Segmentation Fault on calling a function

Question

When I call the print_linklist function I am getting a segmentation fault. Here is the function definition:

//will display the node in a nice string
char * term_to_string(term_t * term){
    int exp = term->exponent;
    int coef = term->coefficient;
    return ("%dx^%d", coef, exp);
}

**//will print the list using the nodde_to_string method
void print_linklist(node_t * curr){
    printf("entering print to list!!!");
    node_t * current = curr;
    while(current != NULL){
        printf("%s +", term_to_string(curr->term));
        current = current->next_node;
    }
}**

And here is the main method where it is being called:

/* This is your main file */
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include"common.h"
#include"buildlinklist.h"
#include"printandcombine.h"
int main() {
    node_t * node_ptr = NULL;
    node_t * new_node_ptr=NULL;
    printf("NAME: SAMPLE OUTPUT\n");
    
    /* Build linklist */
    read_objects(&node_ptr);
    
    /* Print the link list */
    printf("Original: \n");
    print_linklist(node_ptr);
    
    /* Combine like terms in the link list and craeate a new link list */
    new_node_ptr=combine_like_terms(node_ptr);
    printf("\nCombined: : ");
    /* Print new combine linklist */
    print_linklist(new_node_ptr);
    printf("\nNAME: SAMPLE OUTPUT\n");
    free(node_ptr);
    free(new_node_ptr);
    return 0;
}

After the function is called I get "zsh: segmentation fault ./project1". I don't even get the "entering print to list!!!" to print from the print_linklist method.

Answer 1

return ("%dx^%d", coef, exp);

That's not doing what you seem to think it's doing (what I think you think it's doing is to return a string created by some sort of printf functionality).

However, the comma operator in something like a, b evaluates both a and b, but the result is b. The n-variant, like a, b, c, d, evaluates everything and returns the last one (d).

Hence you are returning exp as if it was a character pointer. It almost certainly isn't (since you're trying to printf it with %d) so, if you treat it as such, hilarity may ensue.

Well, less hilarity and more crashing/weirdness, but you get the idea :-)

---

You could create strings in heap memory and pass them around but it's sometimes difficult for newcomers to the language to do that safely. Instead, I would suggest simply printing the thing within the function, with something like:

void print_term(const term_t *term, const char *after){
    printf("%dx^%d%s", term->coefficient, term->exponent, after);
}

void print_linklist(node_t *curr){
    puts("entering print to list!!!");
    node_t *current = curr;
    while (current != NULL){
        print_term(curr->term, " +");
        current = current->next_node;
    }
}

---

You'll notice my "entering print to list" statement is subtly different to yours in that it uses puts, which appends a newline character to the end (you could do that explicitly, with \n, if you wanted to stick with printf).

Since standard output is line-buffered for terminal devices, the reason you're not seeing that is almost certainly due to the fact your code is crashing (as per this answer) before it flushes. In that case, unflushed data is likely to just disappear.

Answer 2

I don't even get the "entering print to list!!!" to print from the print_linklist method.

printf doesn't cause anything to be printed to the screen. It just copies a string into a buffer to be printed at some later time. If you want to force the buffer to be printed, you can call fflush. As mentioned in comments, if stdout is directed to an interactive device, including a \n in the output string will generally also cause the buffer to be flushed to the actual output device.

After the function is called I get "zsh: segmentation fault ./project1".

This probably happens in term_to_string if you call it with an invalid pointer argument, when it tries to dereference its argument.

Answer 3

There could be a number of reasons you are getting a segmentation fault but trying to print the list is not one of them. If your read_objects() declaration is like this read_objects(node_t * curr) then you shouldn't pass as an argument the address of the pointer

read_objects(&node_ptr);

This should be read_objects(node_ptr);

Also, if you want to iterate through the list and print it you should create a pointer to the head of the list in the main() function node_t *head = NULL; and pass it as a parameter in the print_linklist(head);.

After that, make *current point at the head of the list node_t * current = head;.

That way you are starting printing the nodes from the start. You should also check if the list is empty.