Linux基础（一）系统api与库函数的关系

1. 系统api与库函数的关系

 

man 2 open

1.1 open

1.2 read/write

实现cat功能

#include <stdio.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>


int main(int argc, char *argv[])
{

    if (argc != 2)
    {
        printf("./a.out filename
");
        return -1;
    }

    int fd = open(argv[1], O_RDONLY);

    //读，输出到屏幕
    char buff[200];
    int ret = 0;
    while (ret = read(fd, buff, sizeof(buff)))
    {
        write(STDOUT_FILENO, buff, ret);

    }

    close(fd);

    return 0;
}

1.3 lseek

#include <stdio.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>


int main(int argc, char *argv[])
{

    if (argc != 2)
    {
        printf("./a.out filename
");
        return -1;
    }

    int fd = open(argv[1], O_RDWR|O_CREAT, 0666);

    write(fd, "helloword", 11);
    //文件读写的位置此时到末尾
    //需要移动读写的位置
    lseek(fd, 0, SEEK_SET);

    char buf[256] = {0};
    int ret = read(fd, buf, sizeof(buf));

    if (ret)
    {
        write(STDOUT_FILENO, buf, ret);
    }

    close(fd);

    return 0;
}

 计算大小

    int fd = open(argv[1], O_WRONLY);

    int ret = lseek(fd, 0, SEEK_END);

    printf("file size is %d
", ret);

    close(fd);

拓展文件

    //1. open
    int fd = open(argv[1], O_WRONLY|O_CREAT, 0666);

    int ret = lseek(fd, 1024, SEEK_END);

    printf("file size is %d
", ret);

    
    //需要至少写一次，否则不能保存
    write(fd, "a", 1);

    close(fd);

1.4 阻塞

• read函数在读设备或者的读管道，或者读网络的时候。
• 输入输出设备对应 /dev/tty

int fd = open("/dev/tty", O_RDWR|O_CREAT|O_NONBLOCK);

1.5 fcntl函数--设置非阻塞

#include <unistd.h>
#include <fcntl.h>

int fcntl(int fd, int cmd, ... /* arg */ );

int flags = fcntl(fd, F_GETFL);
flags |= O_NONBLOCK;