函数返回值—返回局部变量

看看下面的程序的输出:<wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">#include &lt;stdio.h&gt;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">char *returnStr()</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">{</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> char *p="hello world!";</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> return p;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">}</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">int main()</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">{</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> char *str;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> str=returnStr();</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("%s\n", str);</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> return 0;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">}</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">这个没有任何问题,因为"hello world!"是一个字符串常量,存放在静态数据区,把该字符串常量存放的静态数据区的首地址赋值给了指针,所以returnStr函数退出时,该该字符串常量所在内存不会被回收,故能够通过指针顺利无误的访问。</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">但是,下面的就有问题:</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">#include &lt;stdio.h&gt;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">char *returnStr()</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">{</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> char p[]="hello world!";</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> return p;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">}</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">int main()</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">{</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> char *str;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> str=returnStr();</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("%s\n", str);</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> return 0;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">}</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"></span></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

"hello world!"是一个字符串常量,存放在静态数据区,没错,但是把一个字符串常量赋值给了一个局部变量(char []型数组),该局部变量存放在栈中,这样就有两块内容一样的内存,这是与前着最本质的区别,当returnStr函数退出时,栈要清空,局部变量的内存也被清空了,所以这时的函数返回的是一个已被释放的内存地址,所以打印出来的是乱码。<wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">如果函数的返回值非要是一个局部变量的地址,那么该局部变量一定要申明为static类型。如下:</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">#include &lt;stdio.h&gt;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">char *returnStr()</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">{</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> static char p[]="hello world!";</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> return p;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">}</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">int main()</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">{</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> char *str;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> str=returnStr();</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("%s\n", str);</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> return 0;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">}</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">这个问题可以通过下面的一个例子来更好的说明:</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">#include &lt;stdio.h&gt;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">//返回的是局部变量的地址,该地址位于动态数据区,栈里</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">char *s1()</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">{</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> char p[]="Hello world!";</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("in s1 p=%p\n", p);</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("in s1: string's address: %p\n", &amp;("Hello world!"));</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> return p;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">}</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">//返回的是字符串常量的地址,该地址位于静态数据区</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">char *s2()</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">{</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> char *q="Hello world!";</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("in s2 q=%p\n", q);</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("in s2: string's address: %p\n", &amp;("Hello world!"));</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> return q;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">}</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">//返回的是静态局部变量的地址,该地址位于静态数据区</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">char *s3()</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">{</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> static char r[]="Hello world!";</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("in s3 r=%p\n", r);</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("in s3: string's address: %p\n", &amp;("Hello world!"));</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> return r;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">}</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">int main()</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">{</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> char *t1, *t2, *t3;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> t1=s1();</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> t2=s2();</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> t3=s3();</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("in main:");</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("p=%p, q=%p, r=%p\n", t1, t2, t3);</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("%s\n", t1);</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("%s\n", t2);</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> printf("%s\n", t3);</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"> return 0;</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">}</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em"></span></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

运行输出结果:<wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">in s1 p=0xbff92efb</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">in s1: string's address: 0x80486ac</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">in s2 q=0x80486ac</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">in s2: string's address: 0x80486ac</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">in s3 r=0x804998c</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">in s3: string</span><wbr style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><br style="margin:0px; padding:0px"><span style="font-family:新宋体; margin:0px; padding:0px; line-height:1.3em">这个结果正好应证了上面解释,同时,还可是得出一个结论:字符串常量,之所以称之为常量,因为它可一看作是一个没有命名的字符串且为常量,存放在静态数据区。这里说的静态数据区,是相对于堆、栈等动态数据区而言的。静态数据区存放的是全局变量和静态变量,从这一点上来说,字符串常量又可以称之为一个无名的静态变量,因为"Hello world!"这个字符串在函数 s1和s2 中都引用了,但在内存中却只有一份拷贝,这与静态变量性质相当神似。</span></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

char *p = "abcdefg";//静态存储区

char p[] = "abcdefg"; // p本身是数组名了,数组里放的字符串,是局部变量,内容是原来的静态区域内容的拷贝!因此返回p实际上返回的局部变量地址而不是静态存储区地址,和上面不同!

