String input to flex lexer

I want to create a read-eval-print loop using flex/bison parser. Trouble is, the flex generated lexer wants input of type FILE* and i would like it to be char*. Is there anyway to do this?

One suggestion has been to create a pipe, feed it the string and open the file descriptor and send to the lexer. This is fairly simple but it feels convoluted and not very platform independent. Is there a better way?

The following routines are available for setting up input buffers for scanning in-memory strings instead of files (as yy_create_buffer does):

• YY_BUFFER_STATE yy_scan_string(const char *str): scans a NUL-terminated string`
• YY_BUFFER_STATE yy_scan_bytes(const char *bytes, int len): scans len bytes (including possibly NULs) starting at location bytes

Note that both of these functions create, return a corresponding YY_BUFFER_STATE handle (which you must delete with yy_delete_buffer() when done with it) so yylex() scan a copy of the string or bytes. This behavior may be desirable since yylex() modifies the contents of the buffer it is scanning).

If you want avoid the copy (and yy_delete_buffer) using:

• YY_BUFFER_STATE yy_scan_buffer(char *base, yy_size_t size)

sample main:

int main() {
    yy_scan_buffer("a test string");
    yylex();
}

See this section of Flex's manual for information on how to scan in-memory buffers, such as strings.

flex can parse char * using any one of three functions: yy_scan_string(), yy_scan_buffer(), and yy_scan_bytes() (see the documentation). Here's an example of the first:

typedef struct yy_buffer_state * YY_BUFFER_STATE;
extern int yyparse();
extern YY_BUFFER_STATE yy_scan_string(char * str);
extern void yy_delete_buffer(YY_BUFFER_STATE buffer);

int main(){
	char string[] = "String to be parsed.";
	YY_BUFFER_STATE buffer = yy_scan_string(string);
	yyparse();
	yy_delete_buffer(buffer);
	return 0;
}

The equivalent statements for yy_scan_buffer() (which requires a doubly null-terminated string):

char string[] = "String to be parsed.\0";
YY_BUFFER_STATE buffer = yy_scan_buffer(string, sizeof(string));

My answer reiterates some of the information provided by @dfa and @jlholland, but neither of their answers' code seemed to be working for me.

Here is what I needed to do :

extern yy_buffer_state;
typedef yy_buffer_state *YY_BUFFER_STATE;
extern int yyparse();
extern YY_BUFFER_STATE yy_scan_buffer(char *, size_t);

int main(int argc, char** argv) {

  char tstr[] = "line i want to parse\n\0\0";
  // note yy_scan_buffer is is looking for a double null string
  yy_scan_buffer(tstr, sizeof(tstr));
  yy_parse();
  return 0;
}

you cannot extern the typedef, which make sense when you think about it.

The accepted answer is incorrect. It will cause memory leaks.

Internally, yy_scan_string calls yy_scan_bytes which, in turn, calls yy_scan_buffer.

yy_scan_bytes allocates memory for a COPY of the input buffer.

yy_scan_buffer works directly upon the supplied buffer.

With all three forms, you MUST call yy_delete_buffer to free the flex buffer-state information (YY_BUFFER_STATE).

However, with yy_scan_buffer, you avoid the internal allocation/copy/free of the internal buffer.

The prototype for yy_scan_buffer does NOT take a const char* and you MUST NOT expect the contents to remain unchanged.

If you allocated memory to hold your string, you are responsible for freeing it AFTER you call yy_delete_buffer.

Also, don't forget to have yywrap return 1 (non-zero) when you're parsing JUST this string.

Below is a COMPLETE example.

%%

<<EOF>> return 0;

.   return 1;

%%

int yywrap()
{
    return (1);
}

int main(int argc, const char* const argv[])
{
    FILE* fileHandle = fopen(argv[1], "rb");
    if (fileHandle == NULL) {
        perror("fopen");
        return (EXIT_FAILURE);
    }

    fseek(fileHandle, 0, SEEK_END);
    long fileSize = ftell(fileHandle);
    fseek(fileHandle, 0, SEEK_SET);

    // When using yy_scan_bytes, do not add 2 here ...
    char *string = malloc(fileSize + 2);

    fread(string, fileSize, sizeof(char), fileHandle);

    fclose(fileHandle);

    // Add the two NUL terminators, required by flex.
    // Omit this for yy_scan_bytes(), which allocates, copies and
    // apends these for us.   
    string[fileSize] = '\0';
    string[fileSize + 1] = '\0';

    // Our input file may contain NULs ('\0') so we MUST use
    // yy_scan_buffer() or yy_scan_bytes(). For a normal C (NUL-
    // terminated) string, we are better off using yy_scan_string() and
    // letting flex manage making a copy of it so the original may be a
    // const char (i.e., literal) string.
    YY_BUFFER_STATE buffer = yy_scan_buffer(string, fileSize + 2);

    // This is a flex source file, for yacc/bison call yyparse()
    // here instead ...
    int token;
    do {
        token = yylex(); // MAY modify the contents of the 'string'.
    } while (token != 0);

