为什么我不能从C中的函数传回链表？

我正在尝试通过将指针传递到列表的开头来在函数内部创建链接列表。在函数内部，一切正常。但是，当我回到时main()，指针突然变为NULL。因此，如果我再次调用该函数，则它的行为就像是我第一次添加节点一样。

我的代码有什么问题？

struct course
{
    int c_ID;
    char *c_name;
    struct course *c_next;
};

void new_course(struct course *c_head, struct course *c_tail); // adds a node

int main ( )
{
    // variable declarations
    int choice;
    char y_n;

    // create linked lists
    struct course *c_head = NULL;
    struct course *c_tail = NULL;

    // print out menu, obtain choice, call appropriate function; loop if desired
    do
    {
        printf("\t\t\t***MENU***\n"
               " 1. Add a new course\n\n"
               ................................
               "Enter the number of the menu option you wish to choose: ");
        scanf("%d", &choice);

        switch (choice)
        {
            case 1:
                new_course(c_head, c_tail);
                if (c_tail == NULL)
                {
                    printf("We're screwed.\n"); // this excecutes every time
                }
                break;
            .....................
        }

        printf("Would you like to return to the main menu? Enter y for yes, n for no: ");
        scanf(" %c", &y_n);

    } while (y_n != 'n' && y_n != 'N');

    // free courses
    struct course *c_temp = NULL;
    c_temp = c_head;
    while (c_temp != NULL)
    {
        c_head = c_head->c_next;
        c_temp->c_ID = 0;     // reinitialize the student ID
        c_temp->c_name[0] = '\0'; // reinitialize the student name string
        free(c_temp->c_name);     // return the string memory to the system
        free(c_temp);         // return the node memory to the system
        c_temp = c_head;      // set temp to next item in the list
    }

    return 0;
}

void new_course(struct course *c_head, struct course *c_tail)
{
    // declare variables
    int ID;
    char name[50];

    // obtain user input
    printf("Enter the course ID number and the course name, separated by a space: ");
    scanf("%d%s", &ID, name);

    if(c_head == NULL) // no courses yet
    {
        c_head = (struct course *) malloc(sizeof(struct course)); // allocate memory for c_head
        c_head->c_next = NULL;
        c_tail = c_head; // update c_tail
    }
    else // the list already has nodes
    {
        c_tail->c_next = (struct course *) malloc(sizeof(struct course)); // allocate memory for new node
        c_tail = c_tail->c_next; // update c_tail
        c_tail->c_next = NULL;
    }

    c_tail->c_ID = ID; // assign ID to c_ID component of new node
    c_tail->c_name = (char *) malloc(sizeof(char) * strlen(name) + 1); // allocate memory for c_name component of new node
    strcpy(c_tail->c_name, name); // assign name to c_name component of new node
    printf("%d = %d, %s = %s\n", c_head->c_ID, ID, c_tail->c_name, name); // this always works, proving the list was created and the assignments worked

    return;
}