CODEWITHSUNDEEP
carraysstringsegmentation-fault
Zsolt Bíró
Zsolt Bíró

Reputation: 57

Segmentation fault while operating on user input string

Hi Im new to coding in C and I can't seem to find the problem which causes a segfault after the while loop starts.

int main() {
    char option;
    char nickname;

    printf("Welcome to our chat program. Please select mode (Send,Nickname,Exit): ");

    while (gets(option)) {

        if(!strncmp(option, "Exit", 4)) {
            break;
        }

        if(!strncmp(option, "Nickname", 8)){
            set_nickname(nickname);
        }

        if(!strncmp(option, "Send", 4)){
            if(!nickname){
            nickname = "Anonymous";
            printf("Your nickname was set to Anonymous!");
            }
            send_message(nickname);
        }
    }

Upvotes: 1

Views: 614

Answers (4)

Wouter Verhelst
Wouter Verhelst

Reputation: 1292

Never ever use gets; it cannot be used correctly. Instead, use fgets, with stdin as the third parameter.

You will notice that fgets takes the length of the storage you want to assign as its second parameter. The lack of this parameter in the gets function is why it cannot be used correctly. You should pass it the value 1 in your case, and pass it the address of your char variable. fgets will then read one character into your variable.

If you want to read more, you need to start by allocating more storage for the data you'll read. For instance, you could allocate space for 80 characters:

char string[80];

You can now use fgets with string as the first character, 80 as the second, and stdin as the third. fgets will then read up to 80 characters from stdin.

If you don't want to store that number in two locations in your source code, you can use the sizeof operator instead, or use a macro to predefine the size of your buffer.

Upvotes: 0

Sourav Ghosh
Sourav Ghosh

Reputation: 134316

There are many issues with the code. Let's discuss them one by one.

  • First of all, a char is not sufficient to hold a string, you need either

    • an array
    • a pointer with proper (dynamic) memory allocation.

    Using the first approach, you need to change

    char option;
char nickname;

    to

    #define SIZ 64

char option[SIZ] = {0};
char nicknamep[SIZ] = {0};

  • Then, instead of using gets(), you should be using fgets() to avoid the possibility of buffer overflow. Something like

    fgets(option, SIZ, stdin);

    will do.

  • That said, once option becomes an array, you cannot assign to it. You need to use strcpy() to copy the content into the array. For example,

    strcpy(nickname, "Anonymous");

Upvotes: 3

Danny_ds
Danny_ds

Reputation: 11406

There is no memory allocated for the strings.

You need someting like this:

char option[50];
char nickname[50];

and use strcpy() here:

nickname = "Anonymous";

and check the string with strlen() for example.

if(!nickname){

It is also safer to use fgets() with a length parameter to prevent buffer overflow.

while (gets(option)) {

Upvotes: 1

LoztInSpace
LoztInSpace

Reputation: 5697

option needs to be char[???]

Not sure about nickname but it also needs memory allocated and/or initialisation. How do you set it? What value do you use?

You'll also need to check for strlen(nickname)>0 rather than !nickname which will test for a null pointer, not an empty string.

Upvotes: 0

Related Questions