NAME

stdio — standard input/output library functions

LIBRARY

Standard C Library (libc, -lc)

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.

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 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 read 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.

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:

1. standard input for reading conventional input,
2. standard output for writing conventional output, and
3. 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 an 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.

IMPLEMENTATION NOTES

In multi-threaded applications, operations on streams perform implicit locking, except for the getc_unlocked(), getchar_unlocked(), putc_unlocked(), and putchar_unlocked() functions. Explicit control of stream locking is available through the flockfile(), ftrylockfile(), and funlockfile() functions .

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, SEE_CUR, TMP_MAX, clearerr(), feof(), ferror(), fileno(), freopen(), fwopen(), getc(), getc_unlocked(), getchar(), getchar_unlocked(), putc(), putc_unlocked(), putchar(), putchar_unlocked(), stderr, stdin, stdout.

Function versions of the macro functions feof(), ferror(), clearerr(), fileno(), getc(), getc_unlocked(), getchar(), getchar_unlocked(), putc(), putc_unlocked(), putchar(), and putchar_unlocked() exist and will be used if the macros definitions are explicitly removed.

SEE ALSO

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

STANDARDS

The stdio library conforms to ANSI X3.159-1989 (“ANSI C89”).

LIST OF FUNCTIONS

Function	Description
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
fgetwc	get next wide character from input stream
fileno	check and reset stream status
flockfile	lock a stream
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
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
ftrylockfile	lock a stream (non-blocking)
funlockfile	unlock a stream
funopen	open a stream
fwide	set/get orientation of a stream
fwopen	open a stream
fwrite	binary stream input/output
getc	get next character or word from input stream
getc_unlocked	get next character or word from input stream
	(no implicit locking)
getchar	get next character or word from input stream
getchar_unlocked	get next character or word from input stream
	(no implicit locking)
getdelim	get a delimited record 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
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
putc_unlocked	output a character or word to a stream
	(no implicit locking)
putchar	output a character or word to a stream
putchar_unlocked	output a character or word to a stream
	(no implicit locking)
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
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
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

BUGS

The standard buffered functions do not interact well with certain other library and system functions, especially vfork(2) and abort(3).