What is the difference between NULL and null characters in C language

NULL (pointer) and \0 (null character) are completely different in C language: NULL means that invalid pointer points (memory address 0), while \0 is a character constant, marking the end of the string; mixed use will throw an error (compiler warning).

NULL with null characters: Ghost of pointers and non-existence of characters

You may think that NULL and null characters ( \0 ) both mean "nothing", but in fact they are completely different existences in the C world, just like ghosts and nothingness, one guides the direction and the other takes up space. If you can't figure out their differences, your code may crash silently at runtime, leaving bugs that will make you scratch your head.

Let's start with the most basic ones. NULL is a macro, usually defined as (void *)0 , which represents an invalid pointer. Remember that the pointer is the memory address, and NULL indicates that this pointer does not point to any valid memory location. When you try to access the memory pointed to by NULL pointer, the system will give you a segmentation fault without hesitation.

The empty character \0 , the ASCII code value is 0, is a character constant. It takes up a byte of space, and while it seems nothing, it plays a crucial role in the string: marking the end of the string. C language string processing functions, such as strlen , rely on null characters to determine the length of a string.

The key difference is that: NULL is used for pointers, while \0 is used for character arrays (strings). They are different types of data and cannot be mixed at all. Assigning NULL to a character variable, or assigning \0 to a pointer are all error operations that the compiler will warn you or even report directly.

Let's use code to illustrate:

 <code class="c">#include <stdio.h> int main() { char *ptr = NULL; // ptr is a pointer initialized to NULL char str[] = "Hello\0World"; // str is a character array, '\0' terminates the string "Hello" printf("ptr: %p\n", ptr); // Print the memory address pointed to by ptr (should be NULL) // Attempting to access memory pointed to by ptr would lead to a crash. // printf("Value at ptr: %c\n", *ptr); // Uncomment this to see the crash! printf("str: %s\n", str); // Prints "Hello" because '\0' stops the string printing. printf("Length of str: %zu\n", strlen(str)); // Prints 5, the length of "Hello" char emptyChar = '\0'; printf("Size of emptyChar: %zu bytes\n", sizeof(emptyChar)); // Prints 1, showing it occupies memory. return 0; }</stdio.h></code>

This code shows how NULL and \0 are used and emphasizes their differences. ptr is a pointer to NULL , and attempting to access the memory it points to will cause the program to crash. str is a character array containing null characters. \0 correctly terminates the string "Hello", allowing strlen function to correctly calculate the string length. emptyChar shows the fact that the empty character itself occupies memory space.

Performance and best practices:

When processing strings, be sure to make sure the string ends in \0 . Forgot to add \0 will cause the string processing function to behave abnormally and even the program will crash. Before using a pointer, be sure to check whether it is NULL to avoid accessing invalid memory. This is especially important after dynamic memory allocations (e.g. malloc ).

Experience in trapping:

I have caused my program to crash countless times because I didn't check if the pointer was NULL . When processing strings entered by users, be sure to handle string length carefully to prevent buffer overflow. Remember, although NULL pointers and null characters look similar, they play completely different roles in C, and the consequences of confusing them are unimaginable. Only by developing good programming habits and carefully checking the validity of pointers and strings can you write a robust and reliable C program.

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