Invoking Tcl in C



In my pervious post, I was going to utilize a command-line tool - rsstail - into a script to notify me of Slashdot news, but I finally changed my mind to writing my own tool for monitoring and detecting new entries from Slashdot's RSS feeds. I also decided to use the Tcl programming language for this task because of its simplicity and portability.

The only challange is that Tcl doesn't support running itself as a daemon service in the background, unless using extra libraries (e.g. tcllauncher) or TclX (Extended Tcl), but Tcl is written in C and provides C APIs for calling it from C, in which I can fork a service in daemon mode.

Writing C codes to invoke a Tcl script file

I have implemented my tool in a Tcl script file, the next is to invoke the script file from C, in which several key APIs are supposed to be called.

The first API is to create a Tcl interpreter structure, which is taken as a parameter for the most C APIs:

Tcl_Interp *interp = Tcl_CreateInterp();

After creating the Tcl interpreter, call Tcl_Init to read "init.tcl" from the Tcl script library to set up the script library facility, such that you are able to import packages properly from your Tcl script file (e.g. package require http):


Lastly, reads the given Tcl script file and evaluates its contents:

Tcl_EvalFile(interp, <FILENAME>);

Compiling C codes

To compile C code, it is supposed to specify the include path, where the "tcl.h" locates (i.e. "/usr/local/include/tcl8.6" in my case), and the library path, where the "" file locates (i.e. "/usr/local/lib" in my case). Also, some Tcl packages (e.g. http) relies on the pthread library, otherwise an error undefined symbol 'pthread_attr_init' will raise:

cc -Wl,-L,/usr/local/lib,-l,tcl86,-l,pthread -I/usr/local/include/tcl8.6 <C FILES>

Thanks for reading :)