ctype.h是C标准函数库中的头文档，定义了一批C语言字符分类函数（C character classification functions），用于测试字符是否属于特定的字符类别，如字母字符、控制字符等等。既支持单字节字符，也支持宽字符。
英语百科
C character classification Ctype.h
（重定向自Isblank）
C character classification is an operation provided by a group of functions in the ANSI C Standard Library for the C programming language. These functions are used to test characters for membership in a particular class of characters, such as alphabetic characters, control characters, etc. Both single-byte, and wide characters are supported.