Provided by: tcl9.0-doc_9.0.1+dfsg-1_all 
NAME
Tcl_LinkArray, Tcl_LinkVar, Tcl_UnlinkVar, Tcl_UpdateLinkedVar - link Tcl variable to C variable
SYNOPSIS
#include <tcl.h>
int
Tcl_LinkVar(interp, varName, addr, type)
int 2
Tcl_LinkArray(interp, varName, addr, type, size) 2
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.
void *addr (in) Address of C variable that is to be linked to varName.
In Tcl_LinkArray, may be NULL to tell Tcl to create the storage for the 2
array in the variable.
int type (in) Type of C variable for Tcl_LinkVar or type of array element for
Tcl_LinkArray. 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 one of the extra
ones listed below.
In Tcl_LinkVar, the additional linked type TCL_LINK_STRING may be used.
In Tcl_LinkArray, the additional linked types TCL_LINK_CHARS and 2
TCL_LINK_BINARY may be used.
All the above for both functions may be optionally OR'ed with
TCL_LINK_READ_ONLY to make the Tcl variable read-only.
Tcl_Size size (in) The number of elements in the C array. Must be greater than zero. 2
________________________________________________________________________________________________________________
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.
Tcl_LinkArray is similar, but for arrays of fixed size (given by the size argument). When asked to 2
allocate the backing C array storage (via the addr argument being NULL), it writes the address that it 2
allocated to the Tcl interpreter result.
The type argument specifies the type of the C variable, or the type of the elements of the C array, and
must have one of the following values, optionally OR'ed with TCL_LINK_READ_ONLY:
TCL_LINK_INT
The C variable, or each element of the C array, 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/decimal/binary prefix) are accepted as if they
are valid too.
TCL_LINK_UINT
The C variable, or each element of the C array, 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/decimal/binary prefix) are
accepted as if they are valid too.
TCL_LINK_CHAR
The C variable, or each element of the C array, 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/decimal/binary prefix) are accepted as if they are valid too.
If using an array of these, consider using TCL_LINK_CHARS instead. 2
TCL_LINK_CHARS
The C array is of type char * and is mapped into Tcl as a string. Any value written into the Tcl 2
variable must have the same length as the underlying storage. Only supported with Tcl_LinkArray.
TCL_LINK_UCHAR
The C variable, or each element of the C array, 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/decimal/binary prefix) are
accepted as if they are valid too.
If using an array of these, consider using TCL_LINK_BINARY instead. 2
TCL_LINK_BINARY
The C array is of type unsigned char * and is mapped into Tcl as a bytearray. Any value written 2
into the Tcl variable must have the same length as the underlying storage. Only supported with 2
Tcl_LinkArray.
TCL_LINK_SHORT
The C variable, or each element of the C array, 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/decimal/binary prefix) are accepted as if they are valid too.
TCL_LINK_USHORT
The C variable, or each element of the C array, 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/decimal/binary prefix) are
accepted as if they are valid too.
TCL_LINK_LONG
The C variable, or each element of the C array, 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/decimal/binary prefix) are
accepted as if they are valid too.
TCL_LINK_ULONG
The C variable, or each element of the C array, 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/decimal/binary prefix) are
accepted as if they are valid too.
TCL_LINK_DOUBLE
The C variable, or each element of the C array, 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/decimal/binary prefix) are
accepted as if they are valid too.
TCL_LINK_FLOAT
The C variable, or each element of the C array, 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/decimal/binary prefix) are accepted as if they are
valid too.
TCL_LINK_WIDE_INT
The C variable, or each element of the C array, 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/decimal/binary prefix) are
accepted as if they are valid too.
TCL_LINK_WIDE_UINT
The C variable, or each element of the C array, 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 wideinteger form acceptable to
Tcl_GetWideUIntFromObj; 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/decimal/binary prefix) are accepted as if they are valid too.
TCL_LINK_BOOLEAN
The C variable, or each element of the C array, 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. 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”. This is only
supported by Tcl_LinkVar.
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
Tcl 7.5 Tcl_LinkVar(3tcl)