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Section: Linux Programmer's Manual (3)
Updated: 2001-12-26
Index Return to Main Contents


stdio - standard input/output library functions  


#include <stdio.h>

FILE *stdin;
FILE *stdout;
FILE *stderr;  


The standard I/O library provides a simple and efficient buffered stream I/O interface. Input and output is mapped into logical data streams and the physical I/O characteristics are concealed. The functions and macros are listed below; more information is available from the individual man pages.

A stream is associated with an external file (which may be a physical device) by opening a file, which may involve creating a new file. Creating an existing file causes its former contents to be discarded. If a file can support positioning requests (such as a disk file, as opposed to a terminal), then a file position indicator associated with the stream is positioned at the start of the file (byte zero), unless the file is opened with append mode. If append mode is used, it is unspecified whether the position indicator will be placed at the start or the end of the file. The position indicator is maintained by subsequent reads, writes and positioning requests. All input occurs as if the characters were read by successive calls to the fgetc(3) function; all output takes place as if all characters were written by successive calls to the fputc(3) function.

A file is disassociated from a stream by closing the file. Output streams are flushed (any unwritten buffer contents are transferred to the host environment) before the stream is disassociated from the file. The value of a pointer to a FILE object is indeterminate after a file is closed (garbage).

A file may be subsequently reopened, by the same or another program execution, and its contents reclaimed or modified (if it can be repositioned at the start). If the main function returns to its original caller, or the exit(3) function is called, all open files are closed (hence all output streams are flushed) before program termination. Other methods of program termination, such as abort(3) do not bother about closing files properly.

At program startup, three text streams are predefined and need not be opened explicitly: standard input (for reading conventional input), standard output (for writing conventional output), and standard error (for writing diagnostic output). These streams are abbreviated stdin,stdout and stderr. When opened, the standard error stream is not fully buffered; the standard input and output streams are fully buffered if and only if the streams do not refer to an interactive device.

Output streams that refer to terminal devices are always line buffered by default; pending output to such streams is written automatically whenever an input stream that refers to a terminal device is read. In cases where a large amount of computation is done after printing part of a line on an output terminal, it is necessary to fflush(3) the standard output before going off and computing so that the output will appear.

The stdio library is a part of the library libc and routines are automatically loaded as needed by the compilers cc(1) and pc(1). The SYNOPSIS sections of the following manual pages indicate which include files are to be used, what the compiler declaration for the function looks like and which external variables are of interest.

The following are defined as macros; these names may not be reused without first removing their current definitions with #undef: BUFSIZ, EOF, FILENAME_MAX, FOPEN_MAX, L_cuserid, L_ctermid, L_tmpnam, NULL, SEEK_END, SEEK_SET, SEEK_CUR, TMP_MAX, clearerr, feof, ferror, fileno, getc, getchar, putc, putchar, stderr, stdin, stdout. Function versions of the macro functions feof, ferror, clearerr, fileno, getc, getchar, putc, and putchar exist and will be used if the macros definitions are explicitly removed.  

List of functions


clearerrcheck and reset stream status
fcloseclose a stream
fdopenstream open functions
feofcheck and reset stream status
ferrorcheck and reset stream status
fflushflush a stream
fgetcget next character or word from input stream
fgetposreposition a stream
fgetsget a line from a stream
filenoreturn the integer descriptor of the argument stream
fopenstream open functions
fprintfformatted output conversion
fpurgeflush a stream
fputcoutput a character or word to a stream
fputsoutput a line to a stream
freadbinary stream input/output
freopenstream open functions
fscanfinput format conversion
fseekreposition a stream
fsetposreposition a stream
ftellreposition a stream
fwritebinary stream input/output
getcget next character or word from input stream
getcharget next character or word from input stream
getsget a line from a stream
getwget next character or word from input stream
mktempmake temporary filename (unique)
perrorsystem error messages
printfformatted output conversion
putcoutput a character or word to a stream
putcharoutput a character or word to a stream
putsoutput a line to a stream
putwoutput a character or word to a stream
removeremove directory entry
rewindreposition a stream
scanfinput format conversion
setbufstream buffering operations
setbufferstream buffering operations
setlinebufstream buffering operations
setvbufstream buffering operations
sprintfformatted output conversion
sscanfinput format conversion
strerrorsystem error messages
sys_errlistsystem error messages
sys_nerrsystem error messages
tempnamtemporary file routines
tmpfiletemporary file routines
tmpnamtemporary file routines
ungetcun-get character from input stream
vfprintfformatted output conversion
vfscanfinput format conversion
vprintfformatted output conversion
vscanfinput format conversion
vsprintfformatted output conversion
vsscanfinput format conversion


The stdio library conforms to C89.  


close(2), open(2), read(2), write(2), stdout(3), unlocked_stdio(3)  


This page is part of release 4.13 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at



List of functions

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