Linux-based I/O operations

I/O programming for Linux files

Introduction to files and file descriptors in Linux

In Linux, operations on directories and pens are equivalent to operations on files
There are four main types of files in Linux: ordinary files, directory files, connection files and device files
Under Linux, all operations on devices and files are performed using file descriptors
The startup of a process is generally accompanied by the opening of three files: standard input (STDIN_FILENO), standard output (STDOUT_FILENO), and standard error (STDERR_FILENO)

Basic I/O operation functions --> without buffering:

open、read、write、lseek以及close

open and close functions

The open function is used to open or create a file, and attributes can be specified at the same time
The close function is used to close an open file

Syntax format:

头文件：
    #include<sys/types.h>
    #include<sys/stat.h>
    #include<fcntl.h>
    #include<unistd.h>

open function:

int open(const char *pathname, flags, int perms);
pathname:被打开的文件名(可包含路径)
flag(打开方式):(常用方式)
    O_RDONLY:只读方式
    O_WRONLY:可写方式
    O_RDWR:读写方式
    O_CREATE:如果文件不存在，就创建一个新的文件，并用第三个参数为其设置权限
    --------------------------------
    O_EXCL:用于测试文件是否存在
    O_TRUNC:如果文件已经存在，并且以只读或只写成功打开，那么会先全部删除文件中原有数据
    O+APPEND:以添加的方式打开文件，打开时，文件指针指向文件的末尾
perms：被打开文件的存取权限，为8禁止表示法
flag参数可以通过"|"连接多个组合形式
返回值：
    成功返回文件描述符，失败返回-1

close function:

int close(int fd);
fd:文件描述符
返回值：
    成功返回0，失败返回-1

Such as: mathod_01.c

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

int main(void){
    int fd;     //定义文件描述符
    //调用open函数，以可读写的方式打开，使用“|”符号链接
    if((fd=open("/home/my/myTest.c", O_CREATE | O_TRUNC | O_RDWR, 0666)) < 0){
        perror("open:");    //打印出错信息
        exit(1);
    }else{
        //注意：open函数返回的文件描述符一定是最小的未用文件描述符
        printf("Open file:myTest.c %d\n", fd);
    }

    if(close(fd) <０)｛
        perror("close:");
        exit(1);
    ｝else{
        printf("Close myTest.c\n");
    }
    exit(0);
}

read, write and lseek functions

Function role:
- read: read data from the specified file descriptor
- write: used to write data to the open file, the write operation starts from the current displacement of the file, and returns failure if it exceeds
- lseek: used to locate the file pointer to the corresponding position in the specified file descriptor

Syntax format:

head File

#include<unistd.h>
#include<sys/types.h>

read function

ssize_t read(int fd, void *buf, size_t count);
fd：文件描述符
buf：指定存储器读出数据的缓冲区
count：指定读出的字节数
返回值：成功返回读到的字节数；0表示读到文件尾；-1表示出错

write function

ssize_t write(int fd, void *buf, size_t count);
fd：文件描述符
buf：指定存储器写入数据的缓冲区
count：指定写入的字节数
返回值：成功，返回已写字节数；失败，返回-1

lseek function

off_t lseek(int fd, off_t offset, int whence);
fd：文件描述符
offset：偏移量，每一读写操作所需要移动的距离，单位是字节的数量。可正(前移)可负(后移)
whence(当前位置的基点)：
    SEEK_SET：当前位置为文件的开头，新位置为便宜量的大小
    SEEK_CUR：当前位置为文件指针的位置，新位置为当前位置加上偏移量
    SEEK_END：当前位置为文件的结尾，新位置为文件的大小加上偏移量的大小
返回值：成功，返回当前位移；失败返回-1

Such as: method_02.c

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

#define MAXSIZE

int main(void){
    int i, fd, size, len;
    char *buf = "Hello! Welcom to Linux World!";
    char buf_read[10];
    len = strlen(buf);
    //打开文件并制定权限
    if((fd = open("/home/my/myTest.c", O_CREAT | O_TRUNC | O_RDWR, 666)) < 0 ){
        perror("Open:");
        exit(1);
    }else{
        printf("open file:myTest.c\n", fd);                 
    }

    //write函数将buf中的内容写入到打开的文件中
    if((size = write(fd, buf, len)) < 0){
        perror("write:");
        exit(1);
    }else{
        printf("Write:%s\n", buf);
    }

