Standard I/O (1)

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Preface

      The standard c library is a set of function libraries for the application layer, 标准I/O库就是标准c库当中用于文件I/O操作相关的一套库函数. To use the standard I/O library, you need to include the header file <stdio.h>.

1. The difference between standard I/O and file I/O

      1. Standard I/O is 库函数, and file I/O is 系统调用.
      2. Standard I/O is an encapsulation of file I/O.
      3. Portability: Standard I/O compared to file I/O具有更好的可移植性.
      4. Better efficiency: Standard I/O is at 效率上高于文件I/O. This is because standard I/O is buffered at the application layer (stdio buffer). File I/O is not buffered at the application layer, so standard I/O is more efficient.

2. FILE pointer and fopen() function

    ​ FILE pointer:

      Standard I/O uses the FILE pointer as the file handle, 与文件I/O中的文件描述符相似.

    fopen() function - open a file

      Standard I/O uses the fopen function to open a file:

FILE *fopen(const char *path,const char *mode);

      Detailed explanation of parameters:
      path: Points to the file path, which can be an absolute path or a relative path.
      mode: Specifies the read and write permissions for the file, which is a string.

mode	illustrate	Corresponds to the flags parameter value of the open() function
r	Open file as read-only	O_RDONLY
r+	Open file for reading and writing	O_RDWR
In	Open the file in write-only mode. If the file specified by the parameter path exists, the file length will be truncated to 0; if the specified file does not exist, the file will be created.	O_WRONLY | O_FRAME | O_TRUNC
w+	Open the file in readable and writable mode. If the file specified by the parameter path exists, the file length will be truncated to 0; if the specified file does not exist, the file will be created.	O_RDWR | O_FRAME | O_TRUNC
a	Open the file for writing only, open for appending (writing at the end of the file), create the file if it does not exist	O_WRONLY | O_CREAT | O_APPEND
a+	Open the file for reading and writing, write in append mode (write at the end of the file), create the file if it does not exist	O_RDWR | O_CREAT | O_APPEND

      Memory points:
      有带加号的就是以可读可写形式打开文件, r/r+ has no special function, w/w+ can truncate the file length of existing content to 0 (O_TRUNC), a remember append, in the file Write at the end.

      Return value:
      A successful call returns a pointer to a FILE type object (FILE*), which is associated with the opened or created file. Subsequent standard I/O operations Will be done around the FILE pointer. If it fails, NULL is returned and errno is set to indicate the cause of the error.

      Permissions for new files:
      The fopen function has only two parameters, path and mode. There are no parameters to specify the permissions for new files. When the parameter mode takes the value "w, w+, a , a+", if the specified file does not exist, the file will be created, at this time 新建的文件有一个默认权限, 其权限是0666（八进制）.

    fclose() function - close the file

      Function prototype:

int fclose(FILE *stream);

3. Read files and write files

    Reading files——fread() function

      Function prototype:

size_t fread(void *ptr,size_t size,size_t nmemb,FILE*stream);

      Detailed explanation of parameters:
      The first parameter pointer ptr: points to the buffer;
      The second parameter size: the size of a data packet/data item The byte size, rather than the total byte size of the data to be read;
      The third parameter nmemb: the number of data items
      The fourth parameter stream: FILE pointer, pointing to the file we want to read

      Return value:
      成功时返回读取到的数据项的数目 (not the actual number of bytes read, unless size is equal to 1); if发生错误或到达文件末尾, 则fread()返回的值将小于参数nmemb, at this time 无法区分是到达了文件末尾还是发生错误, 需要使用ferror()或feof()函数 to judge.

    ​Write files——fwrite() function

      Function prototype:

size_t fwrite(const void * ptr,size_t size,size_t nmemb,FILE *stream);

      Its parameters are roughly the same as those of the fread function and will not be described again.
      Return value:
      Call 成功的时候返回写入的数据项的数目; if发生错误，则返回的值将会小于参数nmemb（或等于0）
      It should be noted that, fread和fwrite发生错误的时候是不会设置errno值的，所以不用perror.

    Set the read and write position——fseek() function

      Function prototype:

int fseek(FILE *stream,long offset,int whence);

      has roughly the same function as lseek.
      Detailed explanation of parameters:
      stream: FILE pointer; offset: position offset; whence: set reference value (file head, file end, current pointing position )
      Return value:
      Returns 0 on success, -1 on failure, 与lseek返回值不一样! A successful lseek will return the position offset in bytes from the beginning of the file.

    Determine whether the file reaches the end of the file - feof() function

      Function prototype:

int feof(FILE *stream);

      If the file reaches the end of the file, return a non-zero value (true), otherwise return 0 (false).

    Determine whether an error has occurred——ferror() function

      Function prototype:

int ferror(FILE *stream);

      Return value:
      If an error occurs, it returns true, otherwise it returns false.

    Clear flag——clearerr() function

      Function prototype:

void clearerr(FILE *stream);

      Function:
      For example, when the file read-write pointer reaches the end of the file, 会设置标志位end-of-file, the feof function passes this flag To determine whether the file has reached the end, 如果这个标志位不进行清除的话，就可能对之后的判断造成误判, the clearerr function is to avoid this problem.
      Usage example:

ret = fread(buf,1,11,f);
if(11>ret){
    
    
    if(ferror(f)){
    
    
            printf("fread error\n");
            fclose(f);    
            return 1;
    }
    else {
    
    
        if(feof(f)){
    
    
            printf("fread end-of-file\n");
            fclose(f);
            return 1;        
        }    
    }
    clearerr(f);
}