<think>嗯,用户问的是能否将局部变量的地址作为函数返回值返回。这个问题看起来和C/C++相关,因为在这些语言中,指针和内存管理是关键概念。首先,我需要回忆一下函数返回局部变量的生命周期。 在C/C++中,局部变量是在栈上分配的,当函数执行完毕时,这个栈帧会被销毁,局部变量的内存也就被释放了。所以,如果返回局部变量的地址,这个指针就会成为悬空指针,指向已经释放的内存区域。这时候如果访问它,会导致未定义行为,可能引发程序崩溃或数据错误。 不过,用户可能想知道有没有例外情况或者解决办法。比如,使用静态局部变量或者动态内存分配。静态变量的生命周期是整个程序运行期间,返回它的地址是安全的。而动态分配的内存(比如用malloc或new)需要手动管理,但地址可以作为返回值。不过用户的问题特别提到的是“局部变量”,通常指的是自动存储期的变量,所以静态的可能不算常规局部变量。 另外,用户可能是在编写函数时遇到了问题,比如返回一个指向数组或结构体的指针,但错误地返回局部变量的地址。这时候需要指导正确的做法,比如动态分配或者让调用者传入缓冲区。 还需要考虑不同语言的情况,比如Go语言有逃逸分析,编译器会自动决定变量分配在栈还是堆上。但在C/C++中,必须手动处理这些问题。可能用户对这方面不太清楚,需要明确说明语言环境。 总结回答要点:不能直接返回局部变量的地址,因为会导致悬空指针。解决方法包括使用静态变量、动态分配或让调用者管理内存。同时提醒未定义行为的风险,并提供正确做法的示例。</think>在 C/C++ 等涉及手动内存管理的语言中,**直接返回局部变量的地址是危险的**,会导致未定义行为。原因和解决方案如下: --- ### **为什么不能返回局部变量的地址?** 1. **栈内存生命周期问题** 局部变量存储在栈上,当函数结束时,栈帧被销毁,该变量的内存空间会被回收。返回其地址后,虽然指针依然存在,但指向的内存内容可能被后续操作覆盖。 2. **悬空指针(Dangling Pointer)** 返回的指针会指向已失效的内存区域,访问它可能导致程序崩溃、数据错误或难以调试的问题。 --- ### **如何安全返回指针?** #### 方法 1:使用 `static` 修饰局部变量 ```c int* func() { static int value = 42; // 静态变量,生命周期持续到程序结束 return &value; // 安全,但多次调用会共享同一份数据 } ``` ⚠️ 缺点:所有调用共享同一份数据,可能导致非预期副作用。 #### 方法 2:动态内存分配(堆内存) ```c int* func() { int* value = (int*)malloc(sizeof(int)); // 堆内存需手动释放 *value = 42; return value; // 返回堆地址,需调用者负责 free() } ``` ⚠️ 缺点:调用者必须记得释放内存,否则会导致内存泄漏。 #### 方法 3:由调用者提供内存(推荐) ```c void func(int* output) { *output = 42; // 操作调用者传入的内存 } // 调用方: int main() { int x; func(&x); // 安全,内存由 main 管理 return 0; } ``` 优点:内存管理权明确,避免悬空指针。 --- ### **其他语言的情况** - **Go**:编译器自动进行逃逸分析,若局部变量返回,会将其分配到堆上。 - **Rust**:所有权机制直接禁止返回局部变量的引用,编译时会报错。 --- ### **总结** - **不要返回局部变量的地址**(栈内存地址)。 - 若需返回指针,优先使用动态分配(堆内存)或让调用者管理内存。 - 未定义行为难以调试,务必在代码中避免这类操作。
评论
添加红包

请填写红包祝福语或标题

红包个数最小为10个

红包金额最低5元

当前余额3.43前往充值 >
需支付:10.00
成就一亿技术人!
领取后你会自动成为博主和红包主的粉丝 规则
hope_wisdom
发出的红包
实付
使用余额支付
点击重新获取
扫码支付
钱包余额 0

抵扣说明:

1.余额是钱包充值的虚拟货币,按照1:1的比例进行支付金额的抵扣。
2.余额无法直接购买下载,可以购买VIP、付费专栏及课程。

余额充值