2.11.1 String Representation

In DTrace, strings are represented 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 cannot exceed the length of the predefined string limit. However, the limit can be modified in your D program or on the dtrace command line by tuning the strsize option. See Chapter 10, Options and Tunables for more information about 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 char *, in that it is the address of a sequence of characters, but the D compiler and D functions such as trace provide enhanced capabilities when applied to expressions of type string. For example, the string type removes the ambiguity of type char * when you need to trace the actual bytes of a string.

In the following D statement, if s is of type char *, DTrace traces the value of the pointer s, which means it traces an integer address value:

trace(s);

In the following D statement, by the definition of the * operator, the D compiler dereferences the pointer s and traces the single character at that location:

trace(*s);

These behaviors enable you to manipulate character pointers that 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 next D statement, 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:

trace(s);

You can also perform lexical comparison of expressions of type string. See Section 2.11.5, “String Comparison”.