Unsubscriptions Are Free

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <ctype.h>

#define FLAG_BUFFER 200
#define LINE_BUFFER_SIZE 20


typedef struct {
    uintptr_t (*whatToDo)();
    char *username;
} cmd;

char choice;
cmd *user;

void hahaexploitgobrrr(){
    char buf[FLAG_BUFFER];
    FILE *f = fopen("flag.txt","r");
    fgets(buf,FLAG_BUFFER,f);
    fprintf(stdout,"%s\n",buf);
    fflush(stdout);
}

char * getsline(void) {
    getchar();
    char * line = malloc(100), * linep = line;
    size_t lenmax = 100, len = lenmax;
    int c;
    if(line == NULL)
        return NULL;
    for(;;) {
        c = fgetc(stdin);
        if(c == EOF)
            break;
        if(--len == 0) {
            len = lenmax;
            char * linen = realloc(linep, lenmax *= 2);

            if(linen == NULL) {
                free(linep);
                return NULL;
            }
            line = linen + (line - linep);
            linep = linen;
        }

        if((*line++ = c) == '\n')
            break;
    }
    *line = '\0';
    return linep;
}

void doProcess(cmd* obj) {
    (*obj->whatToDo)();
}

void s(){
    printf("OOP! Memory leak...%p\n",hahaexploitgobrrr);
    puts("Thanks for subsribing! I really recommend becoming a premium member!");
}

void p(){
    puts("Membership pending... (There's also a super-subscription you can also get for twice the price!)");
}

void m(){
    puts("Account created.");
}

void leaveMessage(){
    puts("I only read premium member messages but you can ");
    puts("try anyways:");
    char* msg = (char*)malloc(8);
    read(0, msg, 8);
}

void i(){
    char response;
    puts("You're leaving already(Y/N)?");
    scanf(" %c", &response);
    if(toupper(response)=='Y'){
        puts("Bye!");
        free(user);
    }else{
        puts("Ok. Get premium membership please!");
    }
}

void printMenu(){
    puts("Welcome to my stream! ^W^");
    puts("==========================");
    puts("(S)ubscribe to my channel");
    puts("(I)nquire about account deletion");
    puts("(M)ake an Twixer account");
    puts("(P)ay for premium membership");
    puts("(l)eave a message(with or without logging in)");
    puts("(e)xit");
}

void processInput(){
  scanf(" %c", &choice);
  choice = toupper(choice);
  switch(choice){
    case 'S':
    if(user){
        user->whatToDo = (void*)s;
    }else{
        puts("Not logged in!");
    }
    break;
    case 'P':
    user->whatToDo = (void*)p;
    break;
    case 'I':
    user->whatToDo = (void*)i;
    break;
    case 'M':
    user->whatToDo = (void*)m;
    puts("===========================");
    puts("Registration: Welcome to Twixer!");
    puts("Enter your username: ");
    user->username = getsline();
    break;
   case 'L':
    leaveMessage();
    break;
    case 'E':
    exit(0);
    default:
    puts("Invalid option!");
    exit(1);
      break;
  }
}

int main(){
    setbuf(stdout, NULL);
    user = (cmd *)malloc(sizeof(user));
    while(1){
        printMenu();
        processInput();
        //if(user){
            doProcess(user);
        //}
    }
    return 0;
}