String Representation

Strings are represented in DTrace as an array of characters terminated by a null byte (that is, a byte whose value is zero, usually written as '\0'). The visible part of the string is of variable length, depending on the location of the null byte, but DTrace stores each string in a fixed-size array so that each probe traces a consistent amount of data. Strings may not exceed the length of this predefined string limit, but the limit can be modified in your D program or on the dtrace command line by tuning the strsize option. Refer to Chapter 16, Options and Tunables for more information on tunable DTrace options. The default string limit is 256 bytes.

The D language provides an explicit string type rather than using the type char * to refer to strings. The string type is equivalent to a char * in that it is the address of a sequence of characters, but the D compiler and D functions like trace() provide enhanced capabilities when applied to expressions of type string. For example, the string type removes the ambiguity of the type char * when you need to trace the actual bytes of a string. In the D statement:

trace(s);

if s is of type char *, DTrace will trace the value of the pointer s (that is, it will trace an integer address value). In the D statement:

trace(*s);

by definition of the * operator, the D compiler will dereference the pointer s and trace the single character at that location. These behaviors are essential to permitting you to manipulate character pointers that by design refer to either single characters, or to arrays of byte-sized integers that are not strings and do not end with a null byte. In the D statement:

trace(s);

if s is of type string, the string type indicates to the D compiler that you want DTrace to trace a null terminated string of characters whose address is stored in the variable s. You can also perform lexical comparison of expressions of type string, as described in String Comparison.