Provided by: tcl8.6-doc_8.6.14+dfsg-1build1_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