    //调用lseek函数移动指针，并调用read读取文件
    lseek(fd, 0, SEEK_SET);
    if((size = read(fd, buf_read, 10)) < 0){
        perror("read:");
        exit(0);
    }else{
        printf("read from file:%s\n", buf_read);
    }

    //关闭close()
    if(close(fd) < 0){
        perror("close:");
        exit(1);
    }else{
        printf("Close myTest.c\n");
    }
    exit(0);
}

fcntl function: Manipulate the characteristics of file descriptors

function format

head File

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

Function prototype:

int fcntl(int fd, int cmd, struct flock *lock);
fd：文件描述符
cmd：(常见的有以下几个)
    F_DUPFD：复制文件描述符
    F_GETFL：得到open设置的标志
    F_SETFL：改变open设置的标志
    F_GETTFK：根据lock描述，决定是否上文件锁
    F_SETFK：设置lock描述的文件锁
    F_SETOWN：设置进程号或进程组号
lock：设置记录锁的具体状态
    lock的结构：
        struct flock{
            short l_type;       //取值：F_RDLCK(读取锁→共享锁)、F_WRLCK(写入锁→排斥锁)、F_UNLCK(解锁)
            short l_start;      //相对位移量(字节)
            short l_whence;     
            off_t l_len;        //加锁区域长度
            pid_t l_pid;
        }
返回值：成功，返回0,；失败返回-1

Standard I/O Development -> Buffer Based

Purpose:
Reduce the number of calls using read and write
Three types of buffer storage:
- Full buffering The actual I/O operation takes place
  when the standard I/O buffer is filled.
- Line Buffering The standard I/O library performs I/O operations
  when input and output have new line characters.
- No buffering
  No of characters, e.g. to display error messages as quickly as possible

Open and close files

Open file standard functions: fopen, fdopen, freopen, return a pointer to FILE
- fopen: specify the path and mode to open the file
- fdopen: Specifies that the opened file description matches the pattern
- freopen: You can specify the open file, mode, and specify a specific IO stream

function format definition

head File
```
#include<stdio.h>
```

fopen、fdopen、freopen

FILE* fopen(const char* path, const char* mode);
FILE* fdopen(int fd, const char* mode);
FILE* freopen(const char* path, const char* mode, FILE* stream);
path:包含要打开的文件的路径及文件名
fd:要打开的文件描述符
mode:文件打开状态，取值：
    r/rb:打开只读文件,文件必须存在
    r+/r+b:打开可读写文件,文件必须存在
    w/wb:打开只写文件，覆盖的方式(会清空存在的文件内容)
    w+/w+b:打开可读写文件,覆盖的方式
    a/ab:附加的当时打开只写文件，添加方式(在文件内容后添加内容)
    a+/a+b:附加的当时打开读写文件，添加方式
stream:已打开的文件指针
返回值：成功,返回指向FILE的指针,失败返回NULL

Close the file function: fclose
- Function: Write the data in the buffer to the file and release the file resources
- Format:
```
int fclose(FILE* stream);
stream：已打开的文件指针
返回值：成功：0，失败：EOF
```

Such as:

#include<stdio.h>
int main(void){
    FILE *fp;
    //调用fopen函数
    fp = fopen("/home/test/hello.c", "w");
    if(fp != NULL){
        printf("Open Success!");
    }
    //关闭文件指针
    fclose(fp);
}

file read and write

head File:
```
#include <stdio.h>
```

Read file: fread function

Format:

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
ptr:存放读入记录的缓冲区
size:读取的记录大小
nmemb:读取的记录数
stream:要读取的文件流
返回值：成功，返回实际读到的nmemb，失败返回EOF

Write file: fwrite function

Format

size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);
ptr:存放写入记录的缓冲区
size:写入的记录大小
nmemb：写入的记录数
stream：要写入的文件流
返回值：成功，返回实际写入的nmemb数目，失败返回EOF

Such as:

#include <stdio.h>
int main(void){
    FILE *stream;
    char str[5] = {'H', 'e', 'l', 'l', 'o'};
    //先打开文件
    stream = fopen("test", "w");
    int number = fwrite(str, sizeof(char), nmemb, stream);
    printf("number = %d", number);
    //关闭文件流
    fclose(stream);
}