Home / Questions / 26479
Accepted
PaperBirdMaster
Asked: 2020-10-08 01:34:48 +0800 CST 2020-10-08 01:34:48 +0800 CST

Empty return statement in no-void function undefined behavior?

  • 772

Researching the consequences of misusing the statement returnin functions voidand non- voidin the C standard I realized that it does not specify what happens when a non- function usesvoid an returnempty statement or a function voiduses a returnnon-empty statement:

int devolver_entero() { return; }  // no-void usa una return vacio

What I have been able to find in the standard is the following (translations are mine):

6.8.6.4 The return statement

restrictions

  1. A statement returnwith an expression must not appear in a function whose return type is void. A statement returnwithout an expression must appear only in functions whose return type is void.

The above standard quote states what should be done with statements returnfor return and non-return functions, but elsewhere in the standard it states what happens when the constraints are not followed:

6.9.1 Function definitions

  1. If the function terminator is reached }and the value of the function call is used by the caller, the behavior is undefined.

The above quote from the standard indicates that undefined behavior is incurred if the return value of a function that has reached the closing brace is used }, so the following example has undefined behavior:

int comportamiento_indefinido(int x) { if (x) return x; }

printf("%d", comportamiento_indefinido(1)); // Correcto
printf("%d", comportamiento_indefinido(0)); // Comportamiento indefinido

In the call comportamiento_indefinido(1)the function returns 1through the statement returnunder the conditional if (x); in the call comportamiento_indefinido(0)the condition if (x)is not met and the function ends when the closing brace is reached }, using the return of the call comportamiento_indefinido(0)is undefined behavior (but using the return of is not comportamiento_indefinido(1)). But what happens in this case?

int comportamiento_indefinido(int x) { if (x) return; } // return vacio

printf("%d", comportamiento_indefinido(1)); // Comportamiento indefinido?
printf("%d", comportamiento_indefinido(0)); // Comportamiento indefinido

In the previous example, the call comportamiento_indefinido(1)does not comply with what is indicated in §6.9.1/12to achieve undefined behavior, since the function ends without reaching the closing brace }and does not return any value either.

What part of the C standard takes this into account?

1 Or at least I couldn't find it.

c

2 Answers

  1. Best Answer

    I find the question very interesting and relevant. For example, one does not always have access to correct a code, but it would be interesting to understand what it does. For example, knowing if it is a security flaw.

    It seems to me that it can be understood that both cases are equivalent. Actually, being a " no debe" it doesn't have to compile if it's strictly C99, right? Because otherwise it should say " no debería". Since the original standard (in English) says:

    6.8.6.4 The returnstatements

    constraints

    A returnstatement with an expression shall not appear in a function whose return type is void. A returnstatement without an expression shall only appear in a function whose return type is void.

    And the same standard says (bold mine):

    4.Compliance

    In this International Standard, ''shall'' is to be interpreted as a requirement on an implementation or on a program; conversely, ''shall not'' is to be interpreted as a prohibition . 2 If a ''shall'' or ''shall not'' requirement that appears outside of a constraint is violated, the behavior is undefined . Undefined behavior is otherwise indicated in this International Standard by the words ''undefined behavior'' or by the omission of any explicit definition of behavior. There is no difference in emphasis among these three; they all describe ''behavior that is undefined''.

    That is, if it says nothing about the behavior, then it is undefined .

    What will happen most of the time? I imagine that to the compiler it is equivalent, because a returnno expression compiles to ret(or equivalent to) on most platforms, as does putting none. While with argument, it becomes moving to a register or pushing on the stack the result of the expression, and then returning. For example, a return 8;can be a mov ax,8or a push 8before the ret. If none is specified returnin C, the compiler must still include a retin the assembler, whether or not the function has a type, as if it had been made return;. Do you understand what I say?

    EDITED : I remove reference to another RFC and considerations about de facto standards, finding that the same standard defines it.

    • 4

  2. 4.Compliance

    1. If a ''shall'' or ''shall not'' requirement that appears outside of a constraint is violated, the behavior is undefined. [...].

    (In Spanish) In other words, if a restriction of the standard, of those indicated by shall (= must) or shall not (= must not) is not fulfilled by a program, said program contains undefined behavior.

    And as @ESL quote:

    6.8.6.4 The returnstatements

    constraints

    A returnstatement with an expression shall not appear in a function whose return type is void. A returnstatement without an expression shall only appear in a function whose return type is void.

    (In Spanish) That is, if a function voidtries to return something ( return expr;), or if a function with a type other than voidtries to return nothing ( return;), the behavior is undefined ( undefined behavior ).

    En tu caso, dado que tu sentencia return aparece sin valor en una función que requiere un valor de retorno, es comportamiento indefinido de acuerdo al punto 4.2.

    Conversely, this would also be undefined behavior:

    void f() { return 4; }

    These definitions are taken from:

    • Committee Draft n1570.pdf - April 12, 2011 ISO/IEC 9899:201x
    • 4