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Tcl_LinkVar(3tcl) Tcl Library Procedures Tcl_LinkVar(3tcl) 





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:



  
  
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.

  
  
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.

  
  
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.

  
  
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.

  
  
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.

  
  
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.

  
  
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.

  
  
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.

  
  
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.

  
  
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.

  
  
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.

  
  
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.

  
  
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.

  
  
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







  

    7.5
    Tcl