Have the code examples from K&R ever been conforming?

Question

The C Programming Language by Brian Kernighan and Dennis Ritchie contains a lot of examples such as this famous one (K&R 2nd edition 1.1):

#include <stdio.h>

main()
{
  printf("hello, world\n");
}

Here I note the following issues:

• No return type.

  Writing functions with no return type was allowed in C90 which the second edition of the book claims to follow. These will default to int in C90. It is invalid C in later versions of the language.

• No return statement.

  A function with a return type and no return statement was not well-defined in C90. Writing main() with no return being equivalent to return 0; was a feature added in C99.

• Empty parameter list of main().

  This is valid C still (as of C17) but has always been an obsolescent feature even in C90. (Upcoming C23 talks of finally getting rid of K&R style functions.)

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My question:

Was any code in K&R 2nd edition ever a conforming program, in any version of the standard?

Answer 1

No, the programs in the K&R book were never conforming programs ¹⁾ (C17 4/7) under any verison of the standard.

• In C90 (ISO 9899:1990), the code invoked undefined behavior because of the missing return statement.
• In C99 (ISO 9899:1999) and beyond, the code won't compile because of the implicit int.

Sources below.

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Regarding implicit int, one major difference in function return types between C90 and latter versions can be found here:

C90 6.7.1 Function definitions

The return type of a function shall be void or an object type other than array.
/--/
If the declarator includes an idenfifier list, the types of the parameters may be declared in a following declaration list. Any parameter that is not declared has type int.

C17 6.9.1 Function definitions

The return type of a function shall be void or a complete object type other than array type.
/--/
If the declarator includes an identifier list, the types of the parameters shall be declared in a following declaration list. In either case, the type of each parameter is adjusted as described in 6.7.6.3 for a parameter type list; the resulting type shall be a complete object type.

The main difference being the "complete object type" wording, the definition of complete object type being one of the basic types or a pointer to one (C17 6.2.5). We can conclude that implicit int was allowed in C90 both as the return type or as part of a (non-prototype) parameter list.

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Regarding no return statement, this text was always there for general functions:

C90 6.6.6.4

If a return statement without an expression is executed and the value of the function call is used by the caller, the behavior is undefined. Reaching the } that terminates a function is equivalent to executing a return statement without an expression.

C17 6.9.1/12

If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined.

However, main() is a special case and an exception was added in C99:

C99 5.1.2.2.3

If the return type of the main function is a type compatible with int, a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument; reaching the } that terminates the main function returns a value of 0.

Whereas in C90, the equivalent text says:

C90 5.1.2.2.3

A return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument. If the main function executes a return that specifies no value, the termination status returned to the host environment is undefined.

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Regarding empty parameter lists, it has been marked as obsolescent from C90 to C17. See future language directions, for example C17 6.11 (or C90 6.9, identical text):

6.11.6 Function declarators
The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature.

6.11.7 Function definitions The use of function definitions with separate parameter identifier and declaration lists (not prototypeformat parameter type and identifier declarators) is an obsolescent feature.

This does however not mean that code using the feature isn't conforming, at least up to ISO 9899:2018. It's simply not recommended practice, and was not recommended practice at the point where K&R 2nd edition was released either.

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¹⁾ C17 from chp 4:

A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any strictly conforming program.

A conforming program is one that is acceptable to a conforming implementation.

A strictly conforming program shall use only those features of the language and library specified in this International Standard. It shall not produce output dependent on any unspecified, undefined, or implementation-defined behavior, and shall not exceed any minimum implementation limit.

This means that a conforming program may use features of a conforming implementation that are non-portable, but it may not alter the behavior of a strictly conforming program by for example invoking undefined behavior explicitly listed as such in the standard.

Answer 2

I compiled the program

#include <stdio.h>

main()
{
    printf("hello, world\n");
}

under two well-regarded compilers, gcc and clang. Just for fun I added the --pedantic option also. As far as I know both of these compilers would be considered "conforming", and I believe that's one of the things their authors certainly strive for.

Both compilers produced an executable which printed hello, world. Under the definition that

A conforming program is one that is acceptable to a conforming implementation

, I conclude that the program is conforming.

I pass no judgement on the question of whether the program would have been conforming under C89.

Although I have not studied the code examples in K&R2 in some years, I believe that most/all of the rest of them are similarly conforming, despite various pedagogical or other shortcuts which might render them not strictly conforming.

