segmentation fault (core dumped)?

I am fairly new to using pointers and queues and such, but this is a program we're required to write for class and I finally got it to compile completely without any errors or warnings and now I go to run it with a.out and all I get is the message in the title of this post: "segmentation fault (core dumped)". does anyone know what this means or how to fix it? My code is below. I also included my Makefile because I don't have a lot of experience with those either and thought maybe that was the problem. Thanks in advance for any help!

this is my queue.c

Code:

 
  
#include <stdio.h>
 #include <stdlib.h>
 #include "boolean.h"
 #include "queue.h" 

int menu(void);
 
int main(void){
  queue q;
 int selection, data;
 boolean quit=FALSE;

init_queue(&q);
while(!quit){
   scanf("%d", &selection);
   switch(selection){
     case 1:
       if(is_full()){
         printf("The queue is full\n");
       }
       else{
         printf("Please enter a number to add to the queue.\n");
         scanf("%d", &data);
         add(&q, data);
       }
     break;
     case 2:
       if (is_empty(q)){
          printf("The queue is already empty\n");
        }
        else{
          data = fetch(&q);
          printf("%d was removed\n\n", data);
      }
        break;
     case 3:
       print_queue(q);
       break;
     case 4:
       quit=TRUE;
     default:
       printf("Sorry, but that is not a valid selection.\n\n");
       break;
 }
 if(!quit) menu();
}
 }
  
int menu(void){
 printf("1. Add\n"); printf("2. Fetch\n"); printf("3. List\n"); printf("4. Quit\n"); printf("Enter a selection: "); 
}

This is my queue.h

Code:

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

typedef struct queuenode{
    int data;     struct queuenode *next;
} *node_pointer;

typedef struct endpoint{
    node_pointer front;
    node_pointer back;
}*queue;  

void init_queue(queue *);
boolean is_full(void);
boolean is_empty(queue);
void add(queue *, int);
int fetch (queue *);
void print_queue(queue);

 void init_queue(queue *q){   
(*q) -> front = NULL;   
(*q) -> back = NULL; 
}

 boolean is_full (void){
 node_pointer temp;
 temp = (node_pointer) malloc(sizeof(struct queuenode));
 if (temp == NULL){
   return TRUE;  
}
 else{
   free(temp);
   return FALSE;  
}
}

 boolean is_empty(queue q){
 if ((q -> front == NULL) && ((q -> back == NULL)))
  return TRUE;
 else   
return FALSE;
}  

void add(queue *q, int add){
 queue queue;
 queue = *q;
 node_pointer temp;
 temp = (node_pointer) malloc(sizeof(struct queuenode));
 temp -> data = add;
 temp -> next = NULL;
 if (is_empty(queue)){
   (*q) -> front = temp;
   (*q) -> back = temp;
 }
 else{
   (*q) -> back -> next;
   (*q) -> back = temp;  
}
}

 int fetch (queue *q){
 node_pointer temp;
 int fetch_data;  
temp = (*q) -> front;
 fetch_data = temp -> data;  
if (((*q) -> front) == ((*q) -> back)){
   (*q) -> front = NULL;
   (*q) -> back = NULL;  
}
 else{
   (*q) -> front -> next;
 free(temp);  return fetch_data;
 }
}

void print_queue (queue q){
 node_pointer temp;
 temp = q -> front;
while (temp!=NULL){
printf("%d\n", (temp -> data));
 temp = temp -> next;
 }

and this is my make file

Code:

CC = gcc
CFLAGS = -o a.out
LDFLAGS = -lm

OBJS = queue.o

a.out: $(OBJS)
    $(CC) $(CFLAGS) $(OBJS) $(LDFLAGS)

clean: 
    rm $(OBJS) a.out

print: enscript -Plab01 *.c *.h typescript

is it just on my screen or are the first two parts of my code in a long line? they won't stay the way I wrote them for some reason...

Eh, it is not just on your screen. How exactly did you edit your post? You had it in a text editor and copied and pasted your code, right? If so, which text editor, what line endings, and say, which operating system and web browser?

I sent it to myself in an email through gmail as an attachment, opened it in notepad (which looked wrong like it does here) so i tried internet explorer to open it instead and it looked right there. then copied and pasted using the code tags.

oh, and the text editor where it was all written was gedit. and im not sure what you mean by line endings.

Oh right. My suspicion is that you had gedit set to use *nix line endings on say, a Linux box, but then you opened this on Windows in a text editor that is only aware of Windows line endings (Notepad). Then you copied from Notepad, but because the line endings were not recognised as line endings, when you pasted into the text box input on this forum, you ended up with a result that was all on one line.

My suggestion is for you to use a better text editor on Windows too

For example, install Geany, Notepad++, or a host of other text editors more suited for dealing with source code, then use that text editor to open the file that you sent to yourself through Gmail, and paste it here for us to take a look at the code.

all right sorry guys, I got everything fixed... I just went through and manually put all the spaces in.

Thanks, though it would be good if you could confirm or deny my theory about the many lines becoming a single line.

Originally Posted by tkd_aj

all I get is the message in the title of this post: "segmentation fault (core dumped)". does anyone know what this means or how to fix it?

Typically, this is a symptom of undefined behaviour. Undefined behaviour can be caused by say, accessing an array out of bounds, or more likely in this case, dereferencing a null pointer or an invalid pointer (e.g., to an object that no longer exists, or a pointer that never pointed to an object).

My starting point would be to set breakpoints with your debugger and step through until you find the point where the segmentation fault occurs. The error most probably lies somewhere before that point (as in the flow of control).

Here's the queue code run through a pretty printer.

Code:

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

typedef struct queuenode{
    int data;
    struct queuenode *next;    
} *node_pointer;

typedef struct endpoint{
    node_pointer front;
    node_pointer back;
} *queue;

void init_queue(queue *);
boolean is_full(void);
boolean is_empty(queue);
void add(queue *, int);
int fetch (queue *);
void print_queue(queue);

void init_queue(queue *q){
    (*q)->front = NULL;
    (*q)->back = NULL;    
}

boolean is_full (void){
    node_pointer temp;
    temp = (node_pointer) malloc(sizeof(struct queuenode));
    if (temp == NULL){
        return TRUE;        
    }
    else{
        free(temp);
        return FALSE;   
    }    
}

boolean is_empty(queue q){
    if ((q->front == NULL) && ((q->back == NULL)))
        return TRUE;
    else
        return FALSE;    
}

void add(queue *q, int add){
    queue queue;
    queue = *q;
    node_pointer temp;
    temp = (node_pointer) malloc(sizeof(struct queuenode));
    temp->data = add;
    temp->next = NULL;
    if (is_empty(queue)){
        (*q)->front = temp;
        (*q)->back = temp;        
    }
    else{
        (*q)->back->next;
        (*q)->back = temp;   
    }    
}

int fetch (queue *q){
    node_pointer temp;
    int fetch_data;
    temp = (*q)->front;
    fetch_data = temp->data;
    if (((*q)->front) == ((*q)->back)){
        (*q)->front = NULL;
        (*q)->back = NULL;        
    }
    else{
        (*q)->front->next;
        free(temp);
        return fetch_data;   
    }
}

void print_queue (queue q){
    node_pointer temp;
    temp = q->front;
    while (temp!=NULL){
        printf("%d\n", (temp->data));
        temp = temp->next;   
    }
}

Your is_full function is madness. Get rid of it.

Your is_empty function can just be

Code:

boolean is_empty(queue q) {
    return q->front == NULL;
}

These statements don't do anything.

Code:

// in add
(*q) -> back -> next;
// in fetch
(*q) -> front -> next;

You're not really speaking my language, lol.. This is only my second programming class and I'm not really sure what you mean by "set breakpoints with your debugger". I don't really have a debugger... All we use at school is linux command line style interface, with a text editor and gcc compiler.

Originally Posted by tkd_aj

I don't really have a debugger... All we use at school is linux command line style interface, with a text editor and gcc compiler.

GDB is probably available. Unfortunately, gedit does not seem to have a GDB plugin. You use follow this tutorial to use GDB at the command line, or use a graphical front-end like DDD, if it is installed or if you can install it.

Did you even notice that I showed you a couple of statements that DON'T DO ANYTHING. Could it be that THAT'S YOUR PROBLEM? BTW, compiling with proper warning levels would have pointed those lines out to you. gcc -Wall ....

Also, look at this:

Code:

void add(queue *q, int add){
    queue queue;   // what? 
    queue = *q;    // but why?
    // those lines ultimately do nothing also

Originally Posted by oogabooga

Did you even notice that I showed you a couple of statements that DON'T DO ANYTHING. Could it be that THAT'S YOUR PROBLEM? BTW, compiling with proper warning levels would have pointed those lines out to you. gcc -Wall ....

Also, look at this:

Code:

void add(queue *q, int add){
    queue queue;   // what? 
    queue = *q;    // but why?
    // those lines ultimately do nothing also

lol, I know that looks weird, but the reason i did it was because I thought that *q was a pointer and i kept getting a warning until i did that. I didn't understand why, but it made the warning go away... is it actually hurting me?

I removed the

[code]

queue queue;
queue = *q;

but it didn't do anything. I am still getting the segment error, but there is no longer a warning about that part, so that's good.

Did you fix these lines yet?

Code:

// in add
(*q) -> back -> next;
// in fetch
(*q) -> front -> next;

If you changed your gcc invocation in your makefile by adding -Wall to CFLAGS like this

Code:

CFLAGS = -Wall -o a.out

then the compiler would point out those lines automatically. It will also notify you that something is wrong with your fetch routine's return. Nifty.

(And why would you call your output a.out in a makefile? Still, that's your business.)

Presumably in add you want

Code:

(*q) -> back -> next = temp;

I can't comprehend what fetch is attemting to do.

EDIT: I took another peek at fetch and I get it now. But you presumably want to free(temp) and return fetch_data whether or not it's the last node being removed.