NAME
stdio
—
standard input/output library
functions
SYNOPSIS
#include
<stdio.h>
FILE *stdin;
FILE *stdout;
FILE *stderr;
DESCRIPTION
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”
it. 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 needs and 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 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_cuserid
,
L_ctermid
, L_tmpnam
,
NULL
, SEEK_END
,
SEEK_SET
, SEEK_CUR
,
TMP_MAX
, clearerr
,
feof
, ferror
,
fileno
, freopen
,
fwopen
, getc
,
getchar
, putc
,
putchar
, stderr
,
stdin
, stdout
. Function
versions of the macro functions
feof(3),
ferror(3),
clearerr(3),
fileno(3), getc(3),
getchar(3),
putc(3), and
putchar(3) exist and will be used if the macro definitions are
explicitly removed.
LIST OF FUNCTIONS
Function | Description |
asprintf | formatted output conversion with allocation |
clearerr | check and reset stream status |
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 |
fileno | get a stream's underlying file descriptor |
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 |
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 |
fwopen | open a stream |
fwrite | binary stream input/output |
getc | get next character or word from input stream |
getchar | get next character or word from input stream |
gets | get a line from a stream |
getw | get next character or word from input stream |
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 |
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 |
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 |
vasprintf | formatted output conversion with allocation |
vfprintf | formatted output conversion |
vfscanf | input format conversion |
vprintf | formatted output conversion |
vscanf | input format conversion |
vsnprintf | formatted output conversion |
vsprintf | formatted output conversion |
vsscanf | input format conversion |
SEE ALSO
STANDARDS
The stdio
library conforms to
ANSI X3.159-1989
(“ANSI C89”).
BUGS
The standard buffered functions do not interact well with certain other library and system functions, especially vfork(2) and abort(3).