17.1 Translator Declarations

A translator is a collection of D assignment statements provided by the supplier of an interface. Translators can be used to translate an input expression into an object of the struct type. To understand the need for using translators, consider as an example the ANSI C standard library routines that are defined in stdio.h. These routines operate on a data structure named FILE, which contains implementation artifacts that are abstracted away from C programmers. A standard technique for creating a data structure abstraction is to provide only a forward declaration of a data structure in public header files, while keeping the corresponding struct definition in a separate and private header file.

If you are writing a C program and want to know the file descriptor corresponding to a FILE struct, use the fileno() function to obtain the descriptor rather than dereferencing a member of the FILE struct directly. The Oracle Linux header files enforce this rule by defining FILE as an opaque forward declaration tag so that it cannot be dereferenced directly by C programs that include <stdio.h>.

Inside the /lib/libc.so.6 library, consider the following hypothetical example where fileno is implemented in C, noting that a real-life implementation would not be at all similar to this example:

int
fileno(FILE *fp)
{
  struct file_impl *ip = (struct file_impl *)fp;
 
  return (ip->fd);
}

In the example, the hypothetical fileno takes a FILE pointer as an argument and casts it to a pointer that corresponds to the internal libc structure, struct file_impl, then returns the value of the fd member of the implementation structure.

Unfortunately, observability software like DTrace requires the ability to peer inside the implementation in order to provide useful results. DTrace cannot call arbitrary C functions that are defined in Oracle Linux libraries or in the kernel. You could declare a copy of struct file_impl in your D program to instrument the routines that are declared in stdio.h, but then your D program would rely on Private implementation artifacts of the library that might break in a future micro or minor release, or even in a patch. Ideally, you want to provide a construct for use in D programs that is bound to the implementation of the library and is updated accordingly, yet still provides an additional layer of abstraction associated with greater stability.

A new translator is created by using a declaration of the following form:

translator output-type < input-type input-identifier > {
  member-name = expression ;
  member-name = expression ;
  ...
};

The output-type names a struct that will be the result type for the translation. The input-type specifies the type of the input expression, is surrounded in angle brackets <>, and followed by an input-identifier that can be used in the translator expressions as an alias for the input expression. The body of the translator is surrounded in braces {} and terminated with a semicolon (;), and consists of a list of member-names and identifiers that correspond to translation expressions. Each member declaration must name a unique member of the output-type and must be assigned an expression of a type that is compatible with the member type, according to the rules for the D assignment (=) operator.

For example, you could define a struct of stable information about stdio files based on some of the available libc interfaces:

struct file_info {
  int file_fd;   /* file descriptor from fileno() */
  int file_eof;  /* eof flag from feof() */
};

Then, you could define a hypothetical D translator from FILE to file_info:

translator struct file_info < FILE *F > {
  file_fd = ((struct file_impl *)F)->fd;
  file_eof = ((struct file_impl *)F)->eof;
};

In this hypothetical translator, the input expression is of type FILE * and is assigned the input-identifier F. The identifier F can then be used in the translator member expressions as a variable of type FILE * that is only visible within the body of the translator declaration. To determine the value of the output file_fd member, the translator performs a cast and dereference similar to the hypothetical implementation of fileno() shown in the previous example. A similar translation is performed to obtain the value of the EOF indicator.