Segmentation fault passing struct pointer to function

Question

I'm pretty newb in C and in order to learn about structs I'm building a program which provides a limited set of functionality for a struct ll_string which basically is a linkedlist of strings.

The set of functions I'm trying to implement includes an insert_ll_string() function which should concanate a passed in struct ll_string element to the end of another struct ll_string element but fails to do so because the moment the function is called in my test cases, the program crashes with a sig fault. This is at the STILL WORKS and SIG FAULT comments of the test_insert() function.

This is its header file:

file: ll_string.h

struct ll_string {
    char *string;
    struct ll_string *next;
};

struct ll_string *create_ll_string(char *, struct ll_string *);
void insert_ll_string(struct ll_string *, struct ll_string *);
void remove_item_from_ll_string(struct ll_string *, struct ll_string *);
void free_ll_string(struct ll_string *);
void print_ll_string(struct ll_string *);

and this is it's corresponding .c file missing a few definitions for functions declared in ll_string.h, but I guess my problem probably only revolves around the functions create_ll_string() and insert_ll_string() anyways.

file: ll_string.c

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

#include "ll_string.h"

/* create_ll_string: allocates memory for a new struct ll_string and 
 * initializes it with given arguments returns a pointer to new struct */
struct ll_string *create_ll_string(char *string, struct ll_string *next) {
    struct ll_string *new_ll_string;

    if (!string) {
        printf("string can\'t be NULL\n");
        return NULL;
    }

    if (*string == '\0') {
        printf("string needs to be at least 1 char long\n");
        return NULL;
    }

    if (!(new_ll_string = (struct ll_string *) malloc(sizeof(struct ll_string)))) {
        printf("couldn\'t allocate mem for new ll_string\n");
        exit(EXIT_FAILURE);
    }

    new_ll_string->string = strdup(string);
    new_ll_string->next = next;

    return new_ll_string;
}

/* insert_ll_string: concanates item to the end of dest */
void insert_ll_string(struct ll_string *dest, struct ll_string *item) {
    struct ll_string *cur;

    if (!dest) {
        printf("dest and item can\'t be NULL\n");
        return;
    }
    if (!item) {
        printf("item can\'t be NULL\n");
        return;
    }

    cur = dest;
    while (!cur->next) {
        cur = cur->next;
    }

    cur->next = item;

    return ;
}

/* remove_item_from_ll_string: removes item from list src */
void remove_item_from_ll_string(struct ll_string *src, struct ll_string *item) {
    return ;
}

/* printf_ll_string: prints each string in ll_string */
void print_ll_string(struct ll_string *ll_string) {
    if (!ll_string) {
        printf("ll_string is NULL\n");
        return ;
    }

    do {
        printf("%s\n", ll_string->string);
    } while (!(ll_string = ll_string->next));
}

/* free_ll_string: frees all memory pointed to by ll_string */
void free_ll_string(struct ll_string *ll_string) {
    struct ll_string *next;

    if (!ll_string) {
        return ;
    }

    while ((next = ll_string->next)) {
        free(ll_string->string);
        free(ll_string);
        ll_string = next;
    }
}

and here are my tests. Everything works fine until insert_ll_struct() is evoked by the test_insert() function. (test_create() works as expected) Tests are done using the MinUnit framework.

file: tests_ll_string.c

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

#include "minunit.h"

#include "ll_string.h"

#define MAX_ERROR_MSG_LENGTH 1000

int tests_run = 0;

static char *test_create(void) {
    struct ll_string *test_ll;
    struct ll_string *test_null_ll;
    char *empty_string = strdup("");
    char *null_string = NULL;
    char *correct_string = strdup("this should work");
    char *correct_string2 = strdup("this should also work");
    char *error_msg;

    if (!(error_msg = (char *) malloc(sizeof(char) * MAX_ERROR_MSG_LENGTH))) {
        printf("couldn\'t allocate mem for error msg");
        exit(EXIT_FAILURE);
    }

    // test_ll->string == correct_string
    // test_ll->next == NULL
    test_ll = create_ll_string(correct_string, NULL);
    sprintf(error_msg, "error, test_ll->string != \"%s\" is %s", correct_string, test_ll->string);
    mu_assert(
            error_msg,
            strcmp(test_ll->string, correct_string) == 0);

