diff --git a/libipc.h b/libipc.h
index eb676e8..ed639d7 100644
--- a/libipc.h
+++ b/libipc.h
@@ -106,6 +106,7 @@ int ipc_add_switch (void* ctx, int fd1, int fd2);
 
 // Set IO callbacks for a file descriptor.
 // Returned "char" is a cb_event_types enum.
+// One of the callbacks can be "NULL" to keep the default callback, thus changing only input or output operations.
 int ipc_set_switch_callbacks (void* ctx, int fd
   , char (*in (int orig,       char *payload, size_t *mlen))
   , char (*out(int dest, const char *payload, size_t  mlen)));
diff --git a/src/bindings.zig b/src/bindings.zig
index cca83bc..2683bbf 100644
--- a/src/bindings.zig
+++ b/src/bindings.zig
@@ -152,8 +152,8 @@ export fn ipc_add_switch(ctx: *Context, fd1: i32, fd2: i32) callconv(.C) i32 {
 }
 
 export fn ipc_set_switch_callbacks(ctx: *Context, fd: i32,
-        in: *const fn (origin: i32, mcontent: [*]u8, mlen: *usize) callconv(.C) u8,
-        out: *const fn (origin: i32, mcontent: [*]const u8, mlen: usize) callconv(.C) u8) callconv(.C) i32 {
+        in:  ?*const fn (origin: i32, mcontent:       [*]u8, mlen: *usize) callconv(.C) u8,
+        out: ?*const fn (origin: i32, mcontent: [*]const u8, mlen:  usize) callconv(.C) u8) callconv(.C) i32 {
     ctx.set_switch_callbacks(fd, in, out) catch return -1;
     return 0;
 }
diff --git a/src/context.zig b/src/context.zig
index 734c5ae..cdcb81e 100644
--- a/src/context.zig
+++ b/src/context.zig
@@ -317,8 +317,8 @@ pub const Context = struct {
     }
 
     pub fn set_switch_callbacks(self: *Self, fd: i32,
-            in:  *const fn (origin: i32, mcontent:       [*]u8, mlen: *usize) callconv(.C) u8,
-            out: *const fn (origin: i32, mcontent: [*]const u8, mlen:  usize) callconv(.C) u8) !void {
+            in:  ?*const fn (origin: i32, mcontent:       [*]u8, mlen: *usize) callconv(.C) u8,
+            out: ?*const fn (origin: i32, mcontent: [*]const u8, mlen:  usize) callconv(.C) u8) !void {
         try self.switchdb.set_callbacks(fd, in, out);
     }
 
diff --git a/src/switch.zig b/src/switch.zig
index 203d310..b337def 100644
--- a/src/switch.zig
+++ b/src/switch.zig
@@ -65,11 +65,11 @@ pub const SwitchDB = struct {
     }
 
     pub fn set_callbacks(self: *Self, fd: i32,
-            in: *const fn (origin: i32, mcontent: [*]u8, mlen: *usize) callconv(.C) u8,
-            out: *const fn (origin: i32, mcontent: [*]const u8, mlen: usize) callconv(.C) u8) !void {
+            in:  ?*const fn (origin: i32, mcontent:       [*]u8, mlen: *usize) callconv(.C) u8,
+            out: ?*const fn (origin: i32, mcontent: [*]const u8, mlen:  usize) callconv(.C) u8) !void {
         var managedconnection = self.db.get(fd) orelse return error.unregisteredFD;
-        managedconnection.in = in;
-        managedconnection.out = out;
+        if (in)  |f| { managedconnection.in  = f; }
+        if (out) |f| { managedconnection.out = f; }
         try self.db.put(fd, managedconnection);
     }