未设置的C指针不为空
我正在搞乱C指针。 当我编译并运行以下代码时。
例1：
#include 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); } 我得到输出：
ptr is not NULL ptr value is 1 ptr address is 0x7fff801ace30
如果我没有为k赋值：
例2：
#include 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); }
然后输出就像我期望的那样：
ptr is NULL ptr address is (nil)
同样，如果我在函数外定义变量：
例3：
#include 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)
在第一个示例中，ptr具有地址和值，这是预期的行为吗？ 如果是这样的话：
- 为什么ptr有地址和价值？
- 它们来自哪里，是什么设置它们？
- 如何在本地范围内正确定义空指针并将其保持为空，直到我准备好使用它为止？
我在x64上使用Uccntu 12.04.04上的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
为了清楚起见，我已将上面的例子编号。
基于Dietrich的回答，我做了一些搜索并发现了这个问题： 为什么全局变量总是初始化为’0’，而不是局部变量？ 。
C中的局部变量不会自动初始化，只会自动初始化全局变量。
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! }
未初始化变量的值未指定 。 在实践中，这可能意味着不同的事情。
-
如果您的编译器或运行时在使用之前将内存清零，则它可能为零。
-
它可以填充像
0xdeadbeef或0xeeeeeeee这样的0xeeeeeeee。 -
它可能包含来自该特定内存位置中最后一个的垃圾。 （这种方法最常见。）
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运行该程序（ 强烈推荐 ），它可能会使用未初始化的内存吐出错误消息，但不能保证捕获所有错误。
正确的做事方式
一种选择是如果要使用它们，则显式初始化局部变量。
void function() { int *ptr = NULL; ... }
如果没有使用该值，我更喜欢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certificate使用了未初始化的内存。
永远不要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 。
以上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初始化为NULL ，您将获得正确的输出。
您也许可以尝试使用此代码来了解您想要了解的内容：
#include #include 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"); }