'Segmentation fault (core dumped)' error when trying to alter an array's values

Question

My program is an interactive calculator, where the user may enter something like 'add 2 5' and it runs the add() function, returning '5'.

The way my program works is that it uses strtok() to break the user's input into tokens: the first token determines what the program does (ie adds, divides, etc), and the next two are the values used in the function.

The problem I am having is with my while loop that tries to put the 2nd and 3rd tokens (the values) into an array of integers:

char input[MAX]; 
char *token;
int values[2];
int counter = 0;
        
fgets(input, MAX, stdin);
token = strtok(input, " ");

while (token != NULL) {
    token = strtok(NULL, " ");
    values[counter] = atoi(token);
    counter++;
}

Segmentation fault (core dumped)

An example of how the program is supposed to interpret the information:

if (strcmp(token, "add") == 0) {
    answer = add(values[0], values[1]);
}

An example of the add() function:

int add(int x, int y) {
    int z = x + y;
    return z;
}

Is there something I am doing wrong?

Answer 1

You must check if token is NULL after token is updated. Also, you will want to save the pointer to the first token (command) to interpret it later.

char input[MAX]; 
char *token;
char *command; /* add this */
int values[2];
int counter = 0;
        
fgets(input, MAX, stdin);
token = strtok(input, " ");
command = token; /* add this */

while (token != NULL) {
    token = strtok(NULL, " ");
    if (token == NULL) break; /* add this */
    values[counter] = atoi(token);
    counter++;
}

Then, use the saved pointer to interpret.

if (strcmp(command, "add") == 0) { /* use command, not token */
    answer = add(values[0], values[1]);
}