Answer 3

Some quotes:

"The first edition, published February 22, 1978, was the first widely available book on the C programming language. Its version of C is sometimes termed K&R C (after the book's authors), often to distinguish this early version from the later version of C standardized as ANSI C."

(source Wikipedia)

In other words, K&R edition 1 predates any official C standards. At the time the only specification was "The C Reference Manual" by Dennis M. Ritchie.

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"In April 1988, the second edition of the book was published, updated to cover the changes to the language resulting from the then-new ANSI C standard, particularly with the inclusion of reference material on standard libraries."

(source Wikipedia)

In other words, K&R edition 2 was "aligned with" the first official ANSI C standard otherwise known as C89.

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However, at the time K&R edition 2 was published, C89 was not yet complete. According the the Wikipedia page on ANSI C.

"In 1983, the American National Standards Institute formed a committee, X3J11, to establish a standard specification of C. In 1985, the first Standard Draft was released, sometimes referred to as C85. In 1986, another Draft Standard was released, sometimes referred to as C86. The prerelease Standard C was published in 1988, and sometimes referred to as C88."

(source Wikipedia)

Thus, they may be differences between what K&R says and the ANSI C standard.

Answer 4

By definition, any source text or collection thereof which is "accepted" by a Conforming C Implementation is a "Conforming C Program". Because implementations are given broad latitude to extend the language in any way which does not affect the behavior of any Strictly Conforming C Programs, any source text T which would not otherwise be a Conforming C Program could be turned into a Conforming C Program by modifying a Conforming C Implementation so that if it were given program that doesn't match T, it would process it normally, and if fed a copy of T it would behave as though it were fed some other program that it would accept.

While this may seem an absurdly broad definition, it satisfies one of the major goals of the C Standards Committee, which was to ensure that if any existing programs could accomplish a task, the task could be accomplished by a Conforming C Program.

As for whether the programs were Strictly Conforming under C89, that's a bit harder to answer. The Standard says that if execution falls through the end of main() it will return an Undefined Value to the host environment, and imposes no requirements about the consequence of doing so, which would suggest that such an action would invoke Undefined Behavior. On the other hand, the Standard also imposes no requirements upon what happens if a program returns EXIT_SUCCESS, nor what happens if it returns EXIT_FAILURE, nor if it returns some other value. Thus, all such actions could be viewed as invoking Undefined Behavior. On the other hand, viewing things in such fashion would make it impossible for any program which terminates to be Strictly Conforming.

I think the most reasonable way of interpreting the Standard would be to say that a program whose execution falls through the end of main() waives any control it might have had to affect what the execution environment does once it terminates. If all courses of action the host environment could perform after a program exits would be equally acceptable, a program's failure to do anything to influence which course of action is taken would not be a defect.

In considering whether a program that fails to specify a return value, or any program for that matter, is "Strictly Conforming", one cannot merely examine the source text, but must also consider the application requirements. If one needs a program to output the characters x and y once each, in some order, the following would be a strictly conforming program that accomplishes that:

#include <stdio.h>
int outputx(void) { return printf("x"); }
int outputy(void) { return printf("y"); }
int main(void)
{
  return outputx() + outputy() && printf("\n") && 0;
}

If, on the other hand, one need a program to output "xy", the above would not be a strictly conforming program for that purpose. Thus, I would say that if the application requirements for some task specify that a program must use its return value to influence the host environment, a program that falls through the end of main would not be a Strictly Conforming C Program to accomplish that task. If, however, such influence over the host environment is not part of the application requirements for a task, then a Strictly Conforming C Program could waive such control.

Citations below:

From N1570 section 4 pararaph 7:

A conforming program is one that is acceptable to a conforming implementation. (*) 5) Strictly conforming programs are intended to be maximally portable among conforming implementations. Conforming programs may depend upon nonportable features of a conforming implementation.

Undefined Behavior is defined in 3.4.3:

behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements

From the C99 Rationale, talking about the definition of conformance [emphasis original]:

A strictly conforming program is another term for a maximally portable program. The goal is to give the programmer a fighting chance to make powerful C programs that are also highly portable, without seeming to demean perfectly useful C programs that happen not to be portable, thus the adverb strictly.

The fact that a program exits without setting a return value may make it non-portable, but the Standard deliberately avoids "demeaning" non-portable programs by calling them non-conforming.