Tcl_Obj * Tcl_SubstObj(interp, objPtr, flags)
The Tcl_SubstObj function is used to perform substitutions on strings in the fashion of the subst command. It gets the value of the string contained in objPtr and scans it, copying characters and performing the chosen substitutions as it goes to an output object which is returned as the result of the function. In the event of an error occurring during the execution of a command or variable substitution, the function returns NULL and an error message is left in interp's result.
Three kinds of substitutions are supported. When the TCL_SUBST_BACKSLASHES bit is set in flags, sequences that look like backslash substitutions for Tcl commands are replaced by their corresponding character.
When the TCL_SUBST_VARIABLES bit is set in flags, sequences that look like variable substitutions for Tcl commands are replaced by the contents of the named variable.
When the TCL_SUBST_COMMANDS bit is set in flags, sequences that look like command substitutions for Tcl commands are replaced by the result of evaluating that script. Where an uncaught “continue exception” occurs during the evaluation of a command substitution, an empty string is substituted for the command. Where an uncaught “break exception” occurs during the evaluation of a command substitution, the result of the whole substitution on objPtr will be truncated at the point immediately before the start of the command substitution, and no characters will be added to the result or substitutions performed after that point.