On 24/02/2011, at 3:11 AM, Kari Laitinen wrote:
> A classic book about the C programming language
> begins with a program that contains the statement
>   printf("hello, world");
> It has been said, however, that the printf function
> that is used in the above statement does not
> belong to the language itself but it is a library
> function.

We can draw a parallel to open class and closed class
words in a natural language.  Pronouns, determiners,
conjunctions, prepositions; these are not wholly
unlike keywords in a language.  (Indeed, keywords are
often prepositions, and I do not believe this is accidental.)
Variable and function names are not wholly unlike open class

Would you say that open class words are not part of a language?
If you came across a sentence written using English syntax and
closed-class words but Japanese open class words, would it
still be English?

I say that a type, variable, or function "belongs to the
language" if it is specified in the best available description
of the language.

By that criterion, printf is definitely part of the C language.
A C compiler (like gcc) is fully within its rights to look at
a call of printf() and exploit information in the standard but
not in the type of the identifier to check that the format
string makes sense and agrees with the actual parameters.

> To me, this raises questions: Why
> begin a book about a programming language by
> showing something that does not belong to the language?

Because the claim that it does not is false.

At a minimum, a programming language must provide
 - constants
 - variables
 - user defined functions
 - built in functions
 - combining forms

One important criterion is whether an operation could be provided in
the language if it were not in the standard.  Take fopen() as an
example.  Given only the rest of C, fopen() would not be definable.
Out of putc(), fwrite(), and fprintf(), at least one of them must
be provided in the language; given any one of them the others are

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