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How to prevent caching a .GIF image in browser?

I am dynamically generating a GIF image, but have run into a problem. Because it is always generated under the same filename, my browser's cache gets in the way -- it works great the first time, but each additional time the cached GIF is displayed, not the newly generated GIF. If I clear the browser cache, it works fine again, but obviously only once.

Is there something I can specify on the image tag to prevent the browser (Netscape, in this case) from caching a GIF? The only option I can think of is to dynamically write to a different filename each time, but that complicates things, so I would like to avoid it.

BTW, before you start pointing out problems with this design (like it is CPU intensive, or simultaneous users would clobber each other's images on the server), let me say that this is strictly an admin function, run by a single individual -- I really do have valid reasons to do it this way.


Thanks in advance,
parkerea
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parkerea
Asked:
parkerea
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1 Solution
 
knightEknightCommented:
add a random parameter to the src of your img tag.  For example, using ASP do this:

<IMG src="somepage.asp?rnd=<%=rnd()%>">
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knightEknightCommented:
by "random parameter" I mean something that will change every time ...


if you are generating the URL on the client, to this:

var myImg = new Image();
myImg.src = "somepage.asp?rnd=" + Math.random();
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ahoffmannCommented:
try following:
<META HTTP-EQUIV="Expires"       CONTENT="Fri, Jun 12 1981 08:20:00 GMT">
<META HTTP-EQUIV="Pragma"        CONTENT="no-cache">
<META HTTP-EQUIV="Cache-Control" CONTENT="no-cache">
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parkereaAuthor Commented:
Thank you for your replies.

ahoffmann: the meta tags did not to help. I also did some searching on the web, and found mention of using header info for cache control, but that did not seem to help either. The page itself is apparently not being cached, but the images on it are.

knightEknight: unfortunately, we are running Linux, so ASP is not an option. Also, if I understand your suggestion, you are talking about changing the filename each time, which we would like to avoid. We really want to just prevent caching the image, if possible.


Thanks again,
parkerea
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ahoffmannCommented:
then you need to disable the cache of the browser
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knightEknightCommented:
I don't mean change the name of the file ... I mean add a parameter to the image URL that is unique every time ...

Can I assume that you are using a CGI to generate the HTML?  If so, then have the CGI generate a unique parameter for the image src (like a random number or the time in milliseconds).  The parameter doesn't do anything ... but it does make the URL unique (and therefore not cached).  

If you are not using a CGI then you can do it with client-side script like this ... first, in your code, define the image, but don't give it a src:

<IMG name='myimage' border='0' width='300' height='200'>

then when the page loads, load the image, adding a unique parameter:

<BODY onLoad='document.images["myimage"].src="http://server.com/images/myimage.gif?rnd="+Math.random();'>

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BigRatCommented:
I suppose you use rewrite to get a .gif ending into CGI?

I also suppose that your cgi generates a

     Content-Type: image/gif

line?

If so, generate also :-

    Cache-Control: no-cache
    Expires: Fri, Jun 12 1981 08:20:00 GMT

or anyother date in the past (your birthday for exxample). These two lines (terminated with CR/LF) can be put before or after the Content-Type but BEFORE the famous "blank line".

HTH
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parkereaAuthor Commented:
knightEknight:

Now that I understand it, I think your solution will work, but my development PC just went into the shop for a bit... I will test and let you know the results when I get it back -- probably a week or better.

Yes, both the HTML and the GIF image are being generated by a CGI.


Thanks again,
parkerea
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COBOLdinosaurCommented:
This question has been abandoned. I will make a recommendation to the
moderators on its resolution in a week or two. I appreciate any comments
that would help me to make a recommendation.
<note>
   In the absence of responses, I may recommend DELETE unless it is clear
   to me that it has value as a PAQ.  Silence = you don't care
</note>

Cd&
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parkereaAuthor Commented:
Sorry I took so long -- after I got my machine back I had lost track of things. Thank you for the help.


- parkerea
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