What is it for { } inside the main

Question

I came across a situation in C that I do not know and had never seen anyone use before. Within the method main() there are several keys, I believe that to isolate the code but there is a same variable that in each code block receives a value and even changes type. The variable cfg appears several times, sometimes having the type pjsua_config, now having the type pjsua_transport_config, now having the type pjsua_acc_config.

The code snippet was taken from the tutorial that teaches how to use the pjsip library, the library is in C, this would be some way to simulate an inheritance (making a sort of cast)? Or with each block it creates a new variable and destroys the previous one?

I confess that I am a little lost, because I am trying to pass some things to C++, but I had difficulty understanding this part.

int main(int argc, char *argv[])
{
    pjsua_acc_id acc_id;
    pj_status_t status;


    status = pjsua_create();
    if (status != PJ_SUCCESS) error_exit("Error in pjsua_create()", status);

    if (argc > 1) {
        status = pjsua_verify_url(argv[1]);
        if (status != PJ_SUCCESS) error_exit("Invalid URL in argv", status);
    }

    /* Init pjsua */
    {
         pjsua_config cfg;
         pjsua_logging_config log_cfg;

         pjsua_config_default(&cfg);
         cfg.cb.on_incoming_call = &on_incoming_call;
         cfg.cb.on_call_media_state = &on_call_media_state;
         cfg.cb.on_call_state = &on_call_state;

         pjsua_logging_config_default(&log_cfg);
         log_cfg.console_level = 4;

         status = pjsua_init(&cfg, &log_cfg, NULL);
         if (status != PJ_SUCCESS) error_exit("Error in pjsua_init()", status);
    }

    /* Add UDP transport. */
    {
         pjsua_transport_config cfg;

         pjsua_transport_config_default(&cfg);
         cfg.port = 5060;
         status = pjsua_transport_create(PJSIP_TRANSPORT_UDP, &cfg, NULL);
         if (status != PJ_SUCCESS) error_exit("Error creating transport", status);
    }

    /* Initialization is done, now start pjsua */
    status = pjsua_start();
    if (status != PJ_SUCCESS) error_exit("Error starting pjsua", status);

    /* Register to SIP server by creating SIP account. */
    {
         pjsua_acc_config cfg;

         pjsua_acc_config_default(&cfg);
         cfg.id = pj_str("sip:" SIP_USER "@" SIP_DOMAIN);
         cfg.reg_uri = pj_str("sip:" SIP_DOMAIN);
         cfg.cred_count = 1;
         cfg.cred_info[0].realm = pj_str(SIP_DOMAIN);
         cfg.cred_info[0].scheme = pj_str("digest");
         cfg.cred_info[0].username = pj_str(SIP_USER);
         cfg.cred_info[0].data_type = PJSIP_CRED_DATA_PLAIN_PASSWD;
         cfg.cred_info[0].data = pj_str(SIP_PASSWD);

         status = pjsua_acc_add(&cfg, PJ_TRUE, &acc_id);
         if (status != PJ_SUCCESS) error_exit("Error adding account", status);
    }

    /* If URL is specified, make call to the URL. */
    if (argc > 1) {
         pj_str_t uri = pj_str(argv[1]);
         status = pjsua_call_make_call(acc_id, &uri, 0, NULL, NULL, NULL);
         if (status != PJ_SUCCESS) error_exit("Error making call", status);
    }

    /* Wait until user press "q" to quit. */
    for (;;) {
         char option[10];
         puts("Press 'h' to hangup all calls, 'q' to quit");
         if (fgets(option, sizeof(option), stdin) == NULL) {
             puts("EOF while reading stdin, will quit now..");
             break;
         }

         if (option[0] == 'q')
             break;

         if (option[0] == 'h')
             pjsua_call_hangup_all();
    }

    /* Destroy pjsua */
    pjsua_destroy();

    return 0;
}

Answer 1

Actually, he’s got nothing, he’s just like the one you’ve always used. It only demarcates the beginning and end of a code instruction blocks that should be considered in a unique way for some reason.

One problem I see a lot is that people don’t understand what they use, hence the phrase I created:

While you don’t know what each character of your code does, even white space, you still don’t know how to program.

In this specific case it was used to keep certain variables within a specific scope because the block always generates a new scope. So all variables that were created within it will not exist outside of it, and if there are variables outside of it with the same name, there is no confusion.

Then in the /* Init pjsua */ the variable cfg, for example, not the same variable cfg of /* Add UDP transport. */ or of /* Register to SIP server by creating SIP account. */. Same name but different objects.

Learn more about What is the difference between scope and lifetime?.

It is no different from other uses as in the function, in the if, in the for, etc. You just didn’t know that it was something that existed by itself.

In almost all situations it is demonstration of something wrong in the code and is not usually necessary. If you use this you should review the code, maybe separate parts into another function.

My fan is that now start looking at everything in the code like other eyes and try to learn how things really are and not how they seem to be. Everyone would program better if they did that.

Answer 2

It has no programmatic functionality other than separating the scopes (being able to declare variables of the same name with different types) and probably the author used to organize the code in the editor/IDE he was using. How does the #region in c#.