Thread: Problem passing by reference

Hello

This piece of code is supposed to print X but doesn't . It crashes. Any help in identifying the problem would be greatly appreciated !

Code:

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


typedef struct W {
 char c;
 struct W *next;
} list_node;


list_node *insert_list (char c, list_node *next_node) {
 list_node *temp_node = malloc(sizeof(list_node));
 temp_node->c = c;
 temp_node->next = next_node;
 return temp_node;
}


void read_input (list_node *list) {
 char c = 'X';
 list = insert_list(c, list);
}


int main (void) {
 list_node *list = NULL;
 read_input(list);
 printf("%c", list->c);
 return 0;
}

However, if I change line 27 to

Code:

list = read_input(list);

and change the read_input function to:

Code:

list_node *read_input (list_node *list) {
 char c = 'X';
 list = insert_list(c, list);
 return list;
}

then it does print X. But I do not want to return a value back to main(), I want it to pass by reference like the first example.

Any suggestions?

Originally Posted by Joe Bligh

But I do not want to return a value back to main(), I want it to pass by reference like the first example.

Any suggestions?

Since you want to pass the pointer by reference, pass a pointer to a pointer, e.g.,

Code:

list_node *list = NULL;
read_input(&list);

Of course, read_input has to change accordingly, e.g.,

Code:

void read_input(list_node **list) {
    char c = 'X';
    *list = insert_list(c, *list);
}

Note that you used malloc but neglected to free the memory allocated.