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STDIO(3) Library Functions Manual STDIO(3)

stdiostandard input/output library functions

library “libc”

#include <stdio.h>

FILE *stdin;
FILE *stdout;
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.

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.

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.

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

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

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

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

May 5, 2010 NetBSD-7.0.1