有精通C标准的吗，为什么么一个文件里面一个全局变量初始化过后还可以在定义未初始化的

Question

如图所示，为什么还能编译过？

#include <stdio.h>
int aaa=0,bbb=0,ccc=0;
int aaa,bbb,ccc;
int aaa,bbb,ccc;
int aaa,bbb,ccc;
int main()
{
   /*  Write C code in this online editor and run it. */
   printf("Hello, World! \n");
   aaa = 10;
   return 0;
}

Answer 1

代码中int aaa, bbb, ccc;叫做暂定的定义，相当于声明。 这是为了兼容标准C语言之前的旧代码，那时候编译器把多个重复定义的全局变量合并成一个。

#include <stdio.h>
int aaa = 0, bbb = 0, ccc = 0;  // external definitions
int aaa, bbb, ccc;  // tentative definitions, act as declarations
int aaa, bbb, ccc;  // tentative definitions, act as declarations
int aaa, bbb, ccc;  // tentative definitions, act as declarations
int main()
{
    /*  Write C code in this online editor and run it. */
    printf("Hello, World! \n");
    aaa = 10;
    return 0;
}

From https://en.cppreference.com/w/c/language/extern#Tentative_definitions

A tentative definition is an external declaration without an initializer, and either without a storage-class specifier or with the specifier static.

A tentative definition is a declaration that may or may not act as a definition. If an actual external definition is found earlier or later in the same translation unit, then the tentative definition just acts as a declaration.