Copy-on-write semantics

When multiple variables have the same value, Tcl internally saves space by only storing that value in memory once, and only copies the value in memory when necessary. This is known as . It's useful to keep this in mind when working with large values or with data structures. Each item in a pure list, for example, is a separate value, so extracting it with won't result in the value being copied. In the following example,

set value [string repeat a [expr {2 ** 10}]] for {set i 0} {$i < 60} {incr i} { lappend list1 $value } foreach item $list1 { lappend list2 $value }$value is stored a grand total of 1 time in memory, occupying 1MB, not, as might be supposed, 120MB. All the items in $list1 and $list2 share the same value in memory.A fundamental concept of Tcl is that everything is a string. In a Tcl script, therefore, there are inevitably a large number of strings, and a naive implementation that copied these strings a lot would perform badly. Copy-on-write is the mechanism by which the Tcl C implementation avoids unecessary copies. Each value (Tcl_Obj) has a reference count. Whenever the value is passed to a command or assigned to a variable the reference count is incremented and no copy is made. When a value is to be changed the implementation first checks the reference count. A count of 1 indicates that there is no other reference to the value and it is safe to modify it in-place. A count greater than 1 indicates that there are other references to the value, to preserve Tcl semantics, it should be copied before being modified.An understanding of Tcl's copy-on-write mechanism is often needed when writing a C extension. A good place to start is with the documentation for Tcl_Obj.An extension should always take advantage of Tcl's copy-on-write mechanism where appropriate, so that user expectations at the script level regarding memory usage are met. Correct behaviour by an extension can often be verified using .Those just learning the concepts of call-by-reference and call by value may wonder how to understand Tcl in those terms, and how those concepts relate to Tcl facilities such as upvar. Tcl command arguments are always passed by value at the Tcl script level and by reference at the C implementation level. Passing to a command the name of a value or a command allows it to reference the named variable or value. This script-level facility can be used for the same reasons that a pointer might be used in C, but obviously, the implementation of these things is different, and tangentially related to copy-on-write.