我的C程序没有编译或给出分段错误 [英] My C program does not compile or gives segmentation fault

问题描述

Hi all I am trying to make a sample queue program. I just started with this concept and I am seeing a problem

    #include <stdio.h>
    #include <stdlib.h>
    struct node {
    	int data;
    	struct node *next;
    };
    
    struct queue{
    	struct node *front;
    	struct node *rear;
    };
    
    struct queue *q=NULL;//pointer is initialised
    q->front = NULL;//pointer members are initialised
    q->rear = NULL;//initialised
    
    
    
    int main(void)
    {
    struct node *ptr = NULL;

    /*q->front = NULL;
    q->rear = NULL;*/
    
    ptr = (struct node *)malloc(sizeof(struct node));
    if(ptr == NULL)
    {
    	printf("error allocationg memory\n");
    	return 1;
    }
    ptr->next = NULL;
    ptr->data = 10;
    if(q->front == NULL)
    	{
    		q->front = ptr;
    		q->rear = ptr;
            //rest of the logic
       	}
    free(ptr);
    return 0;
    }

I am getting two problems,

1)If I create 

    q->front = NULL;
    q->rear = NULL;

as global I get this warning

    error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘->’ token
     q->front = NULL;
      ^
    queue.c:15:2: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘->’ token
     q->rear = NULL;

2) If I create same variables inside main, I can compile, but at runtime I am getting segmentation fault.
Why is this happening?
my setup is **gcc version 4.8.4 (Ubuntu 4.8.4-2ubuntu1~14.04.3)** 
maybe this is a stupid question.

我尝试过的事情：



我甚至无法理解变量为NULL，它给我编译错误。

如何理解这个问题并解决它？

What I have tried:

I am not able to understand that even I initialise variables to NULL, its giving me compilation errors.
How to understand this problem and solve it?

推荐答案

引用：

我的C程序没有编译或给出分段错误

My C program does not compile or gives segmentation fault

你需要选择，程序给出编译错误或给出分段错误，但不是两者。



你需要学习

- 动态内存分配（alloc或malloc）

- 指针

- 使用 - >

You need to choose, the program gives compile error or gives segmentation fault, but not both.

You need to study
- Dynamic memory allocation (alloc or malloc)
- pointers
- Usage of ->

Quote：

struct queue *q=NULL;//pointer is initialised

不，它没有初始化，也无法为成员分配值。



以下是该语言作者对C语言参考书的链接。

C编程语言 - 维基百科，免费的百科全书 [ ^ ]

https://hassanolity.files.wordpress.com/2013/ 11 / the_c_programming_language_2.pdf [ ^ ]

http：// www。 ime.usp.br/~pf/Kernighan-Ritchie/C-Programming-Ebook.pdf [ ^ ]

No, it is not initialized, and you can't assign values to members.

Here is links to references books on C by the authors of the language.
The C Programming Language - Wikipedia, the free encyclopedia[^]
https://hassanolity.files.wordpress.com/2013/11/the_c_programming_language_2.pdf[^]
http://www.ime.usp.br/~pf/Kernighan-Ritchie/C-Programming-Ebook.pdf[^]

1）

q->front = NULL;//pointer members are initialised
q->rear = NULL;//initialised

这些是指令。说明必须放在功能中。使用此类函数外，会引发错误。唯一的例外是在声明时初始化变量，如

These are instructions. Instructions must be placed within functions. When using such outside of functions, an error is thrown. The only exception is the initialisation of a variable upon declaration as in

struct queue *q=NULL;

2）

您的 q 指针初始化为 NULL 但它没有指向有效的 struct queue 对象。所以

2)
Your q pointer is initialised with NULL but it is not pointing to a valid struct queue object. So

q->front = NULL;

会尝试将零值写入地址零，这会导致分段错误。 />




你必须先创建一个 struct queue 对象并让 q 指向：

will try to write the value zero to the address zero which results in a segmentation fault.


You have to create a struct queue object first and let q point to that:

/* Base structure initialised with NULL pointers */
struct queue base = { NULL, NULL };
/* Pointer to the base structure */
struct queue *q = &base;

上面解决了两个问题：

它声明了两个全局变量并初始化它们但不是使用任何说明。

The above solves boths problems:
It declares two global variables and initialises them but does not use any instructions.

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