Provided by: tcl8.6-doc_8.6.13+dfsg-2_all 
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.
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_UINT, TCL_LINK_CHAR, TCL_LINK_UCHAR,
TCL_LINK_SHORT, TCL_LINK_USHORT, TCL_LINK_LONG,
TCL_LINK_ULONG, TCL_LINK_WIDE_INT, TCL_LINK_WIDE_UINT,
TCL_LINK_FLOAT, 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 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. Incomplete integer
representations (like the empty string, '+', '-' or the hex/octal/binary prefix)
are accepted as if they are valid too.
TCL_LINK_UINT
The C variable is of type unsigned int. Any value written into the Tcl variable
must have a proper unsigned integer form acceptable to Tcl_GetWideIntFromObj and in
the platform's defined range for the unsigned int type; attempts to write non-
integer values (or values outside the range) into varName will be rejected with Tcl
errors. Incomplete integer representations (like the empty string, '+', '-' or the
hex/octal/binary prefix) are accepted as if they are valid too.
TCL_LINK_CHAR
The C variable is of type char. Any value written into the Tcl variable must have
a proper integer form acceptable to Tcl_GetIntFromObj and be in the range of the
char datatype; attempts to write non-integer or out-of-range values into varName
will be rejected with Tcl errors. Incomplete integer representations (like the
empty string, '+', '-' or the hex/octal/binary prefix) are accepted as if they are
valid too.
TCL_LINK_UCHAR
The C variable is of type unsigned char. Any value written into the Tcl variable
must have a proper unsigned integer form acceptable to Tcl_GetIntFromObj and in the
platform's defined range for the unsigned char type; attempts to write non-integer
values (or values outside the range) into varName will be rejected with Tcl errors.
Incomplete integer representations (like the empty string, '+', '-' or the
hex/octal/binary prefix) are accepted as if they are valid too.
TCL_LINK_SHORT
The C variable is of type short. Any value written into the Tcl variable must have
a proper integer form acceptable to Tcl_GetIntFromObj and be in the range of the
short datatype; attempts to write non-integer or out-of-range values into varName
will be rejected with Tcl errors. Incomplete integer representations (like the
empty string, '+', '-' or the hex/octal/binary prefix) are accepted as if they are
valid too.
TCL_LINK_USHORT
The C variable is of type unsigned short. Any value written into the Tcl variable
must have a proper unsigned integer form acceptable to Tcl_GetIntFromObj and in the
platform's defined range for the unsigned short type; attempts to write non-integer
values (or values outside the range) into varName will be rejected with Tcl errors.
Incomplete integer representations (like the empty string, '+', '-' or the
hex/octal/binary prefix) are accepted as if they are valid too.
TCL_LINK_LONG
The C variable is of type long. Any value written into the Tcl variable must have
a proper integer form acceptable to Tcl_GetLongFromObj; attempts to write non-
integer or out-of-range values into varName will be rejected with Tcl errors.
Incomplete integer representations (like the empty string, '+', '-' or the
hex/octal/binary prefix) are accepted as if they are valid too.
TCL_LINK_ULONG
The C variable is of type unsigned long. Any value written into the Tcl variable
must have a proper unsigned integer form acceptable to Tcl_GetWideIntFromObj and in
the platform's defined range for the unsigned long type; attempts to write non-
integer values (or values outside the range) into varName will be rejected with Tcl
errors. Incomplete integer representations (like the empty string, '+', '-' or the
hex/octal/binary prefix) are accepted as if they are valid too.
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_GetDoubleFromObj; attempts to write non-
real values into varName will be rejected with Tcl errors. Incomplete integer or
real representations (like the empty string, '.', '+', '-' or the hex/octal/binary
prefix) are accepted as if they are valid too.
TCL_LINK_FLOAT
The C variable is of type float. Any value written into the Tcl variable must have
a proper real form acceptable to Tcl_GetDoubleFromObj and must be within the range
acceptable for a float; attempts to write non-real values (or values outside the
range) into varName will be rejected with Tcl errors. Incomplete integer or real
representations (like the empty string, '.', '+', '-' or the hex/octal/binary
prefix) are accepted as if they are valid too.
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.
Incomplete integer representations (like the empty string, '+', '-' or the
hex/octal/binary prefix) are accepted as if they are valid too.
TCL_LINK_WIDE_UINT
The C variable is of type Tcl_WideUInt (which is an unsigned 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 unsigned integer form acceptable to
Tcl_GetWideIntFromObj (it will be cast to unsigned); attempts to write non-integer
values into varName will be rejected with Tcl errors. Incomplete integer
representations (like the empty string, '+', '-' or the hex/octal/binary prefix)
are accepted as if they are valid too.
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 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 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 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.
Note that, as with any call to a Tcl interpreter, Tcl_UpdateLinkedVar must be called from
the same thread that created the interpreter. The safest mechanism is to ensure that the C
variable is only ever updated from the same thread that created the interpreter (possibly
in response to an event posted with Tcl_ThreadQueueEvent), but when it is necessary to
update the variable in a separate thread, it is advised that Tcl_AsyncMark be used to
indicate to the thread hosting the interpreter that it is ready to run
Tcl_UpdateLinkedVar.
SEE ALSO
Tcl_TraceVar(3tcl)
KEYWORDS
boolean, integer, link, read-only, real, string, trace, variable