Tcl_LinkVar(3)

NAME
Tcl_LinkVar, Tcl_UnlinkVar, Tcl_UpdateLinkedVar - link Tcl variable to C variable

SYNOPSIS
#include <tcl.h>

int
Tcl_LinkVar(interp, varName, addr, type)

Tcl_UnlinkVar(interp, varName)

Tcl_UpdateLinkedVar(interp, varName)

ARGUMENTS
Interpreter that contains varName. Also used by Tcl_LinkVar to return error messages. Name of global variable. Must be in writable memory: Tcl may make temporary modifications to it while parsing the variable name. Address of C variable that is to be linked to varName. 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 (e.g. because varName is the name of array) then TCL_ERROR is returned and interp->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.

KEYWORDS
boolean, integer, link, read-only, real, string, traces, variable