The proxy works and prints hexdump of clients' input.

src/main.zig

@@ -1,4 +1,5 @@
 const std = @import("std");
+const net = std.net;
 const hexdump = @import("./hexdump.zig").print_hex;
 const AutoArrayHashMap = std.AutoArrayHashMap;
 const allocator = std.mem.Allocator;
@@ -9,24 +10,7 @@ const ipc = @cImport({
     @cInclude("libipc.h");
 });
 
-// // Return type of callback functions when switching.
-// enum cb_event_types {
-// 	  CB_NO_ERROR   = 0 // No error. A message was generated.
-// 	, CB_ERROR      = 1 // Generic error.
-// 	, CB_FD_CLOSING = 2 // The fd is closing.
-// 	, CB_IGNORE     = 3 // The message should be ignored (protocol specific).
-// };
-//
-// int ipc_write (void* ctx, int servicefd, char* mcontent, uint32_t mlen);
-// int ipc_read_fd (void* ctx, int fd, char* buffer, size_t* buflen);
-// int ipc_read (void* ctx, size_t index, char* buffer, size_t* buflen);
-// void ipc_context_timer (void* ctx, int timer);
-// int ipc_close (void* ctx, size_t index);
-// int ipc_close_all (void* ctx);
-//
-// // Switch functions (for "protocol" services, such as TCPd).
-// int ipc_add_external (void* ctx, int newfd);
-//
+// enum cb_event_types { CB_NO_ERROR, CB_ERROR, CB_FD_CLOSING, CB_IGNORE };
 
 // TODO: noreturn error function: print a error message then crash.
 // TODO: connection to the proxied service
@@ -35,23 +19,41 @@ const ipc = @cImport({
 
 // TODO: find why "in" and "out" functions are detected as
 //       returning "[*c]u8" while they should be returning u8.
+const IPC_MAX_MESSAGE_SIZE = 100_000;
 
 const stdout = std.io.getStdOut().writer();
 
 var context : ?*anyopaque = undefined;
 
+
 // char (*in (int orig, const char *payload, uint32_t *mlen))
-pub fn in(origin : c_int, _ : [*c]const u8, _ : [*c]u32) callconv(.C) [*c]u8 // why this return type?!
+pub fn in(origin : c_int, buffer : [*c]u8, buflen : [*c]usize) callconv(.C) [*c]u8 // why this return type?!
 {
-    stdout.print("we are IN origin is {}\n", .{origin}) catch {};
-    return 1;
+    var stream: net.Stream = .{ .handle = origin };
+    buflen.* = stream.read(buffer[0..buflen.*]) catch { return 1; };
+
+    // Let's handle this as a disconnection.
+    if (buflen.* == 0) { return 2; }
+
+    // Error: received message should be bigger than that!
+    if (buflen.* <= 4) { return 1; }
+
+    const slice = buffer[0..buflen.*];
+    stdout.print("Message received from {}\n", .{origin}) catch {};
+    hexdump("message just received", slice) catch {};
+    return 0;
 }
 
 // char (*out(int dest, char *payload, uint32_t  mlen)));
-pub fn out(dest_fd : c_int, _ : [*c]u8, _ : u32) callconv(.C) [*c]u8 // why this return type?!
+pub fn out(dest_fd : c_int, buffer : [*c]const u8, buflen : usize) callconv(.C) [*c]u8 // why this return type?!
 {
-    stdout.print("we are OUT target is {}\n", .{dest_fd}) catch {};
-    return 1;
+    const slice = buffer[0..buflen];
+    stdout.print("Message to send to {}\n", .{dest_fd}) catch {};
+    hexdump("message to send", slice) catch {};
+
+    var stream = net.Stream{ .handle = dest_fd };
+    _ = stream.write(slice) catch { return 1; };
+    return 0;
 }
 
 var proxy_name      : [:0]const u8 = "proxy";
@@ -82,7 +84,7 @@ pub fn connection_and_link(client_fd : i32) !void
 
     // Returned "char" is a cb_event_types enum.
     try stdout.print("Hello we will put some callbacks\n", .{});
-    ret = ipc.ipc_set_switch_callbacks (context, client_fd, in, out);
+    ret = ipc.ipc_set_switch_callbacks (context, client_fd, in, null); // only print hexdump of input
     if (ret != 0) {
         try stdout.print("Impossible to set the callbacks {} -> {s}\n", .{client_fd, proxied_service});
         return error.CannotAddCallbacks;
@@ -95,7 +97,6 @@ pub fn connection_and_link(client_fd : i32) !void
 pub fn disconnection_and_unlink(fd : i32) !void
 {
     try stdout.print("Disconnection of user ({})\n", .{fd});
-
 }
 
 pub fn message_rx(_ : i32, buffer : [100_000]u8, buflen : u64) !void
@@ -155,8 +156,8 @@ pub fn main() !void {
             ipc.MESSAGE_TX    => try message_tx(origin_fd),                 // Message sent.
             ipc.TIMER         => try stdout.print("TIMER\n",         .{}),  // Timeout in the poll(2) function.
             ipc.EXTERNAL      => try stdout.print("EXTERNAL\n",      .{}),  // Message received from a non IPC socket.
-            ipc.SWITCH_RX     => try stdout.print("SWITCH_RX\n",     .{}),  // Message received from a switched FD.
-            ipc.SWITCH_TX     => try stdout.print("SWITCH_TX\n",     .{}),  // Message sent to a switched fd.
+            ipc.SWITCH_RX     => {},                                        // Message received from a switched fd.
+            ipc.SWITCH_TX     => {},                                        // Message sent to a switched fd.
             else              => try stdout.print("ELSE CASE\n",     .{}),  // Should never happen.
         }
     }