GNU flex unistd.h在VC下的编译问题

问题现象：

...: fatal error C1083: Cannot open include file: 'unistd.h': No such file or directory

原因，flex生成的代码如下：

#ifndef YY_NO_UNISTD_H
/* Special case for "unistd.h", since it is non-ANSI. We include it way
* down here because we want the user's section 1 to have been scanned first.
* The user has a chance to override it with an option.
*/
/* %if-c-only */
#include <unistd.h>
/* %endif */
/* %if-c++-only */
/* %endif */
#endif

如果flex命令行调用加上 --nounistd 或者在词法描述文件中加上 %option nounistd，则可能产生如下错误：

...: error C2447: '{' : missing function header (old-style formal list?)
...: warning C4018: '<' : signed/unsigned mismatch
...: error C3861: 'isatty': identifier not found

解决方法：

1. 包含 <io.h>里面声明了windows版本的isatty函数。

2. 在词法描述文件中加上 %option never-interactive，这个选项向flex表明输入时非交互式的，此时flex不会调用isatty来查询输入设备类型。

isatty is used by the lexer to determine if the input stream is a terminal or a pipe/file. The lexer uses this information to change its caching behavior (the lexer reads large chunks of the input when it is not a terminal). If you know that your program will never be used in an interactive kind, you can add %option never-interactive to you lexer. When the program is run with user input, use %option interactive. When both uses are desired, you can either generate an interactive lexer, which gives a performance loss when used in batch mode, or provide your own isatty function.