Tcl_LinkVar
Tcl_LinkVar(3) Tcl Library Procedures Tcl_LinkVar(3)
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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 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 tempo-
rary modifications to it while pars-
ing 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 vari-
able read-only.
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DESCRIPTION
Tcl_LinkVar uses variable traces to keep the Tcl variable named by var-
Name in sync with the C variable at the address given by addr. When-
ever 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 inter-
preter'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_Get-
Double; 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 proce-
dure 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
Tcl 7.5 Tcl_LinkVar(3)
