Chapter II Flex input and output structure

For a lexical analysis program, generally read files or terminal

A default Lex program looks like this.

Yy_buffer_state Bp;extern file* Yyin; Whatever the program does before the first call to the Scannerif (!yyin) yyin = stdin; Default input is stdin
bp = Yy_create_buffer (yyin,yy_buf_size); Yy_buf_size defined by Flex, typically 16kyy_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)

Create a buffer

2. Yy_switch_to_buffer (BP);

Let Lex read the input from the buffer

Three levels of flex input management

• Set Yyin to read the required files
• Creating and using Yy_buffer_state input buffers
• Redefine Yy_input

Chapter II Flex input and output structure