Chat with POSIX mqueues

Checkpoint 1 (Jun, 4th - 2019)

Implements

• Interactive GUI

  • Blocks special commands such as CTRL + C
  • User can choose options by pressing the correspondent key without pressing enter (When options can be chosen with only one key)
  • Exclusive chat window per queue
  • Messages in a session are stored in a custom coded string map, so the user can navigate to another chat window without losing sent or received messages

• Message protocol

  • User to user messaging
  • Due to the GUI, it is hard to send a message to an unexistant queue (there is a specific situation when this can occur)
  • Chat nickname is the user's unix username
  • Broadcast a message to all users (Jul, 10th - 2019)

Known issues

• When a user A is chatting with user B and user B becomes offline (the queue is deleted), if chat A don't close the current chat window, it will be possible to send a message to an unexistant queue (FIXED)
• If another user chooses the same username as the user's unix username, it is not possible to choose another
• Message broadcasting was 90% implemented but, due to a segfault bug occurring in the GUI thread, it was not released on this checkpoint (FIXED)

Challenges with threading

• User input thread
  • The challenge was to change the tty behaviour to enable stdin flushing without a line break
    • By changing incorrectly, this caused my stdout to ignore lots of formatting and the GUI became unreadable. But the problem was fixed =)
• Message outputting thread
  • I haven't had big issues
• Message receiving thread
  • I haven't had big issues

Challenges unrelated with threading

String manipulation

In C, it is such an overhead to a Python developer to deal with this.

Data structures

In other languages, a data structure to store multiple users messages would look like:

// C++
map<string, vector<string>> messages;

# Python3
messages = {'username': ["message one", "message two"]}

but, in C, it ended looking like:

// string_map.h code snippet
void init_map(int n_keys, int n_values){
    max_value = n_keys;
    sem_init(&map_semaphore, 0, 1);
    int i;
    string_map = (char***) malloc(n_keys * sizeof(char***));

    for(i = 0; i < n_keys; i++){
        char* c = (char*) malloc(sizeof(char));
        *c = 1;
        string_map[i] = (char**) malloc(n_values * sizeof(char**));
        string_map[i][0] = c;
    }
}

and, to convert the strings into the 3-dimentional indexes:

// string_map.h code snippet
unsigned long hash(unsigned char *str){
    unsigned long hash = 5381;
    int c;

    while (c = *str++){
        hash = ((hash << 5) + hash) + c; /* hash * 33 + c */
        if(c == 0){
            break;
        }
    }

    return hash % max_value;
}

but now, it works like magic:

init_map(1000, 11); // Up to 1000 users with 10 messages each

map_insert("victorcmoura", "message one");
char** victorcmoura_messages = map_get("victorcmoura");

int i;
for(i = 1; i < len("victorcmoura"); i++){
    printf("%s\n", messages[i]);
}

Output:

message one