Execute a Dynamic Data Exchange command 

Tclsh Built-In Commands


package require dde 1.1

dde servername ?topic?

dde ?-async? command service topic?data?


This command allows an application to send Dynamic Data Exchange (DDE) command when running under Microsoft Windows. Dynamic Data Exchange is a mechanism where applications can exchange raw data. Each DDE transaction needs a service name and a topic. Both the service name and topic are application defined; Tcl uses the service name TclEval, while the topic name is the name of the interpreter given by dde servername. Other applications have their own service names and topics. For instance, Microsoft Excel has the service name Excel.

The only option to the dde command is:


Requests asynchronous invocation. This is valid only for the execute subcommand. Normally, the dde execute subcommand waits until the command completes, returning appropriate error messages. When the -async option is used, the command returns immediately, and no error information is available.


The following commands are a subset of the full Dynamic Data Exchange set of commands.

dde servername ?topic

dde servername registers the interpreter as a DDE server with the service name TclEval and the topic name specified by topic. If no topic is given, dde servername returns the name of the current topic or the empty string if it is not registered as a service.

dde execute service topic data 

dde execute takes the data and sends it to the server indicated by service with the topic indicated by topic. Typically, service is the name of an application, and topic is a file to work on. The data field is given to the remote application. Typically, the application treats the data field as a script, and the script is run in the application. The command returns an error if the script did not run. If the -async flag was used, the command returns immediately with no error.

dde poke service topic item data 

dde poke passes the data to the server indicated by service using the topic and item specified. Typically, service is the name of an application. topic is application specific but can be a command to the server or the name of a file to work on. The item is also application specific and is often not used, but it must always be non-null. The data field is given to the remote application.

dde request service topic item 

dde request is typically used to get the value of something; the value of a cell in Microsoft Excel or the text of a selection in Microsoft Word. service is typically the name of an application, topic is typically the name of the file, and item is application-specific. The command returns the value of item as defined in the application.

dde services service topic 

dde services returns a list of service-topic pairs that currently exist on the machine. If service and topic are both null strings ({}), then all service-topic pairs currently available on the system are returned. If service is null and topic is not, then all services with the specified topic are returned. If service is not null and topic is, all topics for a given service are returned. If both are not null, if that service-topic pair currently exists, it is returned; otherwise, null is returned.

dde eval topic cmd ?arg arg ...

dde eval evaluates a command and its arguments using the interpreter specified by topic. The DDE service must be the TclEval service. This command can be used to replace send on Windows.


A Tcl interpreter always has a service name of TclEval. Each different interpreter of all running Tcl applications must be given a unique name specified by dde servername. Each interp is available as a DDE topic only if the dde servername command was used to set the name of the topic for each interp. So a dde services TclEval {} command will return a list of service-topic pairs, where each of the currently running interps will be a topic.

When Tcl processes a dde execute command, the data for the execute is run as a script in the interp named by the topic of the dde execute command.

When Tcl processes a dde request command, it returns the value of the variable given in the dde command in the context of the interp named by the dde topic. Tcl reserves the variable $TCLEVAL$EXECUTE$RESULT for internal use, and dde request commands for that variable will give unpredictable results.

An external application which wishes to run a script in Tcl should have that script store its result in a variable, run the dde execute command, and the run dde request to get the value of the variable.

When using DDE, be careful to ensure that the event queue is flushed using either update or vwait. This happens by default when using wish unless a blocking command is called (such as exec without adding the & to place the process in the background). If for any reason the event queue is not flushed, DDE commands may hang until the event queue is flushed. This can create a deadlock situation.


Windows 8.1. Windows Server 2012 R2. Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022.


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PTC MKS Toolkit 10.4 Documentation Build 39.