NULL is essentially a null pointer to an empty address and does not mean invalid. Relying solely on ptr == NULL to determine that the pointer is valid is not enough to capture uninitialized, released or out of boundary memory. More reliable validity checking strategies include: checking the return value after allocating memory, setting the pointer to NULL after freeing memory, checking NULL for function parameters, using assertions and developing good programming habits (initializing pointers, checking validity, setting it to NULL after freeing, be careful of pointer operation).

NULL pointer in C language: truth and trap

Many newbies (even some veterans) think that using ptr == NULL can handle all pointer validity checks. This idea is too naive! This article will explore in-depth the essence of NULL pointers and why relying solely on ptr == NULL to judge the validity of pointers often fall into the pit. After reading it, you will have a deeper understanding of the underlying mechanism of C language pointers and write more robust and less error-prone code.

The nature of NULL

NULL is not a magical magic value, it is essentially a macro, usually defined as (void *)0 . This means that NULL is a null pointer to an empty address. The key is that "empty address" itself does not necessarily mean "invalid". The operating system may reserve certain address areas, for example, for kernel or system calls. Trying to access these addresses will have unimaginable consequences - program crashes, system instability, and even security vulnerabilities.

Why is ptr == NULL not enough

The problem is that ptr == NULL only determines whether the pointer points to an empty address. It cannot distinguish whether the pointer has been initialized, whether it points to freed memory, or points to out-of-bounds memory. These situations will also cause program errors, but ptr == NULL cannot be detected.

For example:

 <code class="c">#include <stdio.h> #include <stdlib.h> int main() { int *ptr; // 未初始化的指針if (ptr == NULL) { printf("ptr is NULL\n"); // 這條語句會執(zhí)行，但ptr并非安全可用的} // 嘗試訪問未初始化的指針*ptr = 10; // 這行代碼很危險！可能導(dǎo)致程序崩潰int *ptr2 = (int *)malloc(sizeof(int)); free(ptr2); // 釋放內(nèi)存*ptr2 = 20; // 使用已釋放的內(nèi)存，程序行為未定義，可能崩潰或出現(xiàn)奇怪的結(jié)果return 0; }</stdlib.h></stdio.h></code>

In this code, although ptr is NULL , it is not initialized, and using it directly will lead to undefined behavior. Although ptr2 is effective at first, it is also dangerous to use it again after release. ptr == NULL cannot catch these errors.

More reliable pointer validity check

So, how to check the validity of pointers more reliably? The answer is: adopt different strategies according to the specific situation .

• For dynamically allocated memory: After malloc , calloc , or realloc , the return value must be checked whether it is NULL . If NULL is returned, it means that memory allocation failed. After freeing memory, set the pointer to NULL immediately to prevent dangling pointer errors.
• For function parameters: If the function receives pointer parameters, it should check whether the pointer is NULL inside the function body to avoid program crashes due to null pointer dereference.
• For global or static variables: Global or static variables are usually automatically initialized to NULL (or a default value) when the program starts, so there is no need to explicitly check NULL . However, before use, be sure to confirm whether the memory it points to is valid.
• Consider using assertions: During the debug phase, you can use assert macro to check the validity of the pointer. assert(ptr != NULL); If the assertion fails, the program will terminate and print the error message.

Talk about experience

When writing C code, pointers are double-edged swords. Proficient in pointers, you can write efficient and elegant code; but if you are not careful, you will fall into the pointer trap. It is crucial to develop good programming habits:

• Always initialize the pointer.
• Before using the pointer, carefully check its validity.
• Once the memory is freed, set the pointer to NULL immediately.
• Make full use of debugging tools to promptly discover and resolve pointer-related problems.
• Use pointer arithmetic with caution to avoid out-of-bounds access.

In short, relying solely on ptr == NULL to judge the validity of a pointer is far from enough. It is necessary to combine multiple methods according to the specific situation to effectively avoid pointer-related errors and write safer and more reliable C code. Remember, be careful to sail the ship for thousands of years!

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