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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_DOUBLE, |
                                         TCL_LINK_WIDE_INT, 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  vari-able variable
       able  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  con-tains contains
       tains 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 var-Name varName
              Name 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_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_BOOLEAN
              The C variable is of type int.  If its value is zero then it will read from Tcl as ``0''; oth-erwise otherwise
              erwise 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.


KEYWORDS
       boolean, integer, link, read-only, real, string, traces, variable



Tcl                                                  7.5                                      Tcl_LinkVar(3)