File descriptor doesn't work with dispatch_read() & dispatch_write() in Objective-C (Creating Email client)

Question

I would like to create Email client working with IMAP protocol. I'm using Apple's Network framework to create TCP connection with imap.gmail.com.

Spoiler! I used the official tutorial (Implementing netcat with Network Framework).

I have very simple interface in my app:

• Email placeholder
• Password placeholder
• Login button

All methods wrote in ViewController.m

When I press Login button:

- (IBAction)loginButtonPressed:(id)sender { 
    const char *hostname = "imap.gmail.com";
    const char *port = "imaps";
    NSString *tempFileTemplate = [NSTemporaryDirectory() stringByAppendingPathComponent:@"myapptempfile.XXXXXX"];
    const char *tempFileTemplateCString = [tempFileTemplate fileSystemRepresentation];
    char *tempFileNameCString = (char *)malloc(strlen(tempFileTemplateCString) + 1);
    strcpy(tempFileNameCString, tempFileTemplateCString);
    dispatch_fd_t fd = mkstemp(tempFileNameCString); // Creating temp file descriptor
    free(tempFileNameCString);
    NSFileHandle *tempFileHandle = [[NSFileHandle alloc] initWithFileDescriptor:fd closeOnDealloc:NO]; // Creating tempFile to use this file descriptor
    nw_parameters_t parameters = nw_parameters_create_secure_tcp(NW_PARAMETERS_DEFAULT_CONFIGURATION, NW_PARAMETERS_DEFAULT_CONFIGURATION);
    nw_endpoint_t endpoint = nw_endpoint_create_host(hostname, port);
    nw_connection_t connection = nw_connection_create(endpoint, parameters);
    [self start_connection:connection: tempFileHandle: fd]; // Start connection
}

Someone will think that it's it not necessary to pass "fd" variable to my connection method. And I guess you are be absolutely right, but I had some problems with file descriptors and it was my last try to understand what's going on.

Then I try to start connection and all is right there.

- (void)start_connection:(nw_connection_t)connection :(NSFileHandle *)file :(dispatch_fd_t)fd {
    nw_connection_set_queue(connection, dispatch_get_main_queue());
    nw_connection_set_state_changed_handler(connection, ^(nw_connection_state_t state, nw_error_t error) {
        switch (state) {
            case nw_connection_state_waiting:
                NSLog(@"waiting");
                break;
            case nw_connection_state_failed:
                NSLog(@"failed");
                break;
            case nw_connection_state_ready:
                NSLog(@"connection is ready");
                [self receive_loop:connection: file :fd];
                break;
            case nw_connection_state_cancelled:
                NSLog(@"connection is cancelled");
                break;
            default:
                break;
        }
    });
    nw_connection_start(connection);
}

My console in Xcode just displays: "connection is ready".

As you can see, we call "receive_loop method with 3 arguments", let's take a look at this method:

- (void)receive_loop:(nw_connection_t)connection :(NSFileHandle *)file :(dispatch_fd_t)fd {
    nw_connection_receive(connection, 1, UINT32_MAX, ^(dispatch_data_t content, nw_content_context_t context, bool is_complete, nw_error_t receive_error) {

        dispatch_block_t schedule_next_receive = ^{
            if (is_complete &&
                context != NULL && nw_content_context_get_is_final(context)) {
                exit(0);
            }
            if (receive_error == NULL) {
                [self receive_loop:connection: file :fd];
            } else {
                NSLog(@"Error");
            }
        };

        if (content != NULL) {
            dispatch_write(fd, content, dispatch_get_main_queue(), ^(__unused dispatch_data_t _Nullable data, int stdout_error) {
                if (stdout_error != 0) {
                    NSLog(@"Error in receive loop");
                } else {
                    NSString* str = @"00000001 LOGIN zurgsrushmber@gmail.com mypasswordhere";
                    NSData* data = [str dataUsingEncoding:NSUTF8StringEncoding];
                    [file writeData:data];
                    [self send_loop:connection: file :fd];
                    schedule_next_receive();
                }
            });
        } else {
            // No content, so directly schedule the next receive
            schedule_next_receive();
        }
    });
}

And now the most interesting thing: At first, I used STDOUT_FILENO instead of "fd" or (very important) "file.fileDescriptor" in my dispath_write() function. Well, when I was using STDOUT_FILENO my console just printing something like that "* OK Gimap ready for requests from 109.252.29.37 m5mb106667441ljg" As you can see, I'm trying to write it in my "fd" or in "file" variables (both doesn't work, I have "NULL" after trying to read fd or file).

So, that's how I try to send info, but it doesn't matter because I can't write to my fd, so I can't send the data to the imap.gmail.com...

My send_loop method:

- (void)send_loop:(nw_connection_t)connection :(NSFileHandle *)file :(dispatch_fd_t)fd {
    dispatch_read(fd, 8192, dispatch_get_main_queue(), ^(dispatch_data_t _Nonnull read_data, int stdin_error) {
        if (stdin_error != 0) {
//            … error logging …
        } else if (read_data == NULL) {
//            … handle end of file …
        } else {
            nw_connection_send(connection, read_data, NW_CONNECTION_DEFAULT_MESSAGE_CONTEXT, true, ^(nw_error_t  _Nullable error) {
                if (error != NULL) {
//                    … error logging …
                } else {
                    NSLog(@"send loop %@", (NSData *)read_data);
                    [self send_loop:connection: file :fd];
                }
            });
        }
    });
}

Someone help me please!

UPDATE: receive_loop method works properly, I understood it, when opened tmp folder and saw my file with content: "* OK Gimap ready for requests from 109.252.29.37 m5mb106667441ljg", but I can't read this file with send_loop method...

Answer 1

EDIT I created a package to help with TLS sockets. You can easily connect to a NodeJS TLS Server, and it takes into account all of the recent iOS 13 restrictions for completing a TLS handshake. Lots of references to helpful articles as well - https://github.com/eamonwhiter73/IOSObjCWebSockets - if you are a developer, feel free to pull and change, I am not an expert on this subject, but I wanted to create this because it doesn't seem there is much like this for an iOS / Objective-C / NodeJS combination on the web.

I think you might be mixing up how to form the send loop , this part in your code:

} else if (read_data == NULL) {
    //… handle end of file …
} else {

The above part of your code is very important for function, I believe your send_loop should look something like this:

- (void)send_loop:(nw_connection_t)connection {
    dispatch_read(STDIN_FILENO, 8192, dispatch_get_main_queue(), ^(dispatch_data_t _Nonnull read_data, int stdin_error) {
        if (stdin_error != 0) {
            //… error logging …
            NSLog(@"error int: %i", stdin_error);
        } else if (read_data == NULL) {
            nw_connection_send(connection, read_data, NW_CONNECTION_FINAL_MESSAGE_CONTEXT, true, ^(nw_error_t  _Nullable error) {
                if (error != NULL) {
                    NSLog(@"error: %@", error);
                }

                NSLog(@"read_data: %@", [read_data description]);
            });
        } else {
            nw_connection_send(connection, read_data, NW_CONNECTION_DEFAULT_MESSAGE_CONTEXT, true, ^(nw_error_t  _Nullable error) {
                if (error != NULL) {
                    NSLog(@"error: %@", error);
                } else {
                    [self send_loop:connection];
                }
            });
        }
    });
}

The part of code I am mentioning is important because of NW_CONNECTION_FINAL_MESSAGE_CONTEXT - when there is nothing left to read, it sends the signal that it is the final message.

I am still working on the package I am creating to make WebSockets easy with IOS and the basic Network package - I will update if I need to when I am done.