Null Character: Difference between revisions

== How to use and detect a null terminator ==

In C, the null terminator is automatically attached at the end of a string when you create it with double quotation marks ("Hello, OSDev!" is an array of 14 characters, as after the '!', there is a '\0' or null byte). The standard library uses this to detect the length of strings and operate consequentially with them, but behind the header files, the code that detects the null terminator is quite easy but important to understand.

size_t strlen(const char *str)

{

}

 

This C code is a quick-and-easy implementation of the strlen() standard function. All it does is increment "n" while the n-th character of the string "str" is not a null terminator, and then return it. If no null byte is found in the string, the function may overflow. Problably it will find one null byte somewhere, but if not, it will read from an invalid address, generating an exception. Keep in mind that detecting a null terminator is only usefull in the context of strings, as memory in general can have arbitrary zeros that don't mean the end of its contents.

 

In most of the dialects of assembly, strings variables (or more accurrately, labels) can be defined to be null terminated when created. Some assemblers accept the tag ".asciiz" followed by an string between double quotation marks to create (unsually in the data section) a string, that then you can treat as a normal C-like string.

Example under 64 bit Intel assembly, with NASM:

bits 64

 

xor rax, rax

call printf

As printf will start reading the characters to print from address "str" until a NULL, the output of the program will be:

<pre>

</pre>

== OS desing considerations ==

== See also ==

=== External Links ===

* [https://elixir.bootlin.com/glibc/glibc-2.17/source/string/strlen.c GLibc's strlen() source]

* [https://p403n1x87.github.io/getting-started-with-x86-64-assembly-on-linux.html x64 Intel assembly basics]