NULtermination

NUL termination is a convention for representing text strings in memory by placing a null character at the end. The terminator, known as the NUL or null byte, has the value zero in ASCII. In this scheme a string consists of a sequence of characters followed by a trailing zero that signals the end to processing routines.

In languages and environments that use NUL-terminated strings, such as C, string length is determined by scanning from the start until the first NUL. Functions like strlen, strcat, and strcpy rely on the terminator to delimit the string. This design avoids storing a length field but requires careful management to preserve the terminating character.

Problems arise if the terminating NUL is overwritten or missing, which can cause buffer overflows, crashes, or security vulnerabilities. Improper handling of user input or mixing binary data with text can exacerbate risks. Furthermore, strings containing a null character within the data cannot be represented directly in a NUL-terminated model.

Alternatives include length-prefixed or bounded strings, and languages that track length separately (for example, C++'s std::string). Some C libraries adopt safer routines that enforce bounds, such as snprintf or strlcpy when available. NUL termination remains a foundational concept in C and in system interfaces that require C compatibility, even as newer abstractions lessen reliance on it.