---
name: "stdio"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/stdio"
---

# stdio(3)

standard input/output library functions

## 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. 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)](/fgetc/3.md) function; all output takes place as if all characters were written by successive calls to the [fputc(3)](/fputc/3.md) 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)](/exit/3.md) 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)](/_exit/2.md) does not flush stdio files. Neither does an exit due to a signal. Buffers are flