How to create an array of struct pointers to NULL

Question

I want to create an array of struct pointers and set each pointer to null. Eventually I want to have the pointers in the array point to my struct. I believed I had the write code to this but I keep getting a seg fault.

Code:

//struct in my .h
struct bin{
 double cac; // current available capacity
 struct node *list //pointer to linked list... bin struct points to linked list struct
}
//main file
void main(){
 struct bin *bArray[20];
 struct bin *binTemp, *pTemp;
 for(i=0;i<20;i++) bArray[i]= NULL;
}

I assumed this would create an array of bin pointers, but I'm getting a seg fault here. Shouldn't I be able to make all the pointers NULL regardless of what type of pointer they are?

Eventually I want to make these all point to bin structs and I thought I could do this without having to make the pointers NULL first so i tried:

for(i=0;i<20;i++){
  binTemp = (struct bin *)malloc(sizeof(struct bin));
    binTemp->cac = 1.0;
    binTemp->list = NULL;
    bArray[i] = binTemp;
}   

Once again I got a seg fault. I have no idea whats going on here. I know seg faults means I'm trying to write to an illegal memory location which makes me think I would have to set the size of the array indexes with malloc. However,

for(i=0;i<20;i++) bArray[i]= malloc(sizeof(struct bin));

also gives me a seg fault. I have no idea what I'm doing wrong.

Actual code I ran:

//Header File

#ifndef hBin_h
#define hBin_h

#include <stdio.h> // stdio used for file io
#include <stdlib.h> // standard c library 

#define MAX_S 20


//bin struc
struct bin {
    double cac; //current available capacity
    struct node *list; // pointer to linked list
};
//linked list node struct
struct node{
    char *name; //name of item
    double size; // weight of item
    struct node *next; //pointer to next
};

//insert the new item into a node in its appropriate location using alphabetical ordering of item names
struct node *oInsert(char *item, double size, struct node *head);
//print the items of the list out along with the list’s capacity
void traverse(struct node *head);
// deallocate the nodes of the list
void destory(struct node *head);
//input info from file - name of object, weight of object
void input(FILE *inFile, char item[], double *weight);

#endif // hBin_h

#include "hBin.h"



void main(){
  FILE *inFile;
  char *item;
  double *weight;
  struct bin *bArray[20];
  int i;

  struct bin *binTemp, *pTemp;


  inFile = fopen("run1.txt", "r"); //open file
  printf("HERE1\n");

  for(i=0;i<20;i++){
      binTemp = (struct bin *)malloc(sizeof(struct bin));
      binTemp->cac = 1.0;
      binTemp->list = NULL;
      bArray[i] = binTemp;
  }   


/*while(!feof(inFile)){
input(inFile, item, weight);
printf("%s, %.2f\n", item, *weight);
}*/
}

I used gdb (not totally sure what I'm doing here):

(gdb) run
Starting program: /home/Christopher/CSC362/Pass_F/Main
[New Thread 813244.0xc7590]
[New Thread 813244.0xc6ce0]
[New Thread 813244.0xc7320]
[New Thread 813244.0xc5994]
HERE1
      0 [main] Main 813244 cygwin_exception::open_stackdumpfile: Dumping stack trace to Main.exe.stackdump
[Thread 813244.0xc7320 exited with code 35584]
[Thread 813244.0xc6ce0 exited with code 35584]
[Inferior 1 (process 813244) exited with code 0105400]
(gdb) where
No stack.
(gdb) for(i=0;i<20;i++){
    binTemp->cac = 1.0;
    binTemp->list = NULL;
    bArray[i] = binTemp;
}   /usr/src/debug/cygwin-2.2.1-1/winsup/cygwin/crt0.c: No such file or     directory.
(gdb)   binTemp = (struct bin *)malloc(sizeof(struct bin));
Undefined command: "binTemp".  Try "help".
(gdb)     binTemp->cac = 1.0;
Undefined command: "binTemp->cac".  Try "help".
(gdb)     binTemp->list = NULL;
Undefined command: "binTemp->list".  Try "help".
(gdb)     bArray[i] = binTemp;
Undefined command: "bArray".  Try "help".
(gdb) }   for(i=0;i<20;i++){
Undefined command: "".  Try "help".
(gdb)   binTemp = (struct bin *)malloc(sizeof(struct bin));
Undefined command: "binTemp".  Try "help".
(gdb)     binTemp->cac = 1.0;
 Undefined command: "binTemp->cac".  Try "help".
 (gdb)     binTemp->list = NULL;
 Undefined command: "binTemp->list".  Try "help".
 (gdb)     bArray[i] = binTemp;
 Undefined command: "bArray".  Try "help".
 (gdb) }   for(i=0;i<20;i++){
 binTemp->cac = 1.0;
 Undefined command: "".  Try "help".
 (gdb)   binTemp = (struct bin *)malloc(sizeof(struct bin));
 Undefined command: "binTemp".  Try "help".
 (gdb)     binTemp->cac = 1.0;
 Undefined command: "binTemp->cac".  Try "help".
 (gdb)     binTemp->list = NULL;
 Undefined command: "binTemp->list".  Try "help".
 (gdb)     bArray[i] = binTemp;
 Undefined command: "bArray".  Try "help".

Answer 1

Reading your code you commented out, you pass the variable weight to the input function and right after dereference it in your call to printf. There is a major problem here, and that is that the variable weight is not initialized, and that passing it to a function passes it by value, which means that the variable is copied and the function only operates on the copy and not the original, meaning that the weight variable in your main function will still be uninitialized when you dereference it and that leads to undefined behavior and a probable crash.

If you intend to emulate pass by reference (you can only emulate since C doesn't have passing by reference), you should declare weight as a normal variable, and use the address-of operator when calling the function:

double weight;

...

input(inFile, item, &weight);
//                  ^
//                  |
// Note ampersand here

---

You have a similar problem with the item variable. It's uninitialized and doesn't point anywhere special. Using it in any way except to initialize it will lead to undefined behavior. And if you try to initialize it in the input function then you have the same problem as described above, and you need to pass a pointer to the pointer using the address-of operator.

If you don't initialize the item pointer in the input function, but use it like it's already pointing to some valid memory (using e.g. strcpy or similar function) then you also have undefined behavior.

---

The actual crash you experience might be totally unrelated to the problems I describe above, since you seem to be doing something involving linked lists, which of course means pointers, and pointers used wrongly will give you many chances for further undefined behaviors and crashes.

The first thing you should do is enable more warning when building, as the compiler is usually very good at finding suspect behavior that can lead to UB. You do that by adding e.g. the flags -Wall -Wextra -pedantic when building.