In DTrace, strings are represented as an array of characters ending in a null byte, which is a byte with a value of 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. 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's 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 aren't strings and don't 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 String Comparison.

String constants are enclosed in pairs of double quotes ("") and are automatically assigned the type string by the D compiler. You can define string constants of any length, limited only by the amount of memory DTrace is permitted to consume on the system and by whatever limit you have set for the strsize DTrace runtime option. The terminating null byte (\0) is added automatically by the D compiler to any string constants that you declare. The size of a string constant object is the number of bytes associated with the string, plus one additional byte for the terminating null byte.

A string constant can't contain a literal newline character. To create strings containing newlines, use the \n escape sequence instead of a literal newline. String constants can also contain any of the special character escape sequences that are defined for character constants.

Unlike the assignment of char * variables, strings are copied by value and not by reference. The string assignment operator = copies the actual bytes of the string from the source operand up to and including the null byte to the variable on the left-hand side, which must be of type string.

You can use a declaration to create a string variable:

string s;

Or you can create a string variable by assigning it an expression of type string.

For example, the D statement:

s = "hello";

creates a variable s of type string and copies the six bytes of the string "hello" into it (five printable characters, plus the null byte).

String assignment is analogous to the C library function strcpy(), with the exception that if the source string exceeds the limit of the storage of the destination string, the resulting string is automatically truncated by a null byte at this limit.

You can also assign to a string variable an expression of a type that's compatible with strings. In this case, the D compiler automatically promotes the source expression to the string type and performs a string assignment. The D compiler permits any expression of type char * or of type char[n], a scalar array of char of any size, to be promoted to a string.

Expressions of other types can be explicitly converted to type string by using a cast expression or by applying the special stringof operator, which are equivalent in the following meaning:

s = (string) expression;
s = stringof (expression);

The expression is interpreted as an address to the string.

The stringof operator binds very tightly to the operand on its right-hand side. You can optionally surround the expression by using parentheses, for clarity.

Scalar type expressions, such as a pointer or integer, or a scalar array address can be converted to strings, in that the scalar is interpreted as an address to a char type. Expressions of other types such as void may not be converted to string. If you erroneously convert an invalid address to a string, the DTrace safety features prevents you from damaging the system or DTrace, but you might end up tracing a sequence of undecipherable characters.

D overloads the binary relational operators and permits them to be used for string comparisons, as well as integer comparisons. The relational operators perform string comparison whenever both operands are of type string or when one operand is of type string and the other operand can be promoted to type string. See String Assignment for a detailed description. See also Table 4-13, which lists the relational operators that can be used to compare strings.

As with integers, each operator evaluates to a value of type int, which is equal to one if the condition is true or zero if it is false.

The relational operators compare the two input strings byte-by-byte, similarly to the C library routine strcmp(). Each byte is compared by using its corresponding integer value in the ASCII character set until a null byte is read or the maximum string length is reached. See the ascii(7) manual page for more information. Some example D string comparisons and their results are shown in the following table.