
   scan_urlencoded2("libstdc++.tar.gz",dest,&len);
   buffer_putmflush(buffer_1,dest,"\n");
+#endif
 #if 0
   static stralloc sa;
   stralloc_copym(&sa,"foo ","bar ","baz.\n");
   write(1,sa.s,sa.len);
 #endif
+#if 0
   buffer_putmflush(buffer_1,"foo ","bar ","baz.\n");
+#endif
 #if 0
   char* c="fnord";
   int fd=open_read(c);