Question

I’m writing a small program that prints a linked list. This list contains a string and a pointer to the next node.

I pass the linked list to a function that adds a new node and fills data field.

When I get back to main function and try to print the list content I get segmentation fault error, although from the function add_node I can print the content of the node.

I want to be able to pass a list and a string to a function and the function must add a new node to the list with the string I passed.

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

struct node
{
    char filename[25];
    struct node *next;
};

typedef struct node LISTNODE;
typedef LISTNODE *LISTNODEPTR;

void add_node(LISTNODEPTR *nFile, char *chaData);

int main (int argc, char *argv[])
{
    LISTNODEPTR nFile = NULL;

    printf("Printing list\n");

    add_node(&nFile, "file.txt");

    printf("%s\n", nFile->filename);

    free(nFile);

    return 0;
}

void add_node(LISTNODEPTR *nFile, char *chaData)
{
    LISTNODEPTR head = *nFile;
    LISTNODEPTR newNode;

    newNode = (LISTNODEPTR) malloc(sizeof (struct node));

    strcpy(newNode->filename, chaData);

    printf("%s\n", newNode->filename);

    newNode->next = head; //link next. newNode next points to head (beginning of the list). At this time (head & newNode)->2nd->3rd->NULL

    head = newNode;
}

Output:
Printing list
file.txt
Segmentation fault

OS:
Linux sisdvb02 2.6.35-28-server #49-Ubuntu SMP Tue Mar 1 14:55:37 UTC 2011 x86_64 GNU/Linux

Compiler: gcc -Wall -o list list.c

Answer 1

nFile in your add_node() function does not change. It still points to NULL and since you try to dereference a NULL you get a segmentation violation error. Btw, use Valgrind to help you with such problems.

Change your function to something like this:

void add_node(LISTNODEPTR *nFile, char *chaData)
{
    LISTNODEPTR head = nFile;
    LISTNODEPTR newNode;

    newNode = (LISTNODEPTR) malloc(sizeof (struct node));

    strcpy(newNode->filename, chaData);

    printf("%s\n", newNode->filename);

    newNode->next = head; //link next. newNode next points to head (beginning of the list). At this time (head & newNode)->2nd->3rd->NULL

    head = newNode;
    nFile = head;
}