postfix calculator having trouble with segfaults

Question

i'm trying to create a postfix calculator using linked-list. when i compile, it doesn't show any errors but when it's executed it would show Segmentation Fault. i don't know how to deal with this, please help. here's my code:

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

typedef struct node {
    int num;
    struct node *next;
} node;

void push(int num, node **head);
int pop(node **head);
void display(node **head);
int is_empty();
int evaluatePostfix();

struct node *head;

int main()  {
    head = NULL;
    char exp[1000]; // = "5 1 2 + 4 * + 3 -";
    printf("Input expression:\t");
    fgets(exp, 1000, stdin);
    for(int i = 1; i <= strlen(exp); i++) {
        if(exp[i] == '\n') {
            exp[i] = '\0';
        }
        else if (exp[0] == '\n') {
            printf("stack is empty\n");
            exit(0);
        }
    }
    printf("%s = %d\n", exp, evaluatePostfix(exp));

    return 0;

}

int evaluatePostfix(char* exp)  {
    char * token;
    int counter = 0;
    char temp[256][256];
    token = strtok(exp, " ");
    while(token != NULL)    {
        strcpy(temp[counter], token);
        counter++;
        token = strtok(NULL, " ");  
    }

    for (int i = 0; temp[i]; ++i)   {
        if (isdigit(*(temp[i]))) {
            int val = atoi(temp[i]);
            push(val, &head);
        }
        else   {
            int val1 = pop(&head);
            int val2 = pop(&head);
            switch (exp[i]) {
                case '+':    push(val2 + val1, &head);
                             printf("%d\n", (*head).num);
                             break;
                case '-':    push(val2 - val1, &head);    break;
                case '*':    push(val2 * val1, &head);    break;
                case '/':    push(val1 / val2, &head);    break;
            }
        }
    }
    return pop(&head);
}


void push (int item, node **head) {
    node *temp;
    node * get_node(int);
    temp = get_node(item);
    temp->next = *head;
    *head = temp;
}

node *get_node(int item) {
    node *temp;
    temp = (node*)malloc(sizeof(node));
    if (temp == NULL)
        printf("\nMemory cannot be allocated");
    temp->num = item;
    temp->next = NULL;
    return(temp);
}


int pop(node **head)    {
    int item;
    node *temp;
    item = (*head)->num;
    temp = *head;
    *head = (*head)->next;
    free(temp);
    return(item);
}

int is_empty(node *temp) {
    if (temp == NULL)
        return 1;
    else
        return 0;
}


void display(node **head)   {
    node *temp;
    temp = *head;
    if(head == NULL)    {
        printf("stack is empty\n");
        return;
    }
    printf("\n");
    printf("=========\n");
    while(temp!=NULL)   {
        printf("%d\n", (*temp).num);
        temp = (*temp).next;
    }
    printf("=========\n");
}

Answer 1

Given this declaration ...

char temp[256][256];

... the loop termination condition here is wrong:

for (int i = 0; temp[i]; ++i)   {

C multi-dimensional arrays are not Java-style arrays of array references. They are arrays of actual arrays. The expression temp[i] will not be false when i exceeds the number of elements of temp[] into which you have written data.

It looks like you want simply

for (int i = 0; i < counter; ++i)   {

. Alternatively, there's no particular need to tokenize before your start the computation. You could as easily combine the two loops in function evaluatePostfix() into one. That would be a bit simpler, and would remove any fixed limit on the number of terms in the expression.

Update: That might look like this:

for (token = strtok(exp, " "); token; token = strtok(NULL, " ")) {

    /* ... use token instead of temp[i] ... */

}

It is conceivable that there are other errors in your code as well, though I didn't spot any on my scan through it.