DString

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.

[Guess by a newcomer: DStrings used to be the main workhorses in Tcl before the introduction of Tcl_Objs, 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.

Initialization

There are two ways:

 /* either this: */
 Tcl_DString *dsPtr;
 dsPtr = malloc(sizeof(Tcl_DString));
 Tcl_DStringInit(dsPtr);

 /* or this: */
 Tcl_DString dsStrng;
 Tcl_DStringInit(&dsStrng);

Which is the preferred way and why. The Tcl sources use both forms.