对于一个词法分析程序，一般读取文件或者终端
一个默认lex程序大致看上去像这样
YY_BUFFER_STATE bp; extern FILE* yyin; ... whatever the program does before the first call to the scanner if(!yyin) yyin = stdin; default input is stdin
bp = yy_create_buffer(yyin,YY_BUF_SIZE ); YY_BUF_SIZE defined by flex, typically 16K yy_switch_to_buffer(bp); tell it to use the buffer we just made
yylex(); or yyparse() or whatever calls the scanner
1. yy_create_buffer(yyin,YY_BUF_SIZE )
创建一个缓冲区
2. yy_switch_to_buffer(bp);
让lex从缓冲区读取输入
flex输入管理的三个层次
原文：http://www.cnblogs.com/xzpp/p/4638119.html