need C help, passing char buffer[] by-value....

Mon Oct 19 22:21:32 UTC 2009

On Mon, Oct 19, 2009 at 11:09:19AM -0700, Patrick Mahan wrote:
> See comments interspaced below -
> 
> Gary,
> 
> Let me restate your problem: You want to read through a file containing tags
> delimited by "<>" and to skip these tags if the user has run your command 
> with
> the "-N" flag.
> 
> In C any thing passed by address is "by reference".  For a your static 
> buffer
> of 1024 characters: you can pass it by reference as:
> 
>            skiptags(buf); /* passes in the starting address of the buffer */
>            skiptags(&buf[0]); /* passes in the starting address of the 
>            buffer */
>            skiptags(&buf[10]); /* passes int the starting address of the 
>            buffer
>                                   at the 11th character position. */
> 
> Arrays and pointers are always by reference.  Individual data types "int",
> "char", etc are by value unless passed in as a pointer.  I think this is 
> where
> your confusion is around.


	You've got it exactly right, Patrick.  There were no "C" classes in 1978--I 
	taught myself.  Obviously, not that well because I have already dreaded 
	pointers.  ---Well, usually.
> 
> A couple of things to keep in mind:
> 
>    1. Remember how fgets() works.  It is entirely possible that you might 
>    have
>       tags that span multiple lines.  You will need to take that into 
>       account.
> 
>    2. You can manipulate the fixed buffer two different ways:
> 
>       A. You can use pointer arithemtic, eg.
> 
>          char buf[1024];
>          char *cp;
> 
>          while ((cp = fgets(buf, sizeof(buf), fp_in))) {
> 
>                if (skiptags) cp = skiptag(buf);
> 
>                /* If NULL, end of line reached */
>                if (!cp) continue;
> 
>          }
> 
>          char *skiptags(char *buf)
>          {
> 	      char *tp = buf;
> 
>               /* find the start of a tag */
>               while (*tp != '\0' && *tp++ != '<');
> 
>               /* if no tag is found return start of buffer */
>               if (*tp == '\0')
>                  return buf;
> 
>               /* Start of tag, find the end of tag */
>               while (*tp != '\0' && *tp != '\n' && *tp++ != '>');
> 
>               /* if end of line reached return NULL */
>               if (*tp == '\0' || *tp == '\n')
>                  return NULL;
> 
>               /* return the next character start after the end tag */
>               return ++tp;
>          }
> 
>        B. Using indexing, eg.
> 
>          char buf[1024];
>          int  i, bsize;
> 
>          while (fgets(buf, sizeof(buf), fp_in)) {
>                i = 0;
>                bsize = strlen(buf);
> 
>                if (skiptags) i = skiptag(buf);
> 
>                /* If NULL, end of line reached */
>                if (i >= bsize) continue;
> 
>          }
> 
>          int skiptags(char *buf)
>          {
> 	      int c = 0;
> 
>               /* find the start of a tag */
>               while (buf[c] != '\0' && buf[c] != '<')
>                      c++;
> 
>               /* if no tag is found return start of buffer */
>               if (buf[c] == '\0')
>                  return 0;
> 
>               /* Start of tag, find the end of tag */
>               while (buf[c] != '\0' && buf[c] != '\n' && buf[c] != '>')
>                      c++;
> 
>               /* if end of line reached return NULL */
>               if (buf[c] == '\0' || buf[c] == '\n')
>                  return strlen(buf);
> 
>               /* return the next character start after the end tag */
>               return ++c;
>          }
> 
> Both methods should allow you to skip past any tags found in the file 
> (provided
> you handle the case of a tag spanning more than one line).
> 
> 
> Hope this clears up your confusion and gets you on your way.


	Your examples help a lot!   Everything works except when there are two or more 
	tags on one line such as:

	<BODY BGCOLOR="#FFFFFF" LINK="#00FFFF" VLINK="#006633"><FONT SIZE="4">


	I think I see where is your skiptags--pointer arithematic function--this can be
	caught.  Thanks much!

	:-)

	gary


> 
> Patrick
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