proc − Create a Tcl procedure

proc name args body

     The proc command creates a new Tcl procedure named
name, replacing any existing command or procedure there may
have been by that name.  Whenever the new command is
invoked, the contents of body will be executed by the Tcl
interpreter.  Normally, name is unqualified (does not
include the names of any containing namespaces), and the new
procedure is created in the current namespace.  If name
includes any namespace qualifiers, the procedure is created
in the specified namespace.  Args specifies the formal
arguments to the procedure.  It consists of a list, possibly
empty, each of whose elements specifies one argument.  Each
argument specifier is also a list with either one or two
fields.  If there is only a single field in the specifier
then it is the name of the argument; if there are two
fields, then the first is the argument name and the second
is its default value.  Arguments with default values that
are followed by non‐defaulted arguments become required
arguments; enough actual arguments must be supplied to allow
all arguments up to and including the last required formal

     When name is invoked a local variable will be created
for each of the formal arguments to the procedure; its value
will be the value of corresponding argument in the invoking
command or the argument’s default value.  Actual arguments
are assigned to formal arguments strictly in order.
Arguments with default values need not be specified in a
procedure invocation.  However, there must be enough actual
arguments for all the formal arguments that do not have
defaults, and there must not be any extra actual arguments.
Arguments with default values that are followed by non‐
defaulted arguments become de‐facto required arguments,
though this may change in a future version of Tcl; portable
code should ensure that all optional arguments come after
all required arguments.

     There is one special case to permit procedures with
variable numbers of arguments.  If the last formal argument
has the name args, then a call to the procedure may contain
more actual arguments than the procedure has formal
arguments.  In this case, all of the actual arguments
starting at the one that would be assigned to args are
combined into a list (as if the list command had been used);
this combined value is assigned to the local variable args.


     When body is being executed, variable names normally
refer to local variables, which are created automatically
when referenced and deleted when the procedure returns.  One
local variable is automatically created for each of the
procedure’s arguments.  Other variables can only be accessed
by invoking one of the global, variable, upvar or namespace
upvar commands.  The current namespace when body is executed
will be the namespace that the procedure’s name exists in,
which will be the namespace that it was created in unless it
has been changed with rename.

     The proc command returns an empty string.  When a
procedure is invoked, the procedure’s return value is the
value specified in a return command.  If the procedure does
not execute an explicit return, then its return value is the
value of the last command executed in the procedure’s body.
If an error occurs while executing the procedure body, then
the procedure‐as‐a‐whole will return that same error.

     This is a procedure that takes two arguments and prints
both their sum and their product. It also returns the string
OK to the caller as an explicit result.

     proc printSumProduct {x y} {
         set sum [expr {$x + $y}]
         set prod [expr {$x * $y}]
         puts "sum is $sum, product is $prod"
         return "OK"

     This is a procedure that accepts arbitrarily many
arguments and prints them out, one by one.

     proc printArguments args {
         foreach arg $args {
             puts $arg

     This procedure is a bit like the incr command, except
it multiplies the contents of the named variable by the
value, which defaults to 2:

     proc mult {varName {multiplier 2}} {
         upvar 1 $varName var
         set var [expr {$var * $multiplier}]

info(3tcl), unknown(3tcl)


argument, procedure