Segmentation fault using argv[1] - c

Question

I created a code that inserts and deletes nodes from a Linked List depending on what a .txt file instructs. I previously used scanf("%s",fname) to open the file but now I want to open it using the command line, specifically argv[1] == file name to open it and read from it. Now that I decided to use this I keep getting a segmentation fault. Any ideas why this is occurring? I just started taking a C course in my University and am brand new to it.
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
typedef struct node{
int val;
struct node *next;
} Node;
void count_node(Node *head){
Node *temp;
int i=0;
temp = head;
while(temp!=NULL){
i++;
temp=temp->next;
}
printf("number of nodes are %d \n ",i);
}
void displayList(Node *head){
Node *temp;
temp = head;
while(temp != NULL){
printf(" %d ",temp->val);
temp = temp->next;
}
}
Node * findBefore(Node *head,int value){
Node *curr = head;
Node *prev = NULL;
while(curr != NULL){
if(value <= curr->val){
return prev;
}
prev = curr;
curr = curr->next;
/*printf("a\n");
break;*/
}
return prev; // 1 2 4
//printf("b\n");
}
void add(Node *head,int newValue){ //return a Node 0 for some reason
ask TA
printf("check \n");
Node *newNode = malloc(sizeof(Node));
if(newNode == NULL){
printf("check \n");
return;
}
newNode->val = newValue;
//if list is empty:
if(head == NULL){
printf("check \n");
head = newNode;
}
//if list is not empty:
Node *prev = findBefore(head,newNode->val); //a -> b
if(prev == NULL){
printf("check \n");
newNode->next = head;
head = newNode;
return;
}
printf("check \n");
Node *temp = prev->next;
prev->next = newNode;
newNode->next = temp;
}
void delete(Node *head, int delVal){
if(head == NULL){
return;
}
Node *prev = NULL;
Node *curr = head;
if(delVal == curr->val && prev !=NULL ){ // 1 2
prev->next = curr->next;
free(curr);
return;
}
else if(delVal == curr->val && prev == NULL){ // 1
head = NULL;
free(curr);
return;
}
while(curr != NULL) {
while(curr != NULL && curr->val != delVal){
prev = curr;
curr = curr->next;
}
if (curr == NULL){
return;
}
prev->next = curr->next;
free(curr);
curr = prev->next;
}
}
int main(int argc,char *argv[]) {
FILE * fp;
char singleLine[150];
char command;
int val;
fp = fopen(argv[1],"r");
if(fp == NULL ){
exit(1);
}
Node *head = malloc(sizeof(Node));
if(head == NULL){ //malloc returns null is there is a memory leak
return 1;
}
while(fgets(singleLine,100,fp)){
//printf("%s\n",singleLine);
sscanf(singleLine,"%c %d",&command,&val);
//printf("command character is: %c\n" , command);
//printf("The value we have to add is: %d\n",val);
if(command == 'i'){
printf("Found i, and add %d\n",val);
add(head,val);
}
else if(command == 'd'){
delete(head,val);
printf("found d and delete %d\n",val);
}
}
free(head);
fclose(fp);
printf("\n");
count_node(head);
printf("The linked list is : \n ");
displayList(head);
return 0;
}

Answer 1

What is the command you are using to run the program? Especially in larger projects, when we need to deal with external input, it's good practice to check it's validity.
Maybe something like:
if (argc < 2) {
println("Invalid input: No arguments");
return 1;
}
char *filename = argv[1];
fp = fopen(filename,"r");
To see what may be the reason the call is failing. Try to add this short debug code before your call to argv[1]:
println("Number of arguments: %d", argc);
for (int i = 0, i < argc, i++) {
println("Argument %d: \"%s\"", i, argv[i]);
}