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.

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.
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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(3)

KEYWORDS

boolean, integer, link, read-only, real, string, trace, variable

Tcl 7.5 Tcl_LinkVar(3)