C: Segmentation fault with printing Link List - c

Question

I'm pretty new to C, but I wanted to build a program that could allows users store ip addresses obtained from a traceroute/tracert that is first stored into a text file. It then allows them to print the next/previous hop. I used a linked list, but the printing gives me a segmentation error.
I tried looking through it but I can't find any error, would someone point out my error and guide me? Thanks in advance!!
(Sorry for the poor indentation!!)
# include <stdio.h>
# include <string.h>
# include <stdlib.h>
int id = 0;
int list = 0;
int nodes = 0;
FILE *readFile(char *fileName)
{
FILE *rawdata = fopen(fileName, "r");
return rawdata;
}
void printMenu()
{
printf(" ========== MENU =============\n\n");
printf("1. Report ID of Previous Hops of a network node\n");
printf("2. Identify the next hops of a network node\n");
printf("3. Quit\n");
}
int getInput()
{
int choice;
printf("Please select your choice (1 to 3) =>: ");
scanf("%d", &choice);
return choice;
}
struct NodeInfo {
int ID;
int IP1;
int IP2;
int IP3;
int IP4;
struct NodeInfo *next;
struct NodeInfo *prev;
};
typedef struct NodeInfo Node;
Node *Topology;
Node *createNode(int ip1, int ip2, int ip3, int ip4)
{
Node *newNode = malloc(sizeof(Node));
newNode->IP1 = ip1;
newNode->IP2 = ip2;
newNode->IP3 = ip3;
newNode->IP4 = ip4;
newNode->next = 0; // NULL Pointer
newNode->prev = 0; // NULL Pointer
return newNode;
}
void addToBack(Node * tempnode)
{
Node *n = Topology;
Node *tail = 0;
while (n != NULL) {
tail = n;
n = n->next;
}
tail->next = tempnode;
tempnode->prev = tail;
}
void printFile(FILE * newFile)
{
char data[256], nth1[50], nth2[50], nth3[50], nth4[50], nth5[50],
nth6[50], nth7[50], ip[50], ip2[15], ip2new[14];
int linecount = -1, strlength;
int ip1, ip2x, ip3, ip4;
int ip11, ip21, ip31, ip41;
if (newFile == NULL) {
printf("There is an error with opening this file\n");
} else {
while (fgets(data, 256, newFile) != NULL) {
if (linecount != 3) {
linecount++;
continue;
} else {
if (linecount == 3 && data[2] != '\0') {
sscanf(data, "%s %s %s %s %s %s %s %s", nth1, nth2, nth3, nth4,
nth5, nth6, nth7, ip);
sscanf(data, "%s %s %s %s %s %s %s %d.%d.%d.%d", nth1, nth2,
nth3, nth4, nth5, nth6, nth7, &ip1, &ip2x, &ip3, &ip4);
if ((ip[0] <= 'z' && ip[0] >= 'a')
|| (ip[0] <= 'Z' && ip[0] >= 'A')) {
sscanf(data, "%s %s %s %s %s %s %s %s %s",
nth1, nth2, nth3, nth4, nth5, nth6, nth7, ip, ip2);
//Rescanning for anomaly results with additional hostname
strncpy(ip2new, ip2 + 1, strlen(ip2) - 2);
ip2new[strlen(ip2) - 2] = '\0';
int i;
char *temp;
char *ipcmp[4];
i = 0;
temp = strtok(ip2new, ".");
while (temp != NULL) {
ipcmp[i++] = temp;
temp = strtok(NULL, ".");
}
Node *tempnode = createNode(ip2new);
if (Topology != 0) {
addToBack(tempnode);
} else {
Topology = tempnode;
}
} else {
printf("%s\n", ip);
printf("%d.%d.%d.%d\n", ip1, ip2x, ip3, ip4);
Node *tempnode2 = createNode(ip);
if (Topology != 0) {
addToBack(tempnode2);
} else {
Topology = tempnode2;
}
continue;
}
}
if (linecount == 3 && data[2] == '\0') {
linecount = -2;
printf("\n");
}
}
}
}
}
void printNodes()
{
Node *n = Topology;
while (n != 0) {
printf("The node is %d.%d.%d.%d\n", n->IP1, n->IP2, n->IP3, n->IP4);
n = n->next; // Jump to next node
}
}
int main(int argc, char *argv[])
{
int option, fail;
FILE *filedata;
char *file;
file = argv[1];
filedata = readFile(file); //open file
printFile(filedata); //prints the ip addresses
do {
printMenu();
option = getInput();
switch (option) {
case 1:
printf("You have selected 1\n\n");
fail = 0;
printNodes();
break;
case 2:
printf("You have selected 2\n\n");
fail = 0;
break;
case 3:
fail = 1;
break;
default:
printf("Please enter a valid choice (1-3) \n");
fail = 0;
break;
}
} while (fail != 1);
while (Topology != 0) {
free(Topology);
Topology = Topology->next;
}
}

Answer 1

Your create node method have 4 parameters:
Node *createNode(int ip1, int ip2, int ip3, int ip4)
But you invoke this method by passing only one parameter:
Node *tempnode = createNode(ip2new);
Node *tempnode2 = createNode(ip);
Also you pass arrays when your method accept only integers.
This is at least two sources of errors in your code.

Answer 2

There are two corrections that I can spot. First is that you are calling createNode() with the wrong number of arguments. You have defined createNode() to take 4 integral arguments, but in each of your calls, you are passing it a string argument. The compiler should have given you a diagnostic about this issue (my compiler refuses to compile the code). You should never ignore a compiler diagnostic (at least, not without first fully understanding what the diagnostic means).
Change those calls to use the 4 integral variables that you scanned.
Node *tempnode = createNode(ip1, ip2x, ip3, ip4);
Node *tempnode2 = createNode(ip1, ip2x, ip3, ip4);
The second error is that your if (linecount != 3) { only allows you to process data when linecount is equal to 3, which is probably not what you want to do. You probably mean to process all lines of data after linecount becomes 3.
if (linecount < 3) {
linecount++;
continue;
} else {
I find it odd that you have initialized linecount to -1, but it just means you won't start processing the file until you reach the fourth line of input.
As has been mentioned by others, you cannot read from memory that has already been freed. Doing so causes undefined behavior. In your loop to release the memory held by Topology, save the pointer to the next item before freeing the current item.
while (Topology != 0) {
void *cur = Topology;
Topology = Topology->next;
free(cur);
}

Answer 3

There's a few issues here.
You mix your usage of NULL and 0. While both will work, in C it is idiomatic to use NULL, and it is best to be consistent in your usage.
Your createNode function expects 4 integers, but your code is calling this function and only providing one argument.
You have undefined behaviour in your loop:
while (Topology != 0) {
free(Topology);
Topology = Topology->next;
}
It is undefined behaviour to access Topology once it has been freed. In your case, you are attempting to access the next member of Topology after it has been freed.