崩溃或“分段错误"；当数据被复制/扫描/读取到未初始化的指针时 [英] Crash or "segmentation fault" when data is copied/scanned/read to an uninitialized pointer
问题描述
此问题旨在作为所有常见问题的参考:
This question is meant to be used as reference for all frequently asked questions of the nature:
当我将数据复制/扫描到未初始化指针指向的地址时，为什么会出现神秘的崩溃或分段错误"?
Why do I get a mysterious crash or "segmentation fault" when I copy/scan data to the address where an uninitialised pointer points to?
例如:
char* ptr;
strcpy(ptr, "hello world"); // crash here!
或
char* ptr;
scanf("%s", ptr); // crash here!
推荐答案
指针是一种特殊类型的变量，它只能包含另一个变量的地址.它不能包含任何数据.您不能将数据复制/存储到指针中"——这没有任何意义.您只能设置一个指针来指向其他地方分配的数据.
A pointer is a special type of variable, which can only contain an address of another variable. It cannot contain any data. You cannot "copy/store data into a pointer" - that doesn't make any sense. You can only set a pointer to point at data allocated elsewhere.
这意味着为了使指针有意义，它必须始终指向有效的内存位置.例如，它可以指向堆栈上分配的内存:
This means that in order for a pointer to be meaningful, it must always point at a valid memory location. For example it could point at memory allocated on the stack:
{
int data = 0;
int* ptr = &data;
...
}
或者在堆上动态分配内存:
Or memory allocated dynamically on the heap:
int* ptr = malloc(sizeof(int));
在初始化之前使用指针总是一个错误.它还没有指向有效的内存.
It is always a bug to use a pointer before it has been initialized. It does not yet point at valid memory.
这些示例都可能导致程序崩溃或其他类型的意外行为，例如分段错误":
These examples could all lead to program crashes or other kinds of unexpected behavior, such as "segmentation faults":
/*** examples of incorrect use of pointers ***/
// 1.
int* bad;
*bad = 42;
// 2.
char* bad;
strcpy(bad, "hello");
相反，您必须确保指针指向(足够)分配的内存:
Instead, you must ensure that the pointer points at (enough) allocated memory:
/*** examples of correct use of pointers ***/
// 1.
int var;
int* good = &var;
*good = 42;
// 2.
char* good = malloc(5 + 1); // allocates memory for 5 characters *and* the null terminator
strcpy(good, "hello");
<小时>
请注意，您还可以设置一个指向明确定义的无处"的指针，方法是让它指向 NULL.这使它成为一个空指针，它是一个保证不指向任何有效内存的指针.这与让指针完全未初始化不同.
Note that you can also set a pointer to point at a well-defined "nowhere", by letting it point to NULL. This makes it a null pointer, which is a pointer that is guaranteed not to point at any valid memory. This is different from leaving the pointer completely uninitialized.
int* p1 = NULL; // pointer to nowhere
int* p2; // uninitialized pointer, pointer to "anywhere", cannot be used yet
然而，如果您尝试访问由空指针指向的内存，您可能会遇到与使用未初始化指针时类似的问题:崩溃或分段错误.在最好的情况下，您的系统会注意到您正在尝试访问地址 null，然后引发空指针异常".
Yet, should you attempt to access the memory pointed at by a null pointer, you can get similar problems as when using an uninitialized pointer: crashes or segmentation faults. In the best case, your system notices that you are trying to access the address null and then throws a "null pointer exception".
空指针异常bug的解决方法是一样的:使用前必须先设置指针指向有效内存.
The solution for null pointer exception bugs is the same: you must set the pointer to point at valid memory before using it.
延伸阅读:
指向无效数据的指针
如何使用指针从不同的函数访问局部变量?
可以在其范围之外访问局部变量的内存吗?
分段错误及原因
什么是分段错误?
为什么写入使用char *s"初始化的字符串时出现分段错误.但不是char s[]"?
char s有什么区别[] 和 char *s?
分段错误常见原因的最终列表
什么是总线错误?
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