fgets - crashes in visual studio, works in gcc

Question

I'm working on something small and I have run into a really small but disturbing problem - the program works in gcc, but crashes in visual studio, when it gets to the fgets command:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void registerUser(char *username);

char path[256] = "C:\\Users\\magshimim\\Desktop\\cproject\\bank\\";

int main() {
    int choice = 0;
    char username[10];
    printf("Welcome to nikitosik's bank program!\n"
           "Choose an option :\n"
           "0 - Register\n"
           "1 - Login to existing account\n");
    scanf("%d", &choice);
    getchar(); // catch enter
    if (!choice) {
        registerUser(username);
    } else {
        //login();
    }
}

void registerUser(char *username) { // username is passed, for verification later
    FILE *userinfo; // file that contains info of all registered users
    FILE *userpathf; // file to open for txt file
    char usertxt[256]; // path of data
    char userpath[256]; // path of new user data file
    char password[15];
    strcat(userpath, path);
    strcat(usertxt, path);
    strcat(usertxt, "users.txt");
    userinfo = fopen(usertxt, "a");
    printf("Choose a username(max 10 letters): ");
    fgets(username, 10, stdin);
    username[strcspn(username, "\n")] = 0;
    strcat(userpath, username);
    strcat(userpath, ".txt");
    userpathf = fopen(userpath, "w");
    fclose(userpathf);
    printf("Choose a password(max 15 letters): ");
    fgets(password, 15, stdin);
    fprintf(userinfo, "%s\n%s", username, password);
    fclose(userinfo);
}

thanks in advance

`strcat(userpath, path)` --> `strcpy(userpath, path)`, `strcat(usertxt, path);` --> `strcpy(usertxt, path);` — BLUEPIXY, Aug 27 '17 at 16:45
I've thought about it but haven't really come to any conclusion if one of the options is better, and why? — Nikita Zaliven, Aug 27 '17 at 16:48
It crashes because `userpath` has not been initialized when you do `strcat(userpath, path);` — Paul Ogilvie, Aug 27 '17 at 16:48
Also `"Choose a password(max 15 letters): "` should be `"max 13 letters"` because `char password[15];` can hold 13 characters plus the newline `'\n'` that you type, and the `NUL` terminator. — Weather Vane, Aug 27 '17 at 16:53
Ditto for `char username[10];` Where `"max 10 letters"` it should be `"max 8 letters"`. You will probably want to [remove the trailing newline character from fgets() input](https://stackoverflow.com/questions/2693776/removing-trailing-newline-character-from-fgets-input/28462221#28462221). — Weather Vane, Aug 27 '17 at 16:56
@NikitaZaliven: you can accept one of the answers by clicking on the grey checkmark below its score. — chqrlie, Sep 03 '17 at 06:39

score 0 · Answer 1 · answered Aug 27 '17 at 17:00

0

As above really or initialize the strings first.

memset(userpath, 0, sizeof(userpath));
memset(usertxt, 0, sizeof(usertxt));

answered Aug 27 '17 at 17:00

AdyB

11
1

1

The usual way is with `char userpath[256] = "";` or with `char userpath[256] = {'\0'};` but as commented above it is better to use `strcpy` anyway. – Weather Vane Aug 27 '17 at 17:07

chqrlie · Answer 2 · 2017-08-29T05:33:57.140

Your program has multiple issues:

you concatenate a string into an uninitialized array: this has undefined behavior and may well explain the observed behavior on different systems as undefined behavior may take multiple forms from no consequence at all to a crash or possibly even worse misfortune.
you do not test the return values of scanf, fopen, fgets(). Any invalid or unexpected input may trigger more undefined behavior.
you do not check for potential buffer overflow in strcat(). You should instead use snprintf().

Here is a safer version:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int registerUser(char *username, size_t size);

char path[] = "C:\\Users\\magshimim\\Desktop\\cproject\\bank\\";

int main(void) {
    int choice = 0;
    int c;
    char username[10];
    printf("Welcome to nikitosik's bank program!\n"
           "Choose an option :\n"
           "0 - Register\n"
           "1 - Login to existing account\n");
    if (scanf("%d", &choice) != 1)
        return 1;
    // consume the rest if the input line
    while ((c = getchar()) != EOF && c != '\n')
        continue;
    if (choice == 0) {
        registerUser(username, sizeof(username));
    } else {
        //login();
    }
    return 0;
}

int registerUser(char *username, size_t size) { // username is passed, for verification later
    char usertxt[256];  // path of data
    char userpath[256]; // path of new user data file
    char password[17];
    FILE *userinfo;     // file that contains info of all registered users
    FILE *userpathf;    // file to open for txt file

    printf("Choose a username(max %d letters): ", (int)(size - 2));
    if (!fgets(username, size, stdin))
        return -1;
    username[strcspn(username, "\n")] = 0;  // strip newline if any

    printf("Choose a password(max 15 letters): ");
    if (!fgets(password, sizeof(password), stdin))
        return -1;
    password[strcspn(password, "\n")] = 0;  // strip newline if any

    if (snprintf(usertxt, sizeof(usertxt), "%s%s", path, "users.txt") >= (int)sizeof(usertxt))
        return -1;
    if (snprintf(userpath, sizeof(userpath), "%s%s.txt", path, username) >= (int)sizeof(userpath))
        return -1;

    userinfo = fopen(usertxt, "a");
    if (userinfo == NULL)
        return -1;

    // create user file
    userpathf = fopen(userpath, "w");
    if (userpathf != NULL)
        fclose(userpathf);

    // add user info
    fprintf(userinfo, "%s\n%s\n", username, password);
    fclose(userinfo);
    return 0;
}

fgets - crashes in visual studio, works in gcc

2 Answers2