APUE_chapter3 文件IO

最新推荐文章于 2019-08-14 18:53:31 发布

shiyang6017

最新推荐文章于 2019-08-14 18:53:31 发布

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分类专栏： APUE LINUX

本文链接：https://blog.youkuaiyun.com/shiyang6017/article/details/52550457

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本文介绍了如何使用C语言进行文件操作，包括打开、读写、关闭等基本操作，并演示了lseek、fsync和fdatasync等功能的具体应用。此外，还探讨了通过fcntl调整文件属性的方法。

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#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>// 对POSIX操作系统API的访问功能的头文件名称
// open lseek open write close
int main(int argc , char* argv[]){// 3.1 - 3.8
    int fd = open (argv[1], O_RDWR | O_APPEND | O_CLOEXEC | O_CREAT); //  open
    if (fd  == -1)
        printf ("fail to open file:%s \n", argv[1]);
    else{
        printf ("succeed to open file:%s \n", argv[1]);

        // 若成功返回新的文件偏移量，出错返回-1；
        off_t curpos = lseek (fd, 0, SEEK_CUR);

        char* buf = (char*)malloc(100 * sizeof(char));
        // 文件尾返回0，出错返回-1
        while (read (fd, buf, 100) > 0){
            printf ("succeed to read the file : %s \n", buf);
        }
        char output[] = "hello world!\n";
        if ( write (fd, output, strlen(output)) == -1)
            printf ("succeed to write the file:%s \n", argv[1]);
        if (!close (fd)) // return 0 when succeed                                      //  close
            printf ("succeed to close file:%s \n", argv[1]);
        else
            printf ("fail to close file:%s \n", argv[1]);
    }
    return 0;
}

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>// 对POSIX操作系统API的访问功能的头文件名称
// fsync fdatasync
int main(int argc , char* argv[]){ // 3.13
    int fd = open (argv[1], O_RDWR | O_APPEND | O_CLOEXEC | O_CREAT);
    char output[] = "hello world!\n";
    time_t start_time = time(NULL);
    for (int i = 0; i < 10000000; ++i)
        write(fd, output, strlen(output));
    printf ("%d\n", time(NULL) - start_time); // 5s
    close(fd);

    fd = open (argv[1], O_RDWR | O_TRUNC | O_CLOEXEC | O_CREAT);
    fdatasync(fd);
     for (int i = 0; i < 10000000; ++i)
        write(fd, output, strlen(output)); //10s
    printf ("%d\n", time(NULL) - start_time);
    close(fd);

    fd = open (argv[1], O_RDWR | O_TRUNC | O_CLOEXEC | O_CREAT);
    fsync(fd);
     for (int i = 0; i < 10000000; ++i)
        write(fd, output, strlen(output)); // 15s
    printf ("%d\n", time(NULL) - start_time);
    close(fd);

    printf("finished");
    return 0;
}

#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
// fctnl函数 : 改变已经打开文件的属性,错误返回 - 1
int main(int argc, char* argv[]){ //3.14
    int fd = open (argv[1], O_RDWR | O_APPEND | O_CLOEXEC | O_CREAT); // O_SYNC 可以指定
    printf( "%d\n", fcntl (fd, F_GETFD, 0)); // return O_CLOEXEC

    int file_state_flag = fcntl(fd, F_GETFL, 0);
    if ( file_state_flag < 0){
        printf ("fctnl error for fd: %s \n", argv[1]);
        exit(1);
    }
    else{
        print_file_state_flag(file_state_flag);
	// 无法设置O_SYNC, 但显示成功设置
        if ( fcntl (fd, F_SETFL,  file_state_flag | O_NONBLOCK | O_SYNC) < 0 ){ 
            printf ("fail to set the file state\n");
            exit(1);
        }
        else{ // 成功返回0
            print_file_state_flag( fcntl(fd, F_GETFL, 0));
        }
    }
    close (fd);
    return 0;
}
void print_file_state_flag(int file_state_flag){
     switch ( file_state_flag & O_ACCMODE){ // O_ACCMODE = 3;
            case O_RDONLY:
                printf ("read only \n");
                break;
            case O_WRONLY:
                printf("write only \n");
                break;
            case O_RDWR:
                printf("read and write \n");
                break;
            default:
                printf ("unknown access mode\n");
    }
    if ( file_state_flag  & O_APPEND)
        printf ("append\n");
    if ( file_state_flag & O_NONBLOCK)
        printf ("nonblock\n");
    if ( file_state_flag & O_SYNC)
        printf ("sync\n");
}
void change_file_state_flag(int fd, int flag){ // set or clear
    int new_flag = fcntl(fd, F_GETFL, 0);
    new_flag |= flag;  // set
    //new_flag &= ~flag;  //clear
    if (fcntl(fd, F_SETFL, new_flag) < 0){
        printf ("fail to set the file state flag\n");
        exit(1);
    }
}

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