Can NULL be used in arrays in C?

In C language, NULL cannot be used directly in ordinary arrays, but can be cleverly used in pointer arrays: assign NULL to pointer array elements to indicate the end or invalid state of the array; when traversing pointer arrays, check whether the element is NULL to stop looping; when allocating pointer arrays, allocate more space to store NULL to avoid out-of-bounds access; when using NULL as the end flag of the array, pay attention to memory allocation and release to prevent memory leakage.

Can NULL be used in arrays in C? The answer is: it cannot be used directly, but it can be used cleverly.

This question seems simple, but it actually has a secret. Many beginners will take it for granted that since NULL represents a null pointer and the array name is essentially a pointer, can't NULL be assigned to an array? wrong! This idea ignores the subtle differences between arrays and pointers in C.

Let's review the basics first. In C, array names in most cases become pointers to their first element. However, the array name itself is not a pointer variable, it does not have independent storage space, and you cannot assign array names, for example array_name = NULL; is illegal. NULL is a macro, usually defined as (void *)0 , which represents a null pointer, pointing to memory address 0. Trying to assign NULL to the array name is equivalent to trying to modify the start address of the array in memory, which is not allowed. The compiler will report an error directly.

So, is NULL completely useless in array-related operations? Not so. We can use NULL to represent the "end" or "invalid" state of an array. This usually occurs in scenarios where arrays are dynamically allocated or arrays of pointers are processed.

Imagine that you dynamically allocate an array of pointers to store some strings. If some strings are not assigned to, or you want to represent the end of the array, NULL will come in handy.

Let’s take a look at an example:

 <code class="c">#include <stdio.h> #include <stdlib.h> #include <string.h> int main() { char **string_array; // 指向字符串的指针数组int num_strings = 5; string_array = (char **)malloc(sizeof(char *) * (num_strings 1)); // 多分配一个空间用于NULL if (string_array == NULL) { fprintf(stderr, "内存分配失败!\n"); return 1; } // 初始化部分字符串string_array[0] = strdup("Hello"); string_array[1] = strdup("World"); string_array[2] = strdup("!"); string_array[3] = NULL; // 用NULL表示数组结束// 遍历并打印字符串int i = 0; while (string_array[i] != NULL) { printf("%s ", string_array[i]); free(string_array[i]); // 释放动态分配的内存i ; } printf("\n"); string_array[4] = strdup("This is a test"); // 使用剩余空间//再次遍历i = 0; while (string_array[i] != NULL) { printf("%s ", string_array[i]); free(string_array[i]); i ; } printf("\n"); free(string_array); // 释放指针数组本身的内存return 0; }</string.h></stdlib.h></stdio.h></code>

In this example, string_array is an array of pointers to a string. We use NULL as the sentinel value to mark the end of the array. When iterating through the array, we check if each element is NULL to stop the loop. This is a common trick to avoid the hassle of needing additional maintenance of array lengths. Note that here we allocate one more element space than num_strings , which is specifically used to store NULL . Forgot this step is a common mistake, which may cause the program to access memory beyond the boundaries and cause crashes.

It should be noted that this method only applies to pointer arrays, not ordinary arrays. For ordinary arrays, NULL is totally useless. Moreover, using NULL as the array end flag requires careful processing of memory allocation and release, otherwise it is easy to cause memory leakage. Be sure to release the dynamically allocated memory in time after using it and develop good memory management habits. This is the only way to avoid C language memory problems.

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