Parse command line arguments in Lex

Question

Suppose I want my Lex and Yacc program to parse the command line arguments like:

./a.out show memory

I want lex to parse the string "show memory". How do I accomplish this?

Answer 1

You'll need to concatenate all the arguments into a big string, by inserting whitespace between them. Then feed the remaining text buffer to Lex/Yacc, by re-defining the YY_INPUT macro so it reads input from your text buffer.

The start could be something like:

#include <stdio.h>
#include <string.h>

char *argbuf;
size_t arglen;

int main(int argc, char *argv[])
{
  int i;

  // Compute total length of all arguments, with a single space between.
  arglen = 0;
  for(i = 1; argv[i] != NULL; i++)
    arglen += 1 + strlen(argv[i]);

  // Allocate buffer space.
  argbuf = malloc(arglen);
  if(argbuf == NULL)
  {
     fprintf(stderr, "No memory for argument buffer, aborting");
     exit(1);
  }

  // Concatenate all arguments. This is inefficient, but simple.
  argbuf[0] = 0;
  for(i = 1; argv[i] != NULL; i++)
  {
    if(i > 1)
      strcat(argbuf, " ");
    strcat(argbuf, argv);
  }

  // Here we should be ready to call yyparse(), if we had implemented YY_INPUT().

  return 0;
}

Answer 2

What's wrong with doing it the old fashioned way?:

if(argc > 1 && !strcmp(argv[1],"show"))
{
    if(argc > 2)
    {
        if(!strcmp(argv[2],"memory"))
            ...
        else if(!strcmp(argv[2],"cpu"))
            ...
        else ...
    }
}

Besides, getopt() and friends are more appropriate.