在C89(也稱ANSI C)中省略了return語句未定義的行為？
考慮以下基本例子：
#include <stdio.h>
int main(void)
{
printf("Hi there!\n");
}
它是否在C89中呼叫未定義的行為？我試圖從ofollow,noindex" target="_blank">this question 獲得一些意義,但大多數高調的答案都表明它是實現定義的,絕對沒有UB這裡(Keith Thompson的評論看起來是矛盾的).
規範在§3.16中定義和約定：
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
or by the omission of any explicit definition of
. There is no difference in emphasis among these three: theyall describe “behavior that is undefined”.
和§5.1.2.2.3程式終止：
A return from the initial call to themain
function is equivalent to
calling theexit
function with the value returned by themain
function
as its argument. If themain
function executes a return that specifies
no value, the termination status returned to the host environment is
undefined.
我的理解是,後一條款並不涵蓋失蹤回報的情況,因為沒有從未援引的回執陳述,因此適用前一條款.
然而,隨之而來的閱讀意味著什麼不同,§6.6.6.4返回語句：
If areturn
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
好的,現在5.1.2.2.3條款適用：
If themain
function executes a return that specifies
no value. the termination status returned to the host environment is
undefined.
“終止狀態未定義”一詞似乎不是UB,也沒有任何具體行為,但更像是C標準範圍之外,更像是“讓主機環境擔心,我們洗手這裡”.這是正確的內省嗎？
幾年前我實際調查了由此引起的問題.如果您有一些程式碼路徑與返回,而沒有其他程式碼路徑,它會變得更有趣.
由於@aruisdante在評論中推測,所展示的行為確實是“未定義”,但唯一未定義的部分是返回的值(它不像許多其他“未定義”的情況可能會導致程式崩潰).
這些日子實際上相當於一個安全風險,因為返回的’未定義’值通常是通常用於返回值(或者在某些實現中的堆疊)的CPU暫存器中發生的任何事情,理論上這可以用於洩漏敏感資料.
http://stackoverflow.com/questions/28203506/is-omitting-return-statement-undefined-behaviour-in-c89-aka-ansi-c