Linux系统中的文件操作

linux中，一切皆文件（网络设备除外）
硬件设备也“是”文件，通过文件来使用设备
目录（文件夹）也是一种文件

预定义的文件描述符：
  0：标准输入，对应于已打开的标准输入设备（键盘）
  1：标准输出，对应于已打开的标准输出设备（控制台）
  2 : 标准错误， 对应于已打开的标准错误输出设备（控制台）
  多次打开同一个文件，可得到多个不同的文件描述符。

// main1.c
#include <errno.h>
#include <string.h>
int main(void)
{
    int len = 0;
    char buff[] = "hello world\n";
    len = write(1, buff, sizeof(buff));
    if (len < 0)
    {
        printf("write to stdout failed. reason: %s\n", strerror(errno));
    }
    else
    {
        printf("write %d bytes.\n", len);
    }
    len = write(2, buff, sizeof(buff));
    if (len < 0)
    {
        printf("write to stderr failed. reason: %s\n", strerror(errno));
    }
    return 0;
}

// main2.c
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
    char buffer[1024];
    int cnt = 0;
    cnt = read(0, buffer, sizeof(buffer));
    write(1, buffer, cnt);
    return 0;
}

// open_demo.c
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#define FILE_RW_LEN 1024
int main(void)
{
    int fd = 0;
    int count = 0;
    char buffer[FILE_RW_LEN] = "I 'm Martin.";
    fd = open("./martin.txt", O_CREAT | O_RDWR | O_APPEND | O_TRUNC, S_IRWXU | S_IRGRP | S _IXGRP | S_IROTH);
    // fd = open("./martin.txt", O_CREAT|O_EXCL|O_RDWR, S_IRWXU|S_IRGRP|S_IXGRP|S                                 _IROTH);
    if (fd < 0)
    {
        printf("open file martin.txt failed. reason: %s\n", strerror(errno));
        exit(-1);
    }
    count = write(fd, buffer, strlen(buffer));
    printf("written: %d bytes.\n", count);
    close(fd);
}

// close_demo.c
#include <stdlib.h>
#include <stdio.h>
#define FILE1_NAME "file1.txt"
#define FILE2_NAME "file2.txt"
int main(void)
{
    int file1, file2;
    char buffer[4096];
    int len = 0;
    file1 = open(FILE1_NAME, O_RDONLY);
    if (file1 < 0)
    {
        printf("open file %s failed\n, reason: %s\n", FILE1_NAME, strerror(errno));
        exit(-1);
    }
    file2 = open(FILE2_NAME, O_CREAT | O_WRONLY, S_IRUSR | S_IWUSR);
    if (file2 < 0)
    {
        printf("open file %s failed\n, reason: %s\n", FILE2_NAME, strerror(errno));
        exit(-1);
    }
    while ((len = read(file1, buffer, sizeof(buffer))) > 0)
    {
        write(file2, buffer, len);
    }
    close(file2);
    close(file1);
    return 0;
}

补充：time命令
          time命令分别输出:
              real - 程序总的执行时间、
              usr - 该程序本身所消耗的时间、
              sys - 系统调用所消耗的时间

// lseek_demo.c
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#define FILE1_NAME "lseek_demo.c"
#define FILE2_NAME "lseek_demo_2.c"
#define SIZE 100
int main(void)
{
    int file1, file2;
    char buffer[1024];
    int ret;
    file1 = open(FILE1_NAME, O_RDONLY);
    if (file1 < 0)
    {
        printf("open file %s failed\n", FILE1_NAME);
        exit(-1);
    }
    file2 = open(FILE2_NAME, O_WRONLY | O_CREAT, S_IRUSR | S_IWUSR);
    if (file2 < 0)
    {
        printf("open file %s failed\n", FILE2_NAME);
        exit(-1);
    }
    // file size
    ret = lseek(file1, 0, SEEK_END);
    printf("file size: %d\n", ret);
    ret = lseek(file1, 100, SEEK_SET);
    printf("lseek ret: %d\n", ret);
    ret = read(file1, buffer, SIZE);
    if (ret > 0)
    {
        buffer[ret] = '\0';
        printf("read[%d]: %s\n", ret, buffer);
        write(file2, buffer, SIZE);
    }
    close(file1);
    close(file2);
    return 0;
}

解决方案: 使用带缓冲功能的标准I/O库（以减少系统调用次数）
/* C语言中的文件操作中已描述 */
1) fwrite
2) fread
3) fopen
4) fclose
5) fseek    
6) fflush

1) /proc/cpuinfo
   CPU详细信息
2) /proc/meminfo
   内存相关信息
3) /proc/version
   版本信息
4) /proc/sys/fs/file-max
   系统中能同时打开的文件总数
   可修改该文件
5) 进程的相关信息
   /proc/32689/ 表示指定进程（进程号为32689)的相关信息
6) /proc/devices
    已分配的字符设备、块设备的设备号

头文件：#include <unistd.h>
         #include <fcntl.h>
函数定义：int fcntl(int fd, int cmd, ... /* arg */ );
    参数： cmd 取值  F_GETLK,   F_SETLK 和 F_SETLKW ,分别表示获取锁、设置锁和同步设置锁.
 文件锁的表示
       struct flock

//
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#define FILE_NAME "test.txt"
int flock_set(int fd, int type)
{
    printf("pid=%d into...\n", getpid());
    struct flock flock;
    flock.l_type = type;
    flock.l_whence = SEEK_SET;
    flock.l_start = 0;
    flock.l_len = 0;
    flock.l_pid = -1;
    fcntl(fd, F_GETLK, &flock);
    if (flock.l_type != F_UNLCK)
    {
        if (flock.l_type == F_RDLCK)
        {
            printf("flock has been set to read lock by %d\n", flock.l_pid);
        }
        else if (flock.l_type == F_WRLCK)
        {
            printf("flock has been set to write lock by %d\n", flock.l_pid);
        }
    }
    flock.l_type = type;
    if (fcntl(fd, F_SETLKW, &flock) < 0)
    {
        printf("set lock failed!\n");
        return -1;
    }
    switch (flock.l_type)
    {
    case F_RDLCK:
        printf("read lock is set by %d\n", getpid());
        break;
    case F_WRLCK:
        printf("write lock is set by %d\n", getpid());
        break;
    case F_UNLCK:
        printf("lock is released by %d\n", getpid());
        break;
    default:
        break;
    }
    printf("pid=%d out.\n", getpid());
    return 0;
}
int main(void)
{
    int fd;
    fd = open(FILE_NAME, O_RDWR | O_CREAT, 0666);
    if (fd < 0)
    {
        printf("open file %s failed!\n", FILE_NAME);
    }
    // flock_set(fd, F_WRLCK);
    flock_set(fd, F_RDLCK);
    getchar();
    flock_set(fd, F_UNLCK);
    getchar();
    close(fd);
    return 0;
}