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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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stdio(3) Library Functions Manual stdio(3)
stdio - standard input/output library functions
Standard C library (libc, -lc)
#include <stdio.h>
extern FILE *stdin;
extern FILE *stdout;
extern 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 cc(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
Function Description
───────────────────────────────────────────────────────────────────
clearerr(3) check and reset stream status
fclose(3) close a stream
fdopen(3) stream open functions
feof(3) check and reset stream status
ferror(3) check and reset stream status
fflush(3) flush a stream
fgetc(3) get next character or word from input stream
fgetpos(3) reposition a stream
fgets(3) get a line from a stream
fileno(3) return the integer descriptor of the argument
stream
fmemopen(3) open memory as stream
fopen(3) stream open functions
fopencookie(3) open a custom stream
fprintf(3) formatted output conversion
fpurge(3) flush a stream
fputc(3) output a character or word to a stream
fputs(3) output a line to a stream
fread(3) binary stream input/output
freopen(3) stream open functions
fscanf(3) input format conversion
fseek(3) reposition a stream
fsetpos(3) reposition a stream
ftell(3) reposition a stream
fwrite(3) binary stream input/output
getc(3) get next character or word from input stream
getchar(3) get next character or word from input stream
gets(3) get a line from a stream
getw(3) get next character or word from input stream
mktemp(3) make temporary filename (unique)
open_memstream(3) open a dynamic memory buffer stream
open_wmemstream(3) open a dynamic memory buffer stream
perror(3) system error messages
printf(3) formatted output conversion
putc(3) output a character or word to a stream
putchar(3) output a character or word to a stream
puts(3) output a line to a stream
putw(3) output a character or word to a stream
remove(3) remove directory entry
rewind(3) reposition a stream
scanf(3) input format conversion
setbuf(3) stream buffering operations
setbuffer(3) stream buffering operations
setlinebuf(3) stream buffering operations
setvbuf(3) stream buffering operations
sprintf(3) formatted output conversion
sscanf(3) input format conversion
strerror(3) system error messages
sys_errlist(3) system error messages
sys_nerr(3) system error messages
tempnam(3) temporary file routines
tmpfile(3) temporary file routines
tmpnam(3) temporary file routines
ungetc(3) un-get character from input stream
vfprintf(3) formatted output conversion
vfscanf(3) input format conversion
vprintf(3) formatted output conversion
vscanf(3) input format conversion
vsprintf(3) formatted output conversion
vsscanf(3) input format conversion
C11, POSIX.1-2008.
C89, POSIX.1-2001.
close(2), open(2), read(2), write(2), stdout(3), unlocked_stdio(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 stdio(3)
Pages that refer to this page: pmsnap(1), _exit(2), fcntl_locking(2), vfork(2), curs_addch(3x), exit(3), ferror(3), FILE(3type), fileno(3), popen(3), printf(3), stdin(3), unlocked_stdio(3), scr_dump(5)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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