@xcombelle noticed malloc calls wasn't followed by free in the library source code.

At writing time, this probably occured:

• static strings was passed to the TCL interpreter
• they was rightly declared as static
• compiler complained a const string doesn't match Tcl_Setresult char* signature
• code switched to char*
• code doesn't make anymore any warranty of immutability, but advertise so to TCL

https://www.tcl.tk/man/tcl/TclLib/Interp.htm says (seems to be for TCL_STATIC):

Normally, results are assumed to be statically allocated, which means that the contents will not change before the next time Tcl_Eval is called or some other command procedure is invoked. In this case, the freeProc field must be zero."

http://tmml.sourceforge.net/doc/tcl/SetResult.html says:

Tcl_ResetResult clears the result for interp, freeing the memory associated with it if the current result was dynamically allocated. It leaves the result in its normal initialized state with interp->result pointing to a static buffer containing TCL_RESULT_SIZE characters, of which the first character is zero. Tcl_ResetResult also clears the error state managed by Tcl_AddErrorInfo and Tcl_SetErrorCode.
Tcl_FreeResult is a macro that performs part of the work of Tcl_ResetResult. It frees up the memory associated with interp's result and sets interp->freeProc to zero, but it doesn't change interp->result or clear error state. Tcl_FreeResult is most commonly used when a procedure is about to replace one result value with another.

This comp.lang.tcl post by Adrian Ho explains very well the difference between TCL_STATIC and TCL_DYNAMIC in term of promises done by the library code to the TCL interpreter.

It seems if we only want to free the pointer, the macro TCL_DYNAMIC could be use. If we need more complex behavior, a callback should be provided.

Relevant files repository:

rRABBITMQTCL