Input and Output
The I/O functions read and write data to and from files and devices. File I/O operations take place in text mode or binary mode. The Microsoft run-time library has three types of I/O functions:
- Stream I/O functions treat data as a stream of individual characters.
- Low-level I/O functions invoke the operating system directly for lower-level operation than that provided by stream I/O.
- Console and port I/O functions read or write directly to a console (keyboard and screen) or an I/O port (such as a printer port).
Warning Because stream functions are buffered and low-level functions are not, these two types of functions are generally incompatible. For processing a particular file, use either stream or low-level functions exclusively.
Low-level I/O
These functions invoke the operating system directly for lower-level operation than that provided by stream I/O. Low-level input and output calls do not buffer or format data.
這些函數直接訪問操作系統進行低級別的操作，而不像流式輸入輸出。低級輸入輸出對數據不進行緩衝或格式化。
Low-level routines can access the standard streams opened at program startup using the following predefined handles:
| Stream | Handle |
| stdin | 0 |
| stdout | 1 |
| stderr | 2 |
Low-level I/O routines set the errno global variable when an error occurs. You must include STDIO.H when you use low-level functions only if your program requires a constant that is defined in STDIO.H, such as the end-of-file indicator (EOF).
Low-Level I/O Functions
| Function | Use |
| _close | Close file |
| _commit | Flush file to disk |
| _creat, _wcreat | Create file |
| _dup | Return next available file handle for given file |
| _dup2 | Create second handle for given file |
| _eof | Test for end of file |
| _lseek, _lseeki64 | Reposition file pointer to given location |
| _open, _wopen | Open file |
| _read | Read data from file |
| _sopen, _wsopen | Open file for file sharing |
| _tell, _telli64 | Get current file-pointer position |
| _umask | Set file-permission mask |
| _write | Write data to file |
_dup and _dup2 are typically used to associate the predefined file handles with different files.
Stream I/O
These functions process data in different sizes and formats, from single characters to large data structures. They also provide buffering, which can improve performance. The default size of a stream buffer is 4K. These routines affect only buffers created by the run-time library routines, and have no effect on buffers created by the operating system.
Stream I/O Routines
| Routine | Use |
| clearerr | Clear error indicator for stream |
| fclose | Close stream |
| _fcloseall | Close all open streams except stdin, stdout, and stderr |
| _fdopen, wfdopen | Associate stream with handle to open file |
| feof | Test for end of file on stream |
| ferror | Test for error on stream |
| fflush | Flush stream to buffer or storage device |
| fgetc, fgetwc | Read character from stream (function versions of getc and getwc) |
| _fgetchar, _fgetwchar | Read character from stdin (function versions of getchar and getwchar) |
| fgetpos | Get position indicator of stream |
| fgets, fgetws | Read string from stream |
| _fileno | Get file handle associated with stream |
| _flushall | Flush all streams to buffer or storage device |
| fopen, _wfopen | Open stream |
| fprintf, fwprintf | Write formatted data to stream |
| fputc, fputwc | Write a character to a stream (function versions of putc and putwc) |
| _fputchar, _fputwchar | Write character to stdout (function versions of putchar and putwchar) |
| fputs, fputws | Write string to stream |
| fread | Read unformatted data from stream |
| freopen, _wfreopen | Reassign FILE stream pointer to new file or device |
| fscanf, fwscanf | Read formatted data from stream |
| fseek | Move file position to given location |
| fsetpos | Set position indicator of stream |
| _fsopen, _wfsopen | Open stream with file sharing |
| ftell | Get current file position |
| fwrite | Write unformatted data items to stream |
| getc, getwc | Read character from stream (macro versions of fgetc and fgetwc) |
| getchar, getwchar | Read character from stdin (macro versions of fgetchar and fgetwchar) |
| gets, getws | Read line from stdin |
| _getw | Read binary int from stream |
| printf, wprintf | Write formatted data to stdout |
| putc, putwc | Write character to a stream (macro versions of fputc and fputwc) |
| putchar, putwchar | Write character to stdout (macro versions of fputchar and fputwchar) |
| puts, _putws | Write line to stream |
| _putw | Write binary int to stream |
| rewind | Move file position to beginning of stream |
| _rmtmp | Remove temporary files created by tmpfile |
| scanf, wscanf | Read formatted data from stdin |
| setbuf | Control stream buffering |
| _setmaxstdio | Set a maximum for the number of simultaneously open files at the stream I/O level. |
| setvbuf | Control stream buffering and buffer size |
| _snprintf, _snwprintf | Write formatted data of specified length to string |
| sprintf, swprintf | Write formatted data to string |
| sscanf, swscanf | Read formatted data from string |
| _tempnam, _wtempnam | Generate temporary filename in given directory |
| tmpfile | Create temporary file |
| tmpnam, _wtmpnam | Generate temporary filename |
| ungetc, ungetwc | Push character back onto stream |
| vfprintf, vfwprintf | Write formatted data to stream |
| vprintf, vwprintf | Write formatted data to stdout |
| _vsnprintf, _vsnwprintf | Write formatted data of specified length to buffer |
| vsprintf, vswprintf | Write formatted data to buffer |
When a program begins execution, the startup code automatically opens several streams: standard input (pointed to by stdin), standard output (pointed to by stdout), and standard error (pointed to by stderr). These streams are directed to the console (keyboard and screen) by default. Use freopen to redirect stdin, stdout, or stderr to a disk file or a device.
Files opened using the stream routines are buffered by default. The stdout and stderr functions are flushed whenever they are full or, if you are writing to a character device, after each library call. If a program terminates abnormally, output buffers may not be flushed, resulting in loss of data. Use fflush or _flushall to ensure that the buffer associated with a specified file or all open buffers are flushed to the operating system, which can cache data before writing it to disk. The commit-to-disk feature ensures that the flushed buffer contents are not lost in the event of a system failure.
There are two ways to commit buffer contents to disk:
- Link with the file COMMODE.OBJ to set a global commit flag. The default setting of the global flag is n, for “no-commit.”
- Set the mode flag to c with fopen or _fdopen.
Any file specifically opened with either the c or the n flag behaves according to the flag, regardless of the state of the global commit/no-commit flag.
If your program does not explicitly close a stream, the stream is automatically closed when the program terminates. However, you should close a stream when your program finishes with it, as the number of streams that can be open at one time is limited. See _setmaxstdio for information on this limit.
Input can follow output directly only with an intervening call to fflush or to a file-positioning function (fseek, fsetpos, or rewind). Output can follow input without an intervening call to a file-positioning function if the input operation encounters the end of the file.
以下摘自：http://www.linuxtopia.org/online_books/programming_books/gnu_libc_guide/Low_002dLevel-I_002fO.html
Stream-level I/O is more flexible and usually more convenient; therefore, programmers generally use the descriptor-level functions only when necessary. These are some of the usual reasons:
- For reading binary files in large chunks.
- For reading an entire file into core before parsing it.
- To perform operations other than data transfer, which can only be done with a descriptor. (You can use
filenoto get the descriptor corresponding to a stream.) - To pass descriptors to a child process. (The child can create its own stream to use a descriptor that it inherits, but cannot inherit a stream directly.)
其他關於LowLevel I/O的文章：
http://www.chemie.fu-berlin.de/chemnet/use/info/libc/libc_8.html
http://www.gnu.org/software/libc/manual/html_node/Low_002dLevel-I_002fO.html
http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=208
http://www.thinkage.ca/english/gcos/expl/c/lib/open.html