未设置的 C 指针不为空

Question

我在搞乱 C 指针。当我编译并运行以下代码时。

示例 1：

#include <stdio.h>

int main()
{
    int k;
    int *ptr;
    k = 555;
    if (ptr == NULL) {
        printf("ptr is NULL\n");
    } else {
        printf("ptr is not NULL\n");
        printf("ptr value is %d\n", *ptr);
    }
    printf("ptr address is %p\n", ptr);
}

我得到了输出：

ptr is not NULL
ptr value is 1
ptr address is 0x7fff801ace30

如果我不给 k 赋值：

示例 2：

#include <stdio.h>

int main()
{
    int k;
    int *ptr;
    if (ptr == NULL) {
        printf("ptr is NULL\n");
    } else {
        printf("ptr is not NULL\n");
        printf("ptr value is %d\n", *ptr);
    }
    printf("ptr address is %p\n", ptr);
}

那么输出如我所料：

ptr is NULL
ptr address is (nil)

同样，如果我在函数之外定义变量：

示例 3：

#include <stdio.h>

int k;
int *ptr;

int main()
{
    k = 555;
    if (ptr == NULL) {
        printf("ptr is NULL\n");
    } else {
        printf("ptr is not NULL\n");
        printf("ptr value is %d\n", *ptr);
    }
    printf("ptr address is %p\n", ptr);
}

输出：

ptr is NULL
ptr address is (nil)

在第一个例子中，ptr 有一个地址和值，这是预期的行为吗？如果是的话：

• 为什么 ptr 有地址和值？
• 这些来自哪里，是什么原因造成的？
• 如何在本地范围内正确定义空指针并保持它们为空，直到我准备好使用？

我在 x64 上的 Ubuntu 12.04.04 上使用 gcc 进行编译：

root@dev:~# gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.6.3-1ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5)

编辑

为清楚起见，我在上面的示例中进行了编号。

根据 Dietrich 的回答，我进行了一些搜索，发现了这个问题：Why are global variables always initialized to '0', but not local variables?。

Answer 1

C中的局部变量不会自动初始化，只有全局变量会自动初始化。

int x; // 0 by default
static int y; // 0 by default

void function()
{
    static int z; // 0 by default
    int w; // Not initialized!  Could be anything!
}

未初始化变量的值是未指定。在实践中，这可能意味着不同的东西。

• 如果您的编译器或运行时在使用内存之前将其归零，则它可能为零。

• 它可以用0xdeadbeef 或0xeeeeeeee 之类的标记来填充。

• 它可能包含来自该特定内存位置最后的垃圾。 （这种方式最为常见。）

您正在使用 GCC，它使用第三种方法，因此您看到最后一个函数使用该内存位置的垃圾。您可以使用 Valgrind 运行程序（强烈推荐），它可能会因为使用未初始化的内存而吐出错误消息，但不能保证捕获所有错误。

正确的做事方式

如果要使用局部变量，一个选项是显式初始化它们。

void function()
{
    int *ptr = NULL;
    ...
}

如果未使用该值，我更愿意保留未初始化的变量，因为编译器和 Valgrind 可以给我诊断消息，让我知道我对代码的理解是不正确的，如果事实证明使用了未初始化的内存。

Answer 2

切勿将间接运算符应用于未初始化的指针变量。如果未初始化，则尝试使用它会调用未定义的行为。
上述两个代码正在调用未定义的行为。在这种情况下，结果可能是预期的或意外的。

最后一个代码工作正常，因为全局变量被隐式初始化为0。因此指针 ptr 被初始化为 NULL 并且你得到了正确的输出。

Answer 3

也许你可以试试这个代码来知道你想知道什么：

#include <stdio.h>
#include <stdlib.h>
typedef struct _appl{
//the pointer
int *ptr_int;
    float *ptr_float;
    double *ptr_double;
    char *ptr_char;

    //the basic data types
    int a_int;
    float a_float;
    double a_double;
    char a_char;

    struct _appl *next;
    } appl;

    //the pointers
    int *ptr_int_global;
    float *ptr_float_global;
    double *ptr_double_global;
    char *ptr_char_global;

    //the basic data types
    int a_int_global;
    float a_float_global;
    double a_double_global;
    char a_char_global;

    //pointer to structure data type
    appl *ptr_struct_global;

    //struct data type;
    appl a_struct_global;

    int main()
    {
    //the pointers
    int *ptr_int_local;
    float *ptr_float_local;
    double *ptr_double_local;
    char *ptr_char_local;

    //the basic data types
    int a_int_local;
    float a_float_local;
    double a_double_local;
    char a_char_local;

    //pointer to structure data type
    appl *ptr_struct_local;

    //struct data type;
    appl a_struct_local;

    //test the basic data types (local variables)
    puts("********");
    puts("****first test : test the basic data types (local variables)");
    puts("********");
    printf("the value of a non-initialized int is : %d\n", a_int_local);
    printf("the value of a non-initialized float is : %f\n", a_float_local);
    printf("the value of a non-initialized double is : %f\n", a_double_local);
    printf("the value of a non-initialized char is : %d\n\n", a_char_local);

