I/O routines input/output API functions

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 ).

WarningBecause stream functions are buffered and low-level functions are not, these two types of functions are generally incompatible. for processing a participant 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 CILS do not buffer or format data.

These functions directly access the operating system for low-level operations, unlike stream input and output. Low-level input/output data is not buffered or formatted.

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

_ DUPAnd_ Dup2Are 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 4 K. 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 release TStdin,Stdout, AndStderr
_ 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 versionsGETCAndGetwc)
_ Fgetchar, _ fgetwchar	Read character fromStdin(Function versionsGetcharAndGetwchar)
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 versionsPutcAndPutwc)
_ Fputchar, _ fputwchar	Write characterStdout(Function versionsPutcharAndPutwchar)
Fputs, fputws	Write string to stream
Fread	Read unformatted data from stream
Freopen, _ wfreopen	ReassignFileStream 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 versionsFgetcAndFgetwc)
Getchar, getwchar	Read character fromStdin(Macro versionsFgetcharAndFgetwchar)
Gets, getws	Read line fromStdin
_ Getw	Read BinaryIntFrom stream
Printf, wprintf	Write formatted dataStdout
Putc, putwc	Write character to a stream (macro versionsFputcAndFputwc)
Putchar, putwchar	Write characterStdout(Macro versionsFputcharAndFputwchar)
Puts, _ putws	Write line to stream
_ Putw	Write binaryIntTo stream
Rewind	Move file position to beginning of stream
_ Rmtmp	Remove temporary files createdTmpfile
Scanf, wscanf	Read formatted data fromStdin
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 dataStdout
_ 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 toStdin), Standard output (pointed toStdout), And standard error (pointed toStderr). These streams are directed to the console (keyboard and screen) by default. UseFreopenTo redirectStdin,Stdout, OrStderrTo a disk file or a device.

Files opened using the stream routines are buffered by default.StdoutAndStderrFunctions 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. useFflushOr_ FlushallTo 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 isN, For "no-commit ."

• Set the mode flagCWithFopenOr_ Fdopen.

Any file specifically opened with eitherCOrNFlag 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 shoshould 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 callFflushOr to a file-positioning function (Fseek,Fsetpos, OrRewind). Output can follow input without an intervening call to a file-positioning function if the input operation encounters the end of the file.

From: 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 Core Before parsing it.
• To perform operations other than data transfer, which can only be done with a descriptor. (You can usefilenoTo 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 .)

Opening and Closing files: how to open and close file descriptors.

I/O primitives: Reading and Writing data.

File position primitive: setting a descriptor's file position.

Descriptors and streams: Converting descriptor to stream or vice-versa.

Stream/descriptor precautions: precautions needed if you use both descriptors and streams.

Scatter-gather: fast I/O to discontinuous buffers.

Memory-mapped I/O: Using files like Memory .

Waiting for I/O: How to check for input or output on multiple file descriptors.

Synchronizing I/O: making sure all I/O Actions completed.

Asynchronous I/O: perform I/O in parallel.

Control Operations: varous other operations on file descriptors.

Duplicating descriptors: fcntl Commands For duplicating file descriptors.

Descriptor flags: fcntl Commands For manipulating flags associated with file descriptors.

File Status flags: fcntl commands for manipulating flags associated with open files.

File locks: fcntl commands for implementing File Locking.

Interrupt input: getting an asynchronous signal when input arrives.

IOCTLs: Generic I/O control operations.

Other articles about lowlevel I/O:

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