STDIO(3) | Library Functions Manual | STDIO(3)

# 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 in previous implementations they were 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 so that the output will appear immediately. 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_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
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 | 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
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 | read a delimited record from a stream
getline | read a delimited record 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 with allocation
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

# SEE ALSO

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

# STANDARDS

The `stdio` library conforms to ISO/IEC 9899:1999 (“ISO C99”).

# BUGS

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

July 3, 2014 | Debian