C:静态结构

问题描述

我对 C 比较陌生，正在复习一些代码以了解散列.

I am fairly new to C and am going over some code to learn about hashing.

我发现了一个包含以下代码行的文件:

I came across a file that contained the following lines of code:

#include <stdio.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>

#include <time.h>
#include <sys/time.h>

// ---------------------------------------------------------------------------

int64_t timing(bool start)
{
    static      struct timeval startw, endw; // What is this?
    int64_t     usecs   = 0;

    if(start) {
        gettimeofday(&startw, NULL);
    }
    else {
        gettimeofday(&endw, NULL);
        usecs   =
               (endw.tv_sec - startw.tv_sec)*1000000 +
               (endw.tv_usec - startw.tv_usec);
    }
    return usecs;
}

我以前从未遇到过以这种方式定义的静态结构.通常结构前面是结构的定义/声明.然而，这似乎只是表明将有类型为 timeval、startw、endw 的静态结构变量.

I have never come across a static struct defined in this manner before. Usually a struct is preceded by the definition/declaration of the struct. However, this just seems to state there are going to be static struct variables of type timeval, startw, endw.

我试图阅读它的作用，但还没有找到足够好的解释.有什么帮助吗?

I have tried to read up on what this does but have not yet found a good enough explanation. Any help?

推荐答案

struct timeval 是一个在 sys/time.h 中声明的结构体.您突出显示的那一行声明了两个名为 startw 和 endw 且类型为 struct timeval 的静态变量.static 关键字适用于声明的变量，而不是结构体(类型).

struct timeval is a struct declared somewhere in sys/time.h. That line you highlighted declares two static variables named startw and endw of type struct timeval. The static keyword applies to the variables declared, not the struct (type).

您可能更习惯于具有 typedef 名称的结构，但这不是必需的.如果你像这样声明一个结构体:

You're probably more used to structs having a typedef'd name, but that's not necessary. If you declare a struct like this:

struct foo { int bar; };

然后您已经声明(并在此处定义)一个名为 struct foo 的类型.每当您想声明该类型的变量(或参数)时，您都需要使用 struct foo.或者使用 typedef 给它另一个名字.

Then you've declared (and defined here) a type called struct foo. You'll need to use struct foo whenever you want to declare a variable (or parameter) of that type. Or use a typedef to give it another name.

foo some_var;              // Error: there is no type named "foo"
struct foo some_other_var; // Ok
typedef struct foo myfoo;
myfoo something_else;      // Ok, typedef'd name
// Or...
typedef struct foo foo;
foo now_this_is_ok_but_confusing;

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