Linux-based I/O operations

Linux file I/O programming Linux files and file descriptors introduction

• Operations on directories and lovemaking in Linux are equivalent to file operations
• There are 4 main types of files in Linux: Normal files, directory files, connection files, and device files
• Under Linux, all operations on devices and files are performed using file descriptors.
• Start of a process, typically with three files open: standard input (Stdin_fileno), standard output (Stdout_fileno), standard error (Stderr_fileno)

Basic I/O operation functions--without caching:

• Open, read, write, Lseek, and close

  • Open, close function

    • open function for opening or creating a file, you can specify properties
    • close function to close open file

    • Syntax format:

       
               
                 header files: #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <unistd.h>  
               

    • Open function:

        int open (const char *pathname, flags, int perms);p athname: file name opened ( Can include a path) flag (open mode):(common way) O_rdonly: Read-only mode o_wronly: Writable O_RDWR: Read and write mode o_create: If the file does not exist, create a new file and set the permissions for it with the third parameter-- ------------------------------o_excl: Used to test whether a file exists O_trunc: If the file already exists and is opened successfully with read-only or write-only, the original data in the file is deleted o+append: Open the text as added When open, the file pointer points to the end of the file perms: Access to the Open file, the 8 Forbidden notation Flag parameter can be passed by "|" Join multiple combined form return value: Return file descriptor successfully, failed return-1  

    • Close function:

        int close (int fd); FD: File Descriptor return value: successfully returned 0 , failed to return-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; Define the file descriptor//Call the Open function to open in a read-write manner, using the "|" Symbolic link if ((Fd=open ("/home/my/mytest.c", O_create | O_trunc |    O_rdwr, 0666)) < 0) {perror ("open:");    Print error message exit (1);    }else{//Note: The file descriptor returned by the Open function must be the smallest unused file descriptor 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:
      
      • read: Reads data from the specified file descriptor
      • write: Used to write data to an open file, write The operation starts at the current offset of the file and is exceeded returns the
      • Lseek: Used to position the file pointer to the appropriate location in the specified file descriptor

    • Syntax format:

      • Header file

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

      • Read function

          ssize_t Read (int fd, void *buf, size_t count); FD: File descriptor buf: Specifies the buffer of the memory readout data count: Specifies the number of bytes read out return value: The number of bytes read successfully returned, 0 for the end of the file; 1 for error  

      • Write function

          ssize_t Write (int fd, void *buf, size_t count), FD: File descriptor buf: Specifies the buffer in which the memory is written to count: Specifies the number of bytes written to return a value: succeeded, returns the number of bytes written; failed, returned-1  

      • Lseek function

          off_t lseek (int fd, off_t offset, int whence); FD: File descriptor offset: offset, per read and write The distance that the operation needs to be moved, in units of bytes. Positive (forward) whence (base point at current position): Seek_set: The current position is the beginning of the file, the new position is a low-cost size seek_cur: The current position is the position of the file pointer, the new position is the current position plus the offset seek_end: The current position is the file's At the end, the new position returns the value of the file plus the size of the offset: succeeds, returns the current offset; 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 Maxsizeint main (void) {int I, FD, size, Len; Char *buf = "hello!    Welcom to Linux world! ";    Char buf_read[10];    Len = strlen (BUF); Open the file and set permissions 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);        The//write function writes the contents of the BUF to the open file if (size = Write (FD, buf, Len)) < 0) {perror ("write:");    Exit (1);    }else{printf ("write:%s\n", buf);    }//Call the Lseek function to move the pointer and call read Read file Lseek (FD, 0, Seek_set);        if (size = Read (FD, Buf_read, ten)) < 0) {perror ("read:");    Exit (0);    }else{printf ("read from file:%s\n", buf_read);        }//Closes Close () if (Close (FD) < 0) {perror ("close:");    Exit (1); }else{        printf ("Close mytest.c\n"); } exit (0);}

• Fcntl function: Properties of the action file descriptor

  • function format

    • Header file

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

    • Function Prototypes:

      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-based buffering

• Objective:
  Reduce the number of use of read and write calls
• Three types of buffered storage:
  
  • Full buffer
    The actual I/O operation is not performed until the standard I/O cache is filled.
  • Row buffers
    The standard I/O library performs I/O operations when the input and output have new line characters.
  • With no buffering
    Do not buffer characters, such as: can display error message as soon as possible

• Open and Close files

  • Open the file standard function: fopen, Fdopen, Freopen, returns a pointer to files
    
    • fopen: Specifies the path and mode of the open file
    • Fdopen: Specifies that open file description conforms to mode
    • Freopen: Can specify open file, mode, specify specific IO stream

  • function format definition

    • Header 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 file function: fclose

    • Function: Data within the buffer is written to the file, and the file resource is freed

    • 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);}

• Reading and writing of files

  • Header 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);}

Linux-based I/O operations