TIP #87 Version 1.1: Allow Tcl Access to the Recursion Limit

This is not necessarily the current version of this TIP.

Abstract

An extension to the [interp] command, [interp recursionlimit], will permit Tcl scripts to control their own recursion limits. Until now, this limit has been changeable from a C API, but not from within Tcl.

Rationale

As of Tcl 8.4a3, Tcl scripts must live with the default recursion depth of 1000 nested calls to the Tcl_Eval family of functions or resort to C code to change the limit. Nevertheless, Tcl programmers may find it useful to reduce the limit when debugging or to increase it for scripts that include deeply recursive functions. The changes proposed in this TIP will make this possible in pure Tcl code.

Specification

generic/tclInterp.c

Add subcommands to [interp] and to the slave interpreter object command with the following syntax:

interp recursionlimit interp ?newlimit?

slave recursionlimit ?newlimit?

The parameter newlimit must be a positive integer. When it is present, the limit is changed to newlimit and the command returns the new recursion limit. If the newlimit parameter is absent, the command returns the current recursion limit.

No maximum value is enforced. It is the programmer's responsibility to ensure the recursion limit will not overflow the process stack.

A safe interpreter is not allowed to change the recursion limit for itself nor for any other interpreter. Attempting to do so will generate an error. Safe interpreters are allowed to query recursion limits.

doc/interp.n

Add documentation for the new subcommands, including a warning about stack overflow, much like the warning in the documentation for ;;Tcl_SetRecursionLimit()''.

test/interp.test

Add tests for the new subcommands.

Comments Received

Using a command or variable ::tcl::recursionLimit to manipulate the limit was initially considered, but Miguel Sofer suggested making the function a subcommand of [interp] because the recursion limit is logically an attribute of each interpreter. Miguel also observed that implementing TclpCheckStackSpace() for Unix (a future TIP?) would mitigate the dangers of setting the recursion limit too high. http://groups.google.com/groups?hl=en&threadm=3C6D0A88.5DC9D8B4%40utdt.edu

Reference Implementation

An implementation is in progress.

Copyright

This document is in the public domain.

This is not necessarily the current version of this TIP.