iscntrl

iscntrl is a function in the C standard library used to test whether a character is a control character. It is declared in the header <ctype.h> and has the prototype int iscntrl(int ch). The function returns a nonzero value if the argument is a control character according to the current C locale, and zero otherwise.

In the common C locale, control characters are those that do not represent printable characters, typically including ASCII codes 0x00 through 0x1F and 0x7F. However, the exact set of characters classified as control characters can vary with the locale used by the program. The classification is performed through the program’s current locale data, so behavior may differ across environments.

The argument ch must be representable as an unsigned char or be the value EOF. Passing a plain char that is negative (on platforms where char is signed) without casting can yield undefined behavior. To use iscntrl safely with character data of type char, cast to unsigned char: iscntrl((unsigned char)ch). The function returns nonzero when ch is a control character and zero otherwise.

In languages such as C++, iscntrl is available in <cctype> within the std namespace. It is related to other character classification utilities such as isspace, isprint, and isalpha, and is commonly used to filter or validate input, skip non-printable characters, or parse protocol data.