Printf function 也打印另一个字符串

Question

Hi i am fairly new in C language and i was trying to understand the strings. As i know, strings are just an array of characters and there shouldn't be a difference between char a[]= "car" and char a[] = {'c','a','r'} .

When i try to print the string as:

char a[] = "car";
char b[] = "testing the cars";
printf("%s", a);

the output is just car and there's no problem.But when i try to print it as:

char a[] = {'c','a','r'};
char b[] = "testing the cars";
printf("%s", a);

it's printing the b too. Can you explain what's the reason of it?

Answer 1

The %s specifier of printf() expects a char* pointer to a null-terminated string.

In the first case, a and b are both null-terminated. Initializing a char[] array of unspecified size with a string literal will include the literal's null-terminator '\0' character at the end. Thus:

char a[] = "car";

is equivalent to:

char a[] = {'c', 'a', 'r', '\0'};

In the second case, a is NOT null-terminated, leading to undefined behavior , as printf("%s", a) will read past the end of a into surrounding memory until it eventually finds a '\0' character. It just happens that, in your case, b exists in that memory following a , but that is not guaranteed, the compiler can put b wherever it wants.