Tcl Server Programming Intro


This is an old page that describes some of the the most commonly used APIs in the server. You should also look at the Learn section that has several more specific write-ups and the manual pages for more complete documentation on each module.

Document Root

The document root is the name of the directory on your server machine that corresponds to the root of server's URL hierarchy. This is defined with the Doc_Root command:

Doc_Root /usr/local/htdocs

With no arguments, Doc_Root returns the current root.

If you have additional directories you need to paste into the hierarchy you can use Doc_AddRoot. This creates sort of a symbolic link from a URL to some point in your server's file system:

Doc_AddRoot virtual directory

The term virtual means a URL path like /cgi-bin or /foobar. The directory is a file pathname like /export/disk3/foobar.

Per-User Directories

If you want users to export their own pages under ~user URLs, then use the Doc_PublicHtml command to define what subdirectory of their home is used.

Doc_PublicHtml public_html

Index Files

The Doc_IndexFile specifies a pattern for the base name of the index file. The pattern can be an exact name, like "index.html", or a pattern that allows any suffix (e.g., index.*) If more than one file matches the pattern, the most recently modified file is used.

Doc_IndexFile index.{html,shtml,htm}

Log Files

The server generates log files in a standard format that is understood by various log-processing utilities. (If you have special needs you can implement a new log module.) The log files are automatically rotated every day, except that there is only one error log that is always appended. You specify the base name of the log file and a different suffix is appended for the daily logs and the error file:

Log_SetFile /usr/local/tclhttpd/log:

The logs are flushed periodically instead of on each log entry, except the error log which is flushed immediately. You can set the period with:

Log_FlushMinutes minutes

Server Name and Port

You set the server's host name and port with the Httpd_Server command. This command opens the socket and activates the server:

Httpd_Server 80 sunscript.sun.com

If you need to specify the server's own IP address, in case you have multiple servers running on different interfaces, just append the IP address to this command:

Httpd_Server 80 sunscript.sun.com

If you are going to run several servers you'll need to clone the startup script and run a different Tcl process for each server.

URL Domain Handlers

The server partitions the URL space into "domains" that are uniquely identified by a prefix of the URL. The longest matching prefix is used. For example, "/" is the prefix of all URLs, but you can also have a "/device" domain and a "/status" domain that are handled by different Tcl procedures. Basic file system service is provided by the Doc domain (i.e., doc.tcl), which makes a call like this:

Url_PrefixInstall / [list DocDomain $Doc(root)]

but the CGI domain makes a call like

Url_PrefixInstall /cgi-bin [list CgiDomain $directory]

This registers a Tcl procedure that is called each time a request matches the prefix. The longest matching prefix is used to resolve conflicts between prefixes like "/" and "/cgi-bin". The domain handler is invoked like this:

DocDomain $directory $sock $suffix

The $sock and $suffix are added by Url_Dispatch when the handler is invoked. The $directory was defined at the time Url_PrefixInstall was called. A domain handler can be installed with any arguments it needs. The $sock is the socket for the connection, and it is used in calls on the Httpd module to return results. The $suffix is the part of the URL after the prefix is stripped off.

Sample domain handlers include DocDomain, CgiDomain, and DirectDomain, which can be found in doc.tcl, cgisrv.tcl, and direct.tcl, respectively.

Application Direct URLs

The Direct module implements a mapping from URLs to Tcl procedure calls. The name of the URL determines the procedure, and the arguments come from query data. Use Direct_Url to define the command prefix associated with a URL subtree. For example:

Direct_Url /status Status
Direct_Url /mail Mail

This declares that URLs beginning with /status are handled by Tcl procedures that begin with Status, and URLS beginning with /mail are handled by Tcl procedures that begin with Mail. The URL must match the Tcl procedure exactly. For example:

proc Status/hello {args} {return hello}

This defines an implementation for the /status/hello URL that returns a page containing the string "hello".

If the procedure has arguments defined, then the Direct module looks for those names in the query data posted (or included) with the URL. The args parameter is used to collect extra form data. If there is no matching form data then the parameter gets its default value or the empty string. For example, the server definesproc Mail/formdata {email subject args} { ... }

If you have an HTML form with email and subject fields, then you can use /mail/formdata as the action URL for your form. The Mail/formdata Tcl procedure is called with email and subject bound to the values from the form. Any additional fields in the form are collected into args as a list that alternates between name and value.

Document Type Handlers

The Doc domain supports handlers for MIME types. The mime.types file establishes a mapping from file suffixes (e.g., html) to MIME types (e.g. text/html). For example, files with suffix .shtml are type application/x-server-include. The Doc module checks to see if a handler exists for a type before returning the file. If it exists, the handler is called like this:

Doc_application/x-server-include $path $suffix $sock

The $path is the absolute pathname of the file. The $suffix is the URL suffix. The $sock is the socket handle for the connection.

Example handlers include Doc_application/x-cgi, Doc_application/x-imagemap, Doc_application/x-tcl-subst, and Doc_application/x-server-include.

HTML+Tcl Templates

The Doc_Subst procedure is a handler for application/x-tcl-subst. This handler simply reads a file and does a Tcl subst on it to replace any embedded Tcl commands and variable references. By default the subst is done in the main interpreter so you have to be careful. However, Doc_Subst is often used by other handler types that use sessions that are maintained in safe slave interpreters.

For example, a simple foo.subst file might contain:

<h1>Hello World</h1>
The current time is Wed May 24 21:27:15 UTC 2017

Sessions and Session State

The Session module keeps track of session state in slave interpreters. A session has a 4-character ID that is dynamically generated when a new session is created. Sessions work by passing around the session ID in URL requests. For example, a URL that starts a new session looks like:


The MIME type handler for .special (e.g., Doc_application/x-whizbang) must process the query data and then process the named file. When it sees the session=new it calls Session_Create to create a new slave interpreter for the session and return the ID. If it sees something like


Then it calls Session_Match to hook up with the existing interpreter state.

The Doc_application/x-tcl-snmp is the best example that uses session state. It uses Url_DecodeQuery to parse incoming form data, Session_Match to hook up to session state, and SnmpProcess to handle the form data in an application-specific way. It then falls through to Doc_Subst to process the page in the context of the session.
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