c - 链接时允许不同的类型

Question

我有两个文件。第一个 a.c 有

extern int i;
int main()
{
    i = 5;
    printf("i=%d",i);
}

另一个文件b.c有

struct i
{
    int a;
    int b;
}

当编译这两个文件并链接它们时，我得到i=5作为输出。

为什么会发生这种情况？ i 在 a.c 和 b.c 中具有单独的类型。

Answer 1

在 C 语言中，相同标识符的不同出现可能具有不同的值相同编译单元中的类型(不用介意链接)。这程序没问题:

#include <stdio.h>

struct i
{
    int i;
};

static int i;

int main(void)
{
    i = 1;
    struct i ii;
    ii.i = i;
    printf("%d\n",ii.i);
    return 0;
}

根据 C99，结构标记和变量名称位于不同的命名空间中标准:

6.2.3 标识符的命名空间

If more than one declaration of a particular identifier is visible at any point in a translation unit, the syntactic context disambiguates uses that refer to different entities. Thus, there are separate name spaces for various categories of identifiers, as follows:

— label names (disambiguated by the syntax of the label declaration and use);

— the tags of structures, unions, and enumerations (disambiguated by following any of the keywords struct, union, or enum);

— the members of structures or unions; each structure or union has a separate name space for its members (disambiguated by the type of the expression used to access the member via the . or -> operator);

— all other identifiers, called ordinary identifiers (declared in ordinary declarators or as enumeration constants).

(该程序也是合法的 C++，将 <stdio.h> 替换为 <cstdio> )