Lexer.l

%{
  #include <stdio.h>
  #include "parser.h"
%}

%%

[ \r\n\t]*   { continue; /* Skip blanks. */ }
[0-9]+       { sscanf(yytext, "%d", &yylval->value); 
               return TOKEN_NUMBER; }
"*"          { return TOKEN_STAR; }
"+"          { return TOKEN_PLUS; }
"("          { return TOKEN_LPAREN; }
")"          { return TOKEN_RPAREN; }
.            { continue; /* Ignore unexpected characters. */}

main.c

#include <stdio.h>

#include "parser.h"
#include "lexer.h"

int yyparse(yyscan_t scanner);


int compile(const char *source) {
    yyscan_t scanner;
    YY_BUFFER_STATE state;

    if (yylex_init(&scanner)) {
        return 1; /* could not initialize */
    }

    state = yy_scan_string(source, scanner);

    if (yyparse(scanner)) {
        return 2; /* error parsing */
    }

    yy_delete_buffer(state, scanner);
    yylex_destroy(scanner);
    return 0;
}

int main(void) {
    char test[] = "4 + 2*10 + 3*(5 + 1)";
    int code = compile(test);
    return code;
}

Makefile

FILES = lexer.c parser.c main.c


main: $(FILES)
	$(CC) $(CFLAGS) $(FILES) -o main

lexer.c lexer.h: lexer.l
	flex --outfile=lexer.c --header-file=lexer.h --nodefault \
		--reentrant --bison-bridge --nounistd --noyywrap --never-interactive --warn Lexer.l

parser.c parser.h: parser.y lexer.c
	bison --output=parser.c --defines=parser.h parser.y

clean:
	rm -f *.o *~ lexer.c lexer.h parser.c parser.h main

Parser.y

%{
  #include "parser.h"
  #include "lexer.h"

  #define emit printf

  void yyerror(yyscan_t scanner, const char *msg) {
      fprintf(stderr, "Error: %s\n", msg);
  }
%}

%pure-parser
%lex-param   { void* scanner }
%parse-param { void* scanner }

%union {
  int value;
}

%token TOKEN_LPAREN   "("
%token TOKEN_RPAREN   ")"
%token TOKEN_PLUS     "+"
%token TOKEN_STAR     "*"
%token <value> TOKEN_NUMBER "number"

%type <void> input
%left "+"
%left "*"

%%

input:
                   { emit(".global main\n");
                     emit("main:\n"); }
  expr             { emit("  pop %%rax\n");
                     emit("  ret\n"); }

expr:
    "number"       { emit("  push $%d\n", $1); }
  | expr "+" expr  { emit("  pop %%rax\n");
                     emit("  pop %%rbx\n");
                     emit("  add %%rbx, %%rax\n");
                     emit("  push %%rax\n"); }
  | expr "*" expr  { emit("  pop %%rax\n");
                     emit("  pop %%rbx\n");
                     emit("  mul %%rbx\n");
                     emit("  push %%rax\n"); }
  | "(" expr ")"   { /* No action. */ }

reference.s

.global main
main:
  push $4
  push $2
  push $10
  pop %rax
  pop %rbx
  mul %rbx
  push %rax
  pop %rax
  pop %rbx
  add %rbx, %rax
  push %rax
  push $3
  push $5
  push $1
  pop %rax
  pop %rbx
  add %rbx, %rax
  push %rax
  pop %rax
  pop %rbx
  mul %rbx
  push %rax
  pop %rax
  pop %rbx
  add %rbx, %rax
  push %rax
  pop %rax
  ret