Meaning of yywrap() in flex

Question

What does this instructions mean in flex (lex) :

#define yywrap() 1

and this [ \t]+$
i find it in the code below:

(%% [ \t]+ putchar('_'); [ \t]+% %%

input "hello world"

output "hello_world"

)

Solution

According to The Lex & Yacc Page :

When the scanner receives an end-of-file indication from YY_INPUT, it then checks the yywrap() function. If yywrap() returns false (zero), then it is assumed that the function has gone ahead and set up yyin to point to another input file, and scanning continues. If it returns true (non-zero), then the scanner terminates, returning 0 to its caller. Note that in either case, the start condition remains unchanged; it does not revert to INITIAL.

The #define is used to simplify building the program (so that no -ll linkage option is needed).

Further reading:

• What are lex and yacc?
• Routines to reprocess input
• 6. How do Lex and YACC work internally (Lex and YACC primer/HOWTO)