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Stream I/O

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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, clearerr_s Clear error indicator for stream
fclose Close stream
_fcloseall Close all open streams except stdin, stdout, and stderr
_fdopen, wfdopen Associate stream with file descriptor of 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 descriptor associated with stream
_flushall Flush all streams to buffer or storage device
fopen, _wfopen, fopen_s, _wfopen_s Open stream
fprintf, _fprintf_l, fwprintf, _fwprintf_l, fprintf_s, _fprintf_s_l, fwprintf_s, _fwprintf_s_l 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, freopen_s, _wfreopen_s Reassign FILE stream pointer to new file or device
fscanf, fwscanf, fscanf_s, _fscanf_s_l, fwscanf_s, _fwscanf_s_l Read formatted data from stream
fseek, _fseeki64 Move file position to given location
fsetpos Set position indicator of stream
_fsopen, _wfsopen Open stream with file sharing
ftell, _ftelli64 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)
_getmaxstdio Returns the number of simultaneously open files permitted at the stream I/O level.
gets_s, _getws_s Read line from stdin
_getw Read binary int from stream
printf, _printf_l, wprintf, _wprintf_l,printf_s, _printf_s_l, wprintf_s, _wprintf_s_l 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, _scanf_l, wscanf, _wscanf_l,scanf_s, _scanf_s_l, wscanf_s, _wscanf_s_l 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, _snprintf_s, _snprintf_s_l, _snwprintf_s, _snwprintf_s_l Write formatted data of specified length to string
_snscanf, _snwscanf, _snscanf_s, _snscanf_s_l, _snwscanf_s, _snwscanf_s_l Read formatted data of a specified length from the standard input stream.
sprintf, swprintf, sprintf_s, _sprintf_s_l, swprintf_s, _swprintf_s_l Write formatted data to string
sscanf, swscanf, sscanf_s, _sscanf_s_l, swscanf_s, _swscanf_s_l Read formatted data from string
_tempnam, _wtempnam Generate temporary filename in given directory
tmpfile, tmpfile_s Create temporary file
tmpnam, _wtmpnam, tmpnam_s, _wtmpnam_s Generate temporary filename
ungetc, ungetwc Push character back onto stream
_vcprintf, _vcwprintf, _vcprintf_s, _vcprintf_s_l, _vcwprintf_s, _vcwprintf_s_l Write formatted data to the console.
vfprintf, vfwprintf, vfprintf_s, _vfprintf_s_l, vfwprintf_s, _vfwprintf_s_l Write formatted data to stream
vprintf, vwprintf, vprintf_s, _vprintf_s_l, vwprintf_s, _vwprintf_s_l Write formatted data to stdout
_vsnprintf, _vsnwprintf, vsnprintf_s, _vsnprintf_s, _vsnprintf_s_l, _vsnwprintf_s, _vsnwprintf_s_l Write formatted data of specified length to buffer
vsprintf, vswprintf, vsprintf_s, _vsprintf_s_l, vswprintf_s, _vswprintf_s_l 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're full or, if you're 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 is flushed to the operating system, or all open buffers are flushed. The operating system can cache data before writing it to disk. The commit-to-disk feature ensures that the flushed buffer contents aren't lost if there's 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 doesn't 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). Input can be followed by output without an intervening call to a file-positioning function, if the input operation encounters the end of the file.

See also

Input and output
Universal C runtime routines by category