Address of a variable: Difference between revisions

=={{header|Tcl}}==

It is highly unusual to want to directly manipulate the address of a variable in Tcl, as it is a thoroughly unsafe operation. Indeed, Tcl does not expose any mechanism to do so at the script level. However, Tcl does contain a C-level API function, <tt>[http://www.tcl.tk/man/tcl8.6/TclLib/LinkVar.htm Tcl_LinkVar]</tt>, to arrange for a variable's value to always reflect the contents of a particular address in memory.

However, that's not the only way of doing it. You can also use the '''critcl''' library to put C code directly inside a Tcl script and so work with addresses directly that way.

<br>

{{libheader|critcl}}

<lang tcl>package require critcl

critcl::cproc peek {int addr} int {

union {

int i;

int *a;

} u;

u.i = addr;

return *u.a;

}

critcl::cproc poke {int addr int value} void {

union {

int i;

int *a;

} u;

u.i = addr;

*u.a = value;

}

package provide poker 1.0</lang>

Have great care with this sort of code; the damage you can do by writing to random locations is considerable and being able to read from anywhere could allow information to flow to otherwise unauthorized programs.