array [index] = 0;的目的是什么？

Question

char dev[20] = "dev_name";
char dest[32];
strncpy(dest,dev,sizeof(dev));
dest[sizeof(dev)-1] = 0;

What does dest[sizeof(dev)-1] = 0; means?

Answer 1

Does it mean all the element of that array are assigned zero?

No it does not mean this.

Assuming you meant strncpy(dest,dev_name,sizeof(dev_name)); /* Extra bracket */ strncpy(dest,dev_name,sizeof(dev_name)); /* Extra bracket */ and dev_name and sizeof in you last line; You are assigning NUL character to the last to mark the end of name array.

When you write a string literal like "foo", it is automatically NUL terminated by the compiler. When you take your own arrays, you sometimes need to mark the end of string manually.

From man strncpy

The strncpy() function is similar, except that at most n bytes of src are copied. Warning: If there is no null byte among the first n bytes of src, the string placed in dest will not be null-terminated .

Explicitly the null-termination is added to handle the warning case in your code snippet.

dev_name[0] , dev_name[1] , dev_name[2] etc are first, second, third ... characters of your string. Assuming device name has less than 31 characters, it is automatically NUL terminated after strncpy and you don't need to do anything.

If the name has exactly 31 character, last character character (32 ^nd ) is already '\\0' (ascii code 0) and writing 0 again over it does not make any difference.

If the name has more than 31 character (corner case), last character character is not NUL and dev_name[sizeof(dev_name)-1] = 0; will make the name NUL terminated.

Answer 2

In your code, assuming size is analogous to sizeof ,

 dev_name[size(dec_name)-1] = 0;

dev_name[size(dec_name)-1] points to the last element of the array, remember C arrays use 0 based indexing.

Then, by definition, a string in c is exxentially a char array with null-termination, so if you want to use a char array as string , you must have the null-termination.

0 is the ASCII value of NUL or null . So, essentially, you're putting a null-terminator to the char array.