什么时候必须添加一个空终止符？

Question

I know that i C , you always have to add a null terminator \\0 so that the processor knows when a word was ended .

But i get hard time to understand when you have to do it . so for example this code works for me without it :

  char connectcmd[50]={0};
  sprintf(connectcmd,"AT+CWJAP=\"%s\",\"%s\"",MYSSID,MYPASS);

How is that possible ? When do you really have to add them ?

Answer 1

sprintf always terminates it with null character , so no need to mannually add it.

From C99 standard -

7.21.6.6 The sprintf function

[...]A null character is written at the end of the characters written; it is not counted as part of the returned value. If copying takes place between objects that overlap, the behavior is undefined.

Answer 2

for example this code works for me without it

It does not (work without it).

The string literal "AT+CWJAP=\\"%s\\",\\"%s\\"" has a null terminator at the end (like every string literal). sprintf copies that null terminator to connectcmd as well.

When do you really have to add them ?

When you're manually building a string, or using a library function whose documentation explicitly states that it isn't going to add a terminating null.

Answer 3

sprintf writes a null terminated string connectcmd , regardless of its initial contents. This works as long as you don't try to write beyond the bounds of the buffer.

On top of that, when you say this:

char connectcmd[50]={0};

you initialize all 50 elements of connectcmd to zero, which is the value of the null-terminator \\0 . So it would be null-terminated even if you wrote characters to it manually, as long as you write less than 50 non-null characters.