    // After flex is done, tell it to release the memory it allocated.    
    yy_delete_buffer(buffer);

    // And now we can release our (now dirty) buffer.
    free(string);

    return (EXIT_SUCCESS);
}

Other-way, you can redefine function YY_INPUT in lex file, and then set your string to LEX's input. As below:

#undef YY_INPUT
#define YY_INPUT(buf) (my_yyinput(buf))

char my_buf[20];

void set_lexbuf(char *org_str)
{  strcpy(my_buf, org_str);  }

void my_yyinput (char *buf)
{  strcpy(buf, my_buf);      } 

In your main.c, before scanning, you need to set lex's buffer first:

set_lexbuf(your_string);
scanning...

here is a small example for using bison / flex as a parser inside your cpp code for parsing string and changing a string value according to it (few parts of the code were removed so there might be irrelevant parts there.) parser.y :

%{
#include "parser.h"
#include "lex.h"
#include <math.h> 
#include <fstream>
#include <iostream> 
#include <string>
#include <vector>
using namespace std;
 int yyerror(yyscan_t scanner, string result, const char *s){  
    (void)scanner;
    std::cout << "yyerror : " << *s << " - " << s << std::endl;
    return 1;
  }
	%}

%code requires{
#define YY_TYPEDEF_YY_SCANNER_T 
typedef void * yyscan_t;
#define YYERROR_VERBOSE 0
#define YYMAXDEPTH 65536*1024 
#include <math.h> 
#include <fstream>
#include <iostream> 
#include <string>
#include <vector>
}
%output "parser.cpp"
%defines "parser.h"
%define api.pure full
%lex-param{ yyscan_t scanner }
%parse-param{ yyscan_t scanner } {std::string & result}

%union {
  std::string *  sval;
}

%token TOKEN_ID TOKEN_ERROR TOKEN_OB TOKEN_CB TOKEN_AND TOKEN_XOR TOKEN_OR TOKEN_NOT
%type <sval>  TOKEN_ID expression unary_expression binary_expression
%left BINARY_PRIO
%left UNARY_PRIO
%%

top:
expression {result = *$1;}
;
expression:
TOKEN_ID  {$$=$1; }
| TOKEN_OB expression TOKEN_CB  {$$=$2;}
| binary_expression  {$$=$1;}
| unary_expression  {$$=$1;}
;

unary_expression:
 TOKEN_NOT expression %prec UNARY_PRIO {result =  " (NOT " + *$2 + " ) " ; $$ = &result;}
;
binary_expression:
expression expression  %prec BINARY_PRIO {result = " ( " + *$1+ " AND " + *$2 + " ) "; $$ = &result;}
| expression TOKEN_AND expression %prec BINARY_PRIO {result = " ( " + *$1+ " AND " + *$3 + " ) "; $$ = &result;} 
| expression TOKEN_OR expression %prec BINARY_PRIO {result = " ( " + *$1 + " OR " + *$3 + " ) "; $$ = &result;} 
| expression TOKEN_XOR expression %prec BINARY_PRIO {result = " ( " + *$1 + " XOR " + *$3 + " ) "; $$ = &result;} 
;

%%

lexer.l : 

%{
#include <string>
#include "parser.h"

%}
%option outfile="lex.cpp" header-file="lex.h"
%option noyywrap never-interactive
%option reentrant
%option bison-bridge

%top{
/* This code goes at the "top" of the generated file. */
#include <stdint.h>
}

id		  ([a-zA-Z][a-zA-Z0-9]*)+
white	  [ \t\r]
newline	  [\n]

%%
{id}                 	{    
	yylval->sval = new std::string(yytext);
	return TOKEN_ID;
}
"(" {return TOKEN_OB;}
")" {return TOKEN_CB;}
"*" {return TOKEN_AND;}
"^" {return TOKEN_XOR;}
"+" {return TOKEN_OR;}
"!" {return TOKEN_NOT;}

{white};  // ignore white spaces
{newline};
. {
return TOKEN_ERROR;
}

%%

usage : 
void parse(std::string& function) {
  string result = "";
  yyscan_t scanner;
  yylex_init_extra(NULL, &scanner);
  YY_BUFFER_STATE state = yy_scan_string(function.c_str() , scanner);
  yyparse(scanner,result);
  yy_delete_buffer(state, scanner);
  yylex_destroy(scanner);
  function = " " + result + " ";  
}

makefile:
parser.h parser.cpp: parser.y
	@ /usr/local/bison/2.7.91/bin/bison -y -d parser.y


lex.h lex.cpp: lexer.l
	@ /usr/local/flex/2.5.39/bin/flex lexer.l

clean:
	- \rm -f *.o parser.h parser.cpp lex.h lex.cpp

There's this funny code in libmatheval:

/* Redefine macro to redirect scanner input from string instead of
 * standard input.  */
#define YY_INPUT( buffer, result, max_size ) \
{ result = input_from_string (buffer, max_size); }