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Man Pages

Manual Reference Pages  -  STDIO (3)


stdio - standard input/output library functions


See Also
List Of Functions


.Lb libc


.In stdio.h
.Vt FILE *stdin ;
.Vt FILE *stdout ;
.Vt 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, the position indicator will be placed at the end-of-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 (garbage) after a file is closed.

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 may not close files properly and hence buffered output may be lost. In particular, _exit(2) does not flush stdio files. Neither does an exit due to a signal. Buffers are flushed by abort(3) as required by POSIX, although previous implementations did not.

This implementation makes no distinction between "text" and "binary" streams. In effect, all streams are binary. No translation is performed and no extra padding appears on any stream.

At program startup, three 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. Initially, the standard error stream is unbuffered; the standard input and output streams are fully buffered if and only if the streams do not refer to an interactive or "terminal" device, as determined by the isatty(3) function. In fact, all freshly-opened streams that refer to terminal devices default to line buffering, and pending output to such streams is written automatically whenever such an input stream is read. Note that this applies only to "true reads"; if the read request can be satisfied by existing buffered data, no automatic flush will occur. In these cases, or when 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. Alternatively, these defaults may be modified via the setvbuf(3) function.

The stdio library is a part of the library libc and routines are automatically loaded as needed by the C compiler. 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 re-used without first removing their current definitions with #undef: BUFSIZ, EOF, FILENAME_MAX, FOPEN_MAX, L_ctermid, L_cuserid, L_tmpnam, NULL, P_tmpdir, SEEK_CUR, SEEK_END, SEEK_SET, TMP_MAX, clearerr, clearerr_unlocked, feof, feof_unlocked, ferror, ferror_unlocked, fileno, fileno_unlocked, fropen, fwopen, getc, getc_unlocked, getchar, getchar_unlocked, putc, putc_unlocked, putchar, putchar_unlocked, stderr, stdin and stdout. Function versions of the macro functions clearerr, clearerr_unlocked, feof, feof_unlocked, ferror, ferror_unlocked, fileno, fileno_unlocked, getc, getc_unlocked, getchar, getchar_unlocked, putc, putc_unlocked, putchar, and putchar_unlocked exist and will be used if the macro definitions are explicitly removed.


close(2), open(2), read(2), write(2)


The stdio library conforms to -isoC-99.


Function       Description
asprintf      formatted output conversion
clearerr      check and reset stream status
dprintf       formatted output conversion
fclose        close a stream
fdopen        stream open functions
feof  check and reset stream status
ferror        check and reset stream status
fflush        flush a stream
fgetc get next character or word from input stream
fgetln        get a line from a stream
fgetpos       reposition a stream
fgets get a line from a stream
fgetwc        get next wide character from input stream
fgetws        get a line of wide characters from a stream
fileno        check and reset stream status
fopen stream open functions
fprintf       formatted output conversion
fpurge        flush a stream
fputc output a character or word to a stream
fputs output a line to a stream
fputwc        output a wide character to a stream
fputws        output a line of wide characters to a stream
fread binary stream input/output
freopen       stream open functions
fropen        open a stream
fscanf        input format conversion
fseek reposition a stream
fsetpos       reposition a stream
ftell reposition a stream
funopen       open a stream
fwide set/get orientation of stream
fwopen        open a stream
fwprintf      formatted wide character output conversion
fwrite        binary stream input/output
getc  get next character or word from input stream
getchar       get next character or word from input stream
getdelim      get a line from a stream
getline       get a line from a stream
gets  get a line from a stream
getw  get next character or word from input stream
getwc get next wide character from input stream
getwchar      get next wide character from input stream
mkdtemp       create unique temporary directory
mkstemp       create unique temporary file
mktemp        create unique temporary file
perror        system error messages
printf        formatted output conversion
putc  output a character or word to a stream
putchar       output a character or word to a stream
puts  output a line to a stream
putw  output a character or word to a stream
putwc output a wide character to a stream
putwchar      output a wide character to a stream
remove        remove directory entry
rewind        reposition a stream
scanf input format conversion
setbuf        stream buffering operations
setbuffer     stream buffering operations
setlinebuf    stream buffering operations
setvbuf       stream buffering operations
snprintf      formatted output conversion
sprintf       formatted output conversion
sscanf        input format conversion
strerror      system error messages
swprintf      formatted wide character output conversion
sys_errlist   system error messages
sys_nerr      system error messages
tempnam       temporary file routines
tmpfile       temporary file routines
tmpnam        temporary file routines
ungetc        un-get character from input stream
ungetwc       un-get wide character from input stream
vasprintf     formatted output conversion
vdprintf      formatted output conversion
vfprintf      formatted output conversion
vfscanf       input format conversion
vfwprintf     formatted wide character output conversion
vprintf       formatted output conversion
vscanf        input format conversion
vsnprintf     formatted output conversion
vsprintf      formatted output conversion
vsscanf       input format conversion
vswprintf     formatted wide character output conversion
vwprintf      formatted wide character output conversion
wprintf       formatted wide character output conversion


The standard buffered functions do not interact well with certain other library and system functions, especially vfork(2).
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