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

Tcl_Interp *interp (in) Interpreter that con

tains varName. Also used by Tcl_LinkVar to return error messages.

CONST char *varName (in) Name of global variable.

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_WIDE_INT,
TCL_LINK_BOOLEAN, or
TCL_LINK_STRING, option
ally OR'ed with
TCL_LINK_READ_ONLY to
make Tcl variable readonly.

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 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_GetIntFromObj; 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 writ
ten into the Tcl variable must have a proper real
form acceptable to Tcl_GetDoubleFromObj; attempts to write non-real values into varName will be rejected with Tcl errors.

TCL_LINK_WIDE_INT

The C variable is of type Tcl_WideInt (which is an integer type at least 64-bits wide on all platforms
that can support it.) Any value written into the
Tcl variable must have a proper integer form
acceptable to Tcl_GetWideIntFromObj; attempts to write non-integer 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_GetBooleanFromObj; attempts to write nonboolean 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 or ckalloc. 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 vari
able 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 imme
diately. 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

Tcl 7.5 Tcl_LinkVar(3)