    //test the basic data types (global variables)
    puts("second test : test the basic data types (global variables)");
    printf("the value of a non-initialized int is : %d\n", a_int_global);
    printf("the value of a non-initialized float is : %f\n", a_float_global);
    printf("the value of a non-initialized double is : %f\n", a_double_global);
    printf("the value of a non-initialized char is : %d\n\n", a_char_global);

    //test the pointers (local variables)
    puts("third test : test basic data type pointers (local variables)");
    if(ptr_int_local == NULL) printf("a non-initialized int pointer is NULL\n");
    else printf("a non-initialized int pointer is not NULL\n");
    if(ptr_float_local == NULL) printf("a non-initialized float pointer is NULL\n");
    else printf("a non-initialized float pointer is not NULL\n");
    if(ptr_double_local == NULL) printf("a non-initialized double pointer is NULL\n");
    else printf("a non-initialized double pointer is not NULL\n");
    if(ptr_char_local == NULL) printf("a non-initialized char pointer is NULL\n\n");
    else printf("a non-initialized char pointer is not NULL\n\n");
    puts("attention ! sometimes a non-initialized char pointer (global variables) is NULL, while sometimes not !");


    //test the pointers (global variables)
    puts("fourth test : test basic data type pointers (global variables)");
    if(ptr_int_global == NULL) printf("a non-initialized int pointer is NULL\n");
    else printf("a non-initialized int pointer is not NULL\n");
    if(ptr_float_global == NULL) printf("a non-initialized float pointer is NULL\n");
    else printf("a non-initialized float pointer is not NULL\n");
    if(ptr_double_global == NULL) printf("a non-initialized double pointer is NULL\n");
    else printf("a non-initialized double pointer is not NULL\n");
    if(ptr_char_global == NULL) printf("a non-initialized char pointer is NULL\n\n");
    else printf("a non-initialized char pointer is not NULL\n\n");

    //test data structure (local variable)
    puts("fifth test : test data structure type (local variables)");
    if(ptr_struct_local == NULL) printf("a non-initialized data structure pointer is NULL\n\n");
    else printf("a non-initialized data structure pointer is not NULL\n\n");

    //test data structure (global variable)
    puts("sixth test : test data structure type (global variables)");
    if(ptr_struct_global == NULL) printf("a non-initialized data structure pointer is NULL\n\n");
    else printf("a non-initialized data structure pointer is not NULL\n\n");

    ptr_struct_local = (struct _appl*)malloc(sizeof(struct _appl*));
    if(ptr_struct_local == NULL) printf("failed to allocate memory\n\n");
    else printf("success of allocating memory\n\n");

    appl *a = ptr_struct_local;
    //test the basic data types (in a local data structure variable)
    puts("\nseventh test : test the basic data types (in a local data structure variable)");
    printf("the value of a non-initialized int is : %d\n", a->a_int);
    printf("the value of a non-initialized float is : %f\n", a->a_float);
    printf("the value of a non-initialized double is : %f\n", a->a_double);
    printf("the value of a non-initialized char is : %d\n\n", a->a_char);

    //test the pointers (in a local data structure variable)
    puts("eigth test : test the pointers (in a local data structure variable)");
    if(a->ptr_int == NULL) printf("a non-initialized int pointer is NULL\n");
    else printf("a non-initialized int pointer is not NULL\n");
    if(a->ptr_float == NULL) printf("a non-initialized float pointer is NULL\n");
    else printf("a non-initialized float pointer is not NULL\n");
    if(a->ptr_double == NULL) printf("a non-initialized double pointer is NULL\n");
    else printf("a non-initialized double pointer is not NULL\n");
    if(a->ptr_char == NULL) printf("a non-initialized char pointer is NULL\n\n");
    else printf("a non-initialized char pointer is not NULL\n\n");

    ptr_struct_global = (struct _appl*)malloc(sizeof(struct _appl*));
    if(ptr_struct_global == NULL) printf("failed to allocate memory\n\n");
    else printf("success of allocating memory\n\n");

    appl *b = ptr_struct_global;
    //test the basic data types (in a global data structure)
    puts("\nninth test : test the basic data types (in a global data structure)");
    printf("the value of a non-initialized int is : %d\n", b->a_int);
    printf("the value of a non-initialized float is : %f\n", b->a_float);
    printf("the value of a non-initialized double is : %f\n", b->a_double);
    printf("the value of a non-initialized char is : %d\n\n", b->a_char);

    //test the pointers (in a global data structure)
    puts("tenth test : test the pointers (in a global data structure)");
    if(b->ptr_int == NULL) printf("a non-initialized int pointer is NULL\n");
    else printf("a non-initialized int pointer is not NULL\n");
    if(b->ptr_float == NULL) printf("a non-initialized float pointer is NULL\n");
    else printf("a non-initialized float pointer is not NULL\n");
    if(b->ptr_double == NULL) printf("a non-initialized double pointer is NULL\n");
    else printf("a non-initialized double pointer is not NULL\n");
    if(b->ptr_char == NULL) printf("a non-initialized char pointer is NULL\n\n");
    else printf("a non-initialized char pointer is not NULL\n\n");

}