Tcl_LinkVar(), Tcl_UnlinkVar(), Tcl_UpdateLinkedVar()

link Tcl variable to C variable 

Tcl Library Procedures

SYNOPSIS

#include <tcl.h>

int Tcl_LinkVar(interp, varName, addr, type)

Tcl_UnlinkVar(interp, varName)

Tcl_UpdateLinkedVar(interp, varName)

ARGUMENTS

Tcl_Interp *interp (in) 

Interpreter that contains varName. Also used by Tcl_LinkVar() to return error messages.

char *varName (in) 

Name of global variable. Must be in writable memory: Tcl may make temporary modifications to it while parsing the variable name.

char *addr (in) 

Address of C variable that is to be linked to varName.

int type (in) 

Type of C variable. Must be one of TCL_LINK_INT, TCL_LINK_DOUBLE, TCL_LINK_BOOLEAN, or TCL_LINK_STRING, optionally OR'ed with TCL_LINK_READ_ONLY to make Tcl variable read-only.

DESCRIPTION

Tcl_LinkVar() uses variable traces to keep the Tcl variable named by varName in sync with the C variable at the address given by addr. Whenever the Tcl variable is read the value of the C variable will be returned, and whenever the Tcl variable is written the C variable will be updated to have the same value. Tcl_LinkVar() normally returns TCL_OK; if an error occurs while setting up the link (for example, because varName is the name of array) then TCL_ERROR is returned and the interpreter's result contains an error message.

The type argument specifies the type of the C variable, and must have one of the following values, optionally OR'ed with TCL_LINK_READ_ONLY:

TCL_LINK_INT 

The C variable is of type int. Any value written into the Tcl variable must have a proper integer form acceptable to Tcl_GetInt(); attempts to write non-integer values into varName will be rejected with Tcl errors.

TCL_LINK_DOUBLE 

The C variable is of type double. Any value written into the Tcl variable must have a proper real form acceptable to Tcl_GetDouble(); attempts to write non-real values into varName will be rejected with Tcl errors.

TCL_LINK_BOOLEAN 

The C variable is of type int. If its value is zero then it will read from Tcl as 0; otherwise it will read from Tcl as 1. Whenever varName is modified, the C variable will be set to a 0 or 1 value. Any value written into the Tcl variable must have a proper boolean form acceptable to Tcl_GetBoolean(); attempts to write non-boolean values into varName will be rejected with Tcl errors.

TCL_LINK_STRING 

The C variable is of type char *. If its value is not null then it must be a pointer to a string allocated with Tcl_Alloc(). Whenever the Tcl variable is modified the current C string will be freed and new memory will be allocated to hold a copy of the variable's new value. If the C variable contains a null pointer then the Tcl variable will read as NULL.

If the TCL_LINK_READ_ONLY flag is present in type then the variable will be read-only from Tcl, so that its value can only be changed by modifying the C variable. Attempts to write the variable from Tcl will be rejected with errors.

Tcl_UnlinkVar() removes the link previously set up for the variable given by varName. If there does not exist a link for varName then the procedure has no effect.

Tcl_UpdateLinkedVar() may be invoked after the C variable has changed to force the Tcl variable to be updated immediately. In many cases this procedure is not needed, since any attempt to read the Tcl variable will return the latest value of the C variable. However, if a trace has been set on the Tcl variable (such as a Tk widget that wishes to display the value of the variable), the trace will not trigger when the C variable has changed. Tcl_UpdateLinkedVar() ensures that any traces on the Tcl variable are invoked.

PORTABILITY

Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022. Windows Server 2025.

AVAILABILITY

PTC MKS Toolkit for Professional Developers
PTC MKS Toolkit for Enterprise Developers
PTC MKS Toolkit for Enterprise Developers 64-Bit Edition

PTC MKS Toolkit 10.5 Documentation Build 40.