Tcl_Eval(3)

NAME
Tcl_Eval, Tcl_VarEval, Tcl_EvalFile, Tcl_GlobalEval - execute Tcl commands

SYNOPSIS
#include <tcl.h>

int
Tcl_Eval(interp, cmd)

int
Tcl_VarEval(interp, string, string, ... (char *) NULL)

int
Tcl_EvalFile(interp, fileName)

int
Tcl_GlobalEval(interp, cmd)

ARGUMENTS
Interpreter in which to execute the command. A string result will be stored in interp->result. Command (or sequence of commands) to execute. Must be in writable memory (Tcl_Eval makes temporary modifications to the command). String forming part of Tcl command. Name of file containing Tcl command string.

DESCRIPTION

All four of these procedures execute Tcl commands. Tcl_Eval is the core procedure and is used by all the others. It executes the commands in the script held by cmd until either an error occurs or it reaches the end of the script.

Note that Tcl_Eval and Tcl_GlobalEval have been largely replaced by the object-based procedures Tcl_EvalObj and Tcl_GlobalEvalObj. Those object-based procedures evaluate a script held in a Tcl object instead of a string. The object argument can retain the bytecode instructions for the script and so avoid reparsing the script each time it is executed. Tcl_Eval is implemented using Tcl_EvalObj but is slower because it must reparse the script each time since there is no object to retain the bytecode instructions.

The return value from Tcl_Eval is one of the Tcl return codes TCL_OK, TCL_ERROR, TCL_RETURN, TCL_BREAK, or TCL_CONTINUE, and interp->result will point to a string with additional information (a result value or error message). If an error occurs during compilation, this return information describes the error. Otherwise, this return information corresponds to the last command executed from cmd.

Tcl_VarEval takes any number of string arguments of any length, concatenates them into a single string, then calls Tcl_Eval to execute that string as a Tcl command. It returns the result of the command and also modifies interp->result in the usual fashion for Tcl commands. The last argument to Tcl_VarEval must be NULL to indicate the end of arguments.

Tcl_EvalFile reads the file given by fileName and evaluates its contents as a Tcl command by calling Tcl_Eval. It returns a standard Tcl result that reflects the result of evaluating the file. If the file couldn't be read then a Tcl error is returned to describe why the file couldn't be read.

During the processing of a Tcl command it is legal to make nested calls to evaluate other commands (this is how procedures and some control structures are implemented). If a code other than TCL_OK is returned from a nested Tcl_Eval invocation, then the caller should normally return immediately, passing that same return code back to its caller, and so on until the top-level application is reached. A few commands, like for, will check for certain return codes, like TCL_BREAK and TCL_CONTINUE, and process them specially without returning.

Tcl_Eval keeps track of how many nested Tcl_Eval invocations are in progress for interp. If a code of TCL_RETURN, TCL_BREAK, or TCL_CONTINUE is about to be returned from the topmost Tcl_Eval invocation for interp, it converts the return code to TCL_ERROR and sets interp->result to point to an error message indicating that the return, break, or continue command was invoked in an inappropriate place. This means that top-level applications should never see a return code from Tcl_Eval other then TCL_OK or TCL_ERROR.

SEE ALSO
Tcl_EvalObj, Tcl_GlobalEvalObj

KEYWORDS
command, execute, file, global, object, object result, variable