diff --git a/io.h b/io.h
index 505e2d8..0f3c370 100644
--- a/io.h
+++ b/io.h
@@ -77,6 +77,8 @@ void* io_getcookie(int64 d);
 
 /* put descriptor in non-blocking mode */
 void io_nonblock(int64 d);
+/* put descriptor in blocking mode */
+void io_block(int64 d);
 /* put descriptor in close-on-exec mode */
 void io_closeonexec(int64 d);
 
diff --git a/io/io_block.3 b/io/io_block.3
new file mode 100644
index 0000000..b5fd11e
--- /dev/null
+++ b/io/io_block.3
@@ -0,0 +1,15 @@
+.TH io_block 3
+.SH NAME
+io_block \- switch to blocking I/O
+.SH SYNTAX
+.B #include <io.h>
+
+void \fBio_block\fP(int64 fd);
+.SH DESCRIPTION
+io_block puts UNIX descriptor fd into ``blocking mode.''
+
+File descriptors are normally in blocking mode, except if they come from
+accept() or io_accept() and the listening socket was in non-blocking
+mode.
+.SH "SEE ALSO"
+io_nonblock(3)
diff --git a/io/io_block.c b/io/io_block.c
new file mode 100644
index 0000000..1deb265
--- /dev/null
+++ b/io/io_block.c
@@ -0,0 +1,25 @@
+#include <unistd.h>
+#include <fcntl.h>
+#include <errno.h>
+#include "io_internal.h"
+
+#ifdef __MINGW32__
+#include <winsock2.h>
+#include "windoze.h"
+#endif
+
+#ifndef O_NDELAY
+#define O_NDELAY O_NONBLOCK
+#endif
+
+void io_block(int64 d) {
+  io_entry* e=array_get(&io_fds,sizeof(io_entry),d);
+#ifdef __MINGW32__
+  unsigned long i=0;
+  if (ioctlsocket( d, FIONBIO, &i)==0)
+    if (e) e->nonblock=0;
+#else
+  if (fcntl(d,F_SETFL,fcntl(d,F_GETFL,0) & ~O_NDELAY)==0)
+    if (e) e->nonblock=0;
+#endif
+}