## Tcl8.6.0/Tk8.6.0 Documentation > Tcl C API, version 8.6.0 > LinkVar

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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
  DESCRIPTION
    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_DOUBLE
    TCL_LINK_FLOAT
    TCL_LINK_WIDE_INT
    TCL_LINK_WIDE_UINT
    TCL_LINK_BOOLEAN
    TCL_LINK_STRING
  SEE ALSO
  KEYWORDS

### 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

### KEYWORDS

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