The DString (formally, a ''Tcl_DString'') is a data type defined in the Tcl C library. It is similar to a C string, but it can dynamically (hence the D) grow to be as large as it needs to be. Using DStrings thus avoids a lot of risks for buffer overflow that plague many "ordinary" C programs.
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[[Guess by a newcomer: DStrings used to be the main workhorses in Tcl before the introduction of [Tcl_Obj]s, and thus have a bunch of features for constructing DStrings with list structure. These features are not so interesting now, but DStrings remain useful for constructing complex strings of other sorts (e.g. error messages).]]

[DKF]: They're good for things like temporary buffers, and are used in quite a lot of places in the Tcl core still. The list-related stuff is rare though; that's usually done with [Tcl_Obj]-based list routines nowadays.
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[[[Category Internals]|[Category Tcl Library]]]