int Tcl_ExprLong(interp, string, longPtr)
int Tcl_ExprDouble(interp, string, doublePtr)
int Tcl_ExprBoolean(interp, string, booleanPtr)
int Tcl_ExprString(interp, string)
These four procedures all evaluate the expression given by the string argument and return the result in one of four different forms. The expression can have any of the forms accepted by the expr command. Note that these procedures have been largely replaced by the object-based procedures Tcl_ExprLongObj, Tcl_ExprDoubleObj, Tcl_ExprBooleanObj, and Tcl_ExprObj. Those object-based procedures evaluate an expression held in a Tcl object instead of a string. The object argument can retain an internal representation that is more efficient to execute.
The interp argument refers to an interpreter used to evaluate the expression (e.g. for variables and nested Tcl commands) and to return error information.
For all of these procedures the return value is a standard Tcl result: TCL_OK means the expression was successfully evaluated, and TCL_ERROR means that an error occurred while evaluating the expression. If TCL_ERROR is returned then the interpreter's result will hold a message describing the error. If an error occurs while executing a Tcl command embedded in the expression then that error will be returned.
If the expression is successfully evaluated, then its value is returned in one of four forms, depending on which procedure is invoked. Tcl_ExprLong stores an integer value at *longPtr. If the expression's actual value is a floating-point number, then it is truncated to an integer. If the expression's actual value is a non-numeric string then an error is returned.
Tcl_ExprDouble stores a floating-point value at *doublePtr. If the expression's actual value is an integer, it is converted to floating-point. If the expression's actual value is a non-numeric string then an error is returned.
Tcl_ExprBoolean stores a 0/1 integer value at *booleanPtr. If the expression's actual value is an integer or floating-point number, then they store 0 at *booleanPtr if the value was zero and 1 otherwise. If the expression's actual value is a non-numeric string then it must be one of the values accepted by Tcl_GetBoolean such as ``yes'' or ``no'', or else an error occurs.
Tcl_ExprString returns the value of the expression as a string stored in the interpreter's result.