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,
_exit2
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
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
#undefBUFSIZEOFFILENAME_MAXFOPEN_MAXL_ctermidL_cuseridL_tmpnamNULLP_tmpdirSEEK_CURSEEK_ENDSEEK_SETTMP_MAXclearerrclearerr_unlockedfeoffeof_unlockedferrorferror_unlockedfilenofileno_unlockedfropenfwopengetcgetc_unlockedgetchargetchar_unlockedputcputc_unlockedputcharputchar_unlockedstderrstdin
and
stdout
Function versions of the macro functions
clearerrclearerr_unlockedfeoffeof_unlockedferrorferror_unlockedfilenofileno_unlockedgetcgetc_unlockedgetchargetchar_unlockedputcputc_unlockedputchar
and
putchar_unlocked
exist and will be used if the macro
definitions are explicitly removed.
