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By default (and for purposes of efficiency), the scanner uses block-reads rather than simple `getc()' calls to read characters from yyin. The nature of how it gets its input can be controlled by defining the YY_INPUT macro. YY_INPUT's calling sequence is "YY_INPUT(buf,result,max_size)". Its action is to place up to max_size characters in the character array buf and return in the integer variable result either the number of characters read or the constant YY_NULL (0 on Unix systems) to indicate EOF. The default YY_INPUT reads from the global file-pointer "yyin".

A sample definition of YY_INPUT (in the definitions section of the input file):

%{
#define YY_INPUT(buf,result,max_size) { int c = getchar(); result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); }
%}
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FILE * file = fopen(filename, "r");if (!file) { /* handle error somehow */ }yyin = file;