[SOLVED] C linked list...why am i getting segfault?

[S15_88]

hi i'm wondering why i'm getting a segfault before the user can input data. I had the program running fine earlier before i tried to make a list. Just wondering if i'm doing something wrong with respect to the pointers. If anyone could give me some tips on how to make a list and stuff that would be great as i'm pretty confused right now.

i'm guessing that it has something to do with setting the pointers to NULL?

Thanks, ALain



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

#define MAX_LEN 250

struct node
{
char *name;
int age;
struct node *nextName;
struct node *nextAge;
};

typedef struct node NODE;

int drain_stdin()
{
int c;

while((c = getchar()) != '\n' && c != EOF );

return c;
}

void addStruct(NODE *root, NODE *ptr)
{
root = NULL;
ptr = NULL;

if (drain_stdin() != EOF)
{
printf("Enter name: ");
fgets(ptr->name, MAX_LEN + 1, stdin);

printf("Enter age: ");
scanf("%d", &ptr->age);

ptr->nextName = root;
ptr->nextAge = root;
root = ptr;
}
else
{
printf("An Error Has Occured\n");
}

}

void removeStruct(NODE *root, NODE *ptr)
{
char nameRemove[MAX_LEN + 1];

root = NULL;
ptr = NULL;

if (drain_stdin() != EOF )
{
printf("Enter Name Of Desired Record To Be Removed: ");
fgets(nameRemove, MAX_LEN + 1, stdin);

if(strcmp(nameRemove, ptr->name) == 0)
{
printf("Found The Record\n");
}
else
{
printf("There Is No Record With That Name\n");
}

}
else
{
printf("An Error Has Occured\n");
}
}

void printName(NODE *root, NODE *ptr)
{
root = NULL;
ptr = NULL;

printf("%s %d\n", ptr->name, ptr->age);
}

void printAge(NODE *root, NODE *ptr)
{
root = NULL;
ptr = NULL;

printf("%s %d\n", ptr->name, ptr->age);
}


void progExit(NODE *root, NODE *ptr)
{

}


int main()
{
int choice;

NODE *root, *ptr;

ptr = malloc(sizeof(NODE));
ptr->name = malloc(sizeof(char)*MAX_LEN);

do
{
printf("1. Add structure\n");
printf("2. Remove structure\n");
printf("3. Print names\n");
printf("4. Print ages\n");
printf("5. Exit\n");
scanf("%d", &choice);

if(choice == 1)
{
addStruct(root, ptr);
}
else if(choice == 2)
{
removeStruct(root, ptr);
}

else if(choice == 3)
{
printName(root, ptr);
}
else if(choice == 4)
{
printAge(root, ptr);
}
else if(choice == 5)
{
progExit(root, ptr);
}
else
{
return 0;
}

}while(choice != 5);

return 0;
}

[Jessehk]

EDIT: I skimmed your post the first time, so I didn't realize you suspected the NULL pointers were the problem. Do you actually have a good idea of how pointers work in C? I ask not be critical, but randomly changing things and settings pointers to NULL (ie, trial and error) will likely not get you anywhere. My favorite reference for pointers is here (http://www.eternallyconfuzzled.com/tuts/languages/jsw_tut_pointers.aspx), but be prepared to learn.
If your looking at writing linked lists, the same site has a great (and lengthy, but in a good way) guide on them here (http://www.eternallyconfuzzled.com/tuts/datastructures/jsw_tut_linklist.aspx). Good luck, and try to ask better questions in the future. :)

Despite the terrible way in which you asked this question (no line numbers, no indications of expected behavior, etc), I'll tell you that in
addStruct(), you set


ptr = NULL;


and then dereference it when you call fgets(). Dereferencing a NULL pointer will usually result in a segfault, but AFAIK is actually undefined behavior. You do the same thing in removeStruct(), and printName().

[S15_88]

sorry about the phrasing of the question, and i figured the problem out, my apologies i didn't look hard enough before i posted.

Thanks, ALain

[stevescripts]

The OP was on the right track to suspect a NULL pointer.

A little levity - for any of you who may never have seen this -

The Ten Commandments for C Programmers : http://www.lysator.liu.se/c/ten-commandments.html

Hope somebody gets a smile - I know I certainly did, the first time somebody
pointed me to this link.

Steve

[Jessehk]

my apologies i didn't look hard enough before i posted.

That's ok. Read the edit. :)