Segmentation fault (core dumped) while using pointers in c

Question

I'm working on a project and trying to perform a similar operation as given below, but getting segmentation fault error. I don't understand why it is giving this error, even though I assigned the memory using malloc. Any help on this error is appreciated.

#include <stdio.h>  
 struct hello{  
  int i;  
};  
struct proc{  
  int j;  
  struct hello *hello[20];  
};  
int main()
{  
struct proc *proc;

proc->hello[0] = malloc(sizeof(struct hello));
proc->hello[0]->i =10; 

printf("value of i: %d\n",proc->hello[0]->i);

return 0;
}

What I find strange, is that you know you should mallocate something for `proc->hello[0]` to point at, and yet are completely fine with not setting `proc` itself to point at valid memory. — StoryTeller - Unslander Monica, Mar 03 '17 at 22:45
when you declare `struct proc *proc;`, the variable proc is an uninitialized pointer that doesn't have any memory associated. When you dereference it, i.e. `proc->hello[0]` you get a segmentation fault. Better to have your variable declared like this: `struct proc proc;`, and then use it like this: `proc.hello[0]`. — bruceg, Mar 03 '17 at 22:49
Also will be less confusing to name your variables, fields and struct types with different names. — bruceg, Mar 03 '17 at 22:50

brian.reeder · Answer 1 · 2017-03-04T04:11:23.490

1

It would be best practice for you to give your variables names separate from their typing.

so

struct proc *proc;

I would recommend something like

struct proc *my_proc;

However, the reason why you're seg faulting is that you're trying to access your *proc before it has been allocated any memory.

It might be NULL, but, more likely, it contains the memory address of a value equal to whatever left over memory is occupying the space that you should be storing a memory address.

So is it we assume it is NULL then what you have programmed is saying

Start Program
Give me a pointer at NULL
Go to NULL and malloc
-- SEG FAULT--

You just need to malloc your *proc

int NUM_PROC = 1;
struct proc *my_proc = malloc(sizeof(struct proc) * NUM_PROC);

edited Mar 04 '17 at 04:11

answered Mar 03 '17 at 23:14

brian.reeder

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1
5

1

The SO consensus is to _not_ cast the result of `malloc` and friends: http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc – yano Mar 03 '17 at 23:36
Heh, yeah. One of many things I learned on SO. In school, I was always taught to cast. In c++ you have to cast or you get a compiler error I believe – yano Mar 04 '17 at 04:16

Segmentation fault (core dumped) while using pointers in c

1 Answers1