Program crashing when creating linked lists in C

Question

I'm having problems creating a function that creates two separate linked lists in C.

In my program, the user enters an equation such as 7 + 9 * 8, character by character, the program then makes lists with them.

The code is as follows: (Where place is where is should be in the list and data is the number/operator itself. Both come from another part of the program)

struct trees {

    char data;
    int posicion;
    struct trees *next;
    struct trees *childI;
    struct trees *childD;

};

    struct trees *root;
    struct trees *operador_uno;
    struct trees *numero_uno;

    char *numeros;
    char *operadores;
    int num, num_operadores;


void crearLista(int place, char data) {

    int i;

    struct trees *temp1 = (struct trees *)calloc(1, sizeof(struct trees));

    temp1->data = data;

    if(place == 0) {
        if((data == '/') || (data == '*') || (data == '+') || (data == '-')){
            temp1->next = operador_uno;
            operador_uno = temp1;
        }
        else {
            temp1->next = numero_uno;
            numero_uno = temp1;
        }

    }

    else {

        struct trees *temp2;

        if((data == '/') || (data == '*') || (data == '+') || (data == '-')) {
            struct trees *temp2 = operador_uno;
        }
        else {
            struct trees *temp2 = numero_uno;
        }

        for(i = 0; i < place - 1; i++) {
            temp2 = temp2->next; // [CRASH]
        }

        temp1->next = temp2->next;
        temp2->next = temp1; // [CRASH]

    }


    for(i = 0; i < place && place != 0; i++) {
        struct trees *temp1 = operador_uno;
        temp1 = temp1->next;
    }

    for(i = 0; i < place + 1; i++) {
        struct trees *temp2 = numero_uno;
        temp2 = temp2->next;
    }

}

I identified through a tonne of printf statements that it will add the first number in the equation to the list successfully, how it does not add the first operator and with the second number the program crashes altogether.

The crashing problem appears to occur in the places I have written [CRASH], when I put temp2->next = temp1.

Any help greatly appreciated!

Learning to use gdb would be a great gift to yourself in terms of time saved finding where things crash. — Richard
– Richard, Commented Aug 1, 2018 at 21:15

Jean-François Fabre · Accepted Answer · 2018-08-01 21:38:59Z

3

maybe not the only issue but:

    struct trees *temp2;

    if((data == '/') || (data == '*') || (data == '+') || (data == '-')) {
        struct trees *temp2 = operador_uno;  // not the same "temp2" as above
    }
    else {
        struct trees *temp2 = numero_uno;  // not the same "temp2" as above
    }

    for(i = 0; i < place - 1; i++) {
        temp2 = temp2->next; // [CRASH] because temp2 isn't initialized
    }

struct trees *temp2 = operador_uno; is shadowing temp2 declared in the outer scope. So the outer temp2 is never initialized, your initialization sets a value to a variable that goes out of scope.

So remove struct trees * so the same temp2 variable is used (and initialised), like this (I'd prefer a ternary expression, though):

 if((data == '/') || (data == '*') || (data == '+') || (data == '-')) 
 {
    temp2 = operador_uno;
 }
else 
 {
    temp2 = numero_uno;
 }

And turn on compiler warnings which would have told you that:

you're using the outer temp2 uninitialized
you're not using the inner temp2 variables

edited Aug 1, 2018 at 21:38

answered Aug 1, 2018 at 21:33

Jean-François Fabre♦

141k24 gold badges179 silver badges246 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

bruceg Over a year ago

Note that with gcc at least, you could turn on -Wshadow to see the warning about shadows, and then you'd get even more warnings than the ones you've mentioned.

Collectives™ on Stack Overflow

Program crashing when creating linked lists in C

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related