    // test_ll->next->string == correct_string
    // test_ll->string == correct_string2
    test_ll = create_ll_string(correct_string2, test_ll);
    sprintf(error_msg, "error, test_ll->string != \"%s\" is %s", correct_string2, test_ll->string);
    mu_assert(
            error_msg,
            strcmp(test_ll->string, correct_string2) == 0);

    sprintf(error_msg, "error, test_ll->next->string != \"%s\" is \"%s\"", correct_string, test_ll->next->string);
    mu_assert(
            error_msg,
            strcmp(test_ll->next->string, correct_string) == 0);

    test_null_ll = test_ll;
    test_null_ll = create_ll_string(empty_string, test_ll);
    // test_null_ll == NULL
    mu_assert(
            "error, test_null_ll != NULL",
            test_null_ll == NULL);

    test_null_ll = test_ll;
    test_null_ll = create_ll_string(null_string, test_ll);
    // test_null_ll == NULL
    mu_assert(
            "error, test_null_ll != NULL",
            test_null_ll == NULL);

    sprintf(error_msg, "error, test_ll->string != \"%s\" is \"%s\"", correct_string2, test_ll->string);
    mu_assert(
            error_msg,
            strcmp(test_ll->string, correct_string2) == 0);

    sprintf(error_msg, "error, test_ll->next->string != \"%s\" is \"%s\"", correct_string, test_ll->next->string);
    mu_assert(
            error_msg,
            strcmp(test_ll->next->string, correct_string) == 0);

    free_ll_string(test_ll);
    free(correct_string);
    free(correct_string2);
    free(empty_string);
    free(error_msg);

    return 0;
}

static char *test_insert(void) {
    struct ll_string *ll_test1;
    struct ll_string *ll_test2;
    struct ll_string *ll_test3;

    char *test_string1 = strdup("test_string1");
    char *test_string2 = strdup("test_string2");
    char *test_string3 = strdup("test_string3");
    char *error_msg;

    if (!(error_msg = (char *) malloc(sizeof(char) * MAX_ERROR_MSG_LENGTH))) {
        printf("couldn\'t allocate mem for error msg");
        exit(EXIT_FAILURE);
    }


    ll_test1 = create_ll_string(test_string1, NULL);
    ll_test2 = create_ll_string(test_string2, NULL);
    ll_test3 = create_ll_string(test_string3, NULL);

    // STILL WORKS

    insert_ll_string(ll_test1, ll_test2); // SEG FAULT
    insert_ll_string(ll_test1, ll_test3);

    sprintf(error_msg, "error, ll_test1->string != \"%s\" is \"%s\"", test_string1, ll_test1->string);
    mu_assert(
            error_msg,
            strcmp(ll_test1->string, test_string1) == 0);
    sprintf(error_msg, "error, ll_test1->next->string != \"%s\" is \"%s\"", test_string2, ll_test1->next->string);
    mu_assert(
            error_msg,
            strcmp(ll_test1->next->string, test_string2) == 0);
    sprintf(error_msg, "error, ll_test1->next->next->string != \"%s\" is \"%s\"", test_string1, ll_test1->next->next->string);
    mu_assert(
            error_msg,
            strcmp(ll_test1->next->next->string, test_string3) == 0);

    free_ll_string(ll_test1);
    free_ll_string(ll_test2);
    free_ll_string(ll_test3);
    free(test_string1);
    free(test_string2);
    free(test_string3);

    return 0;
}

static char *all_tests(void) {
    mu_run_test(test_create);
    mu_run_test(test_insert);
    return 0;
}

int main(int argc, char* argv[]) {
    char *result = all_tests();
    if (result != 0) {
        printf("%s\n", result);
    } else {
        printf("ALL TESTS PASSED\n");
    }
    printf("Tests run: %d\n", tests_run);

    return result != 0;
}

and this is the output of compilation and execution:

>> gcc -Wall -o test ll_string.c tests_ll_string.c
>> ./test
string needs to be at least 1 char long
string can't be NULL
[1]    6789 segmentation fault (core dumped)  ./test

What's causing this Sigmentation fault? I'm not accessing any memory besides local variables in the section the program crashes. I'm not dereferencing the pointers I'm passing to insert_ll_struct() at least not immediately after the function has been evoked.

Thanks in advance for the help

Answer 1

while (!cur->next) { cur = cur->next; } This code will cause crash suppose cur->next return null than you are trying to acess null next that is invalid you should put null check for cur in while loop like thiswhile (null!=cur&& !cur->next)

Answer 2

I think the answer is staring us in the face. In insert_ll_string() :

while (!cur->next) {

should be

while (cur->next) {

Answer 3

I'd look at the logic in free_ll_string(). Are you sure you aren't freeing memory twice? In the code it looks like if frees up all strings in the chain. Therefore I think you will free test_ll multiple times in test_create. See if you still get the error when disabling test_create, and if not then you issue I think is probably resulting from undefined behaviour because you are free-ing things more than once...

It is good practice to set any freed pointer to NULL after freeing the memory that it is pointing to, then you will avoid this problem.

/* free_ll_string: frees all memory pointed to by ll_string */
void free_ll_string(struct ll_string *ll_string) {
    struct ll_string *next;

    if (!ll_string) {
        return ;
    }

    while ((next = ll_string->next)) {
        free(ll_string->string);
        free(ll_string);
        ll_string = next;
    }
}