c - 每次在 C 中执行程序时，指针地址都会改变吗？

Question

我试图理解指针，我遇到了这段代码，每当我编译和执行它时，地址都会改变。它是一些垃圾值还是指针实际上在移动中分配内存？

我的命令提示符:

kaushik@IntelliBox:~/Desktop/Learn_C$ ./Practice
nNUmber is equal to : 15
nNumber is equal to : 25
0xbf98fd64

kaushik@IntelliBox:~/Desktop/Learn_C$ make Practice
make: 'Practice' is up to date.

kaushik@IntelliBox:~/Desktop/Learn_C$ ./Practice
nNUmber is equal to : 15
nNumber is equal to : 25
0xbfcce2a4

kaushik@IntelliBox:~/Desktop/Learn_C$ ./Practice
nNUmber is equal to : 15
nNumber is equal to : 25
0xbfa25df4

kaushik@IntelliBox:~/Desktop/Learn_C$ ./Practice
nNUmber is equal to : 15
nNumber is equal to : 25
0xbfecf104

我的 C 代码是:

#include <stdio.h>

int main()
{
    int nNumber;
    int *pPointer;

    nNumber = 15;
    pPointer = &nNumber;

    printf("nNUmber is equal to : %d\n", nNumber );

    *pPointer = 25;

    printf("nNumber is equal to : %d\n", nNumber );

    printf("%p\n", pPointer );

    return 0;
}

提前谢谢你。

Answer 1

指针值的表示或确切构成是一个实现细节。 C标准没有说明任何要求。无法保证每次运行代码时该值是相同还是不同。

C 标准只定义了有效指针之间的指针运算(例如，比较数组对象中的两个指针)。

顺便说一句，您应该将指针转换为 void* 以按照 C 标准的要求使用 %p 进行打印:

printf("%p\n", (void*) pPointer );

---

如评论中所述，某些操作系统会执行 address space layout randamization . Linux 默认这样做。对于您的代码，我使用 ASLR 得到以下输出:

$ ./a.out
nNUmber is equal to : 15
nNumber is equal to : 25
0x7fffde18ba7c
$ ./a.out
nNUmber is equal to : 15
nNumber is equal to : 25
0x7fff981efe0c
$ ./a.out
nNUmber is equal to : 15
nNumber is equal to : 25
0x7ffdade6837c
$ ./a.out
nNUmber is equal to : 15
nNumber is equal to : 25
0x7ffced208b4c

如果我禁用它:

echo 0 > /proc/sys/kernel/randomize_va_space

然后它输出相同的值:

$ ./a.out
nNUmber is equal to : 15
nNumber is equal to : 25
0x7fffffffeaec
$ ./a.out
nNUmber is equal to : 15
nNumber is equal to : 25
0x7fffffffeaec
$ ./a.out
nNUmber is equal to : 15
nNumber is equal to : 25
0x7fffffffeaec
$ ./a.out
nNUmber is equal to : 15
nNumber is equal to : 25
0x7fffffffeaec
$ ./a.out
nNUmber is equal to : 15
nNumber is equal to : 25
0x7fffffffeaec
$ ./a.out
nNUmber is equal to : 15
nNumber is equal to : 25
0x7fffffffeaec
$ ./a.out
nNUmber is equal to : 15
nNumber is equal to : 25
0x7fffffffeaec
$ ./a.out
nNUmber is equal to : 15
nNumber is equal to : 25
0x7fffffffeaec
$ ./a.out
nNUmber is equal to : 15
nNumber is equal to : 25
0x7fffffffeaec

但就 C 标准而言，绝对不能保证值。