/*
** $Id: tol.c 772 2008-02-15 23:54:05Z phf $
**
** Example where pointers are handy: How to properly (well,
** almost properly anyway) use the strtol() function. Lots
** of older material uses atoi(), but you should not do so
** anymore these days. :-)
**
** Some atoi() man pages say that atoi() is about the same
** thing as
**
**   ... (int) strtol(nptr, NULL, 10) ...
**
** and that's quite close. But of course we want to take
** advantage of all strtol() has to offer, which includes
** decent error checking ("is it really an integer?").
**
** The program below will go through the command line and
** try to convert every argument into a long (assuming we
** use base 10!). It'll tell us (as part of this) which
** arguments could be converted flawlessly. Watch for the
** double-pointer stunts! :-)
*/

#include <stdlib.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
  // go through arguments (except program name) one by one
  for(int i = 1; i < argc; i++) {
    // just for debugging...
    printf("argv[%d] == \"%s\"\n", i, argv[i]);

    // set up the "error pointer" for strtol()
    char *err;

    // call strtol(); we need to pass the *address* of the
    // error pointer, since strtol() must be able to set it
    // for us; it'll be made to point to the first char in
    // the string that could *not* be converted using the
    // given base; so if we are completely successful, it
    // will point to the '\0' at the end; if we succeed to
    // some extent, it'll point into the string somewhere;
    // if we are not successful at all, it'll point to the
    // first char; kinda cool... :-)
    long n = strtol(argv[i], &err, 10);

    // print the value we got back for this argument string
    printf("n == %ld\n", n);
    // print the value (and the string) we get trough err
    printf("err == %p or \"%s\"\n", err, err);

    if (*err == '\0') {
      // we know that everything was good if we point to '\0'
      printf("Sunny day, we converted that one flawlessly.\n");
    }
    else if (err == argv[i]) {
      // we know all is lost if we point to the beginning
      printf("Stormy night, can't do anything with that!!!\n");
    }
    else {
      // we're (hopefully) somewhere in the middle of the string,
      // so there was some trash we couldn't convert at the end
      printf("Cloudy dusk, got some stuff done but not all!\n");
    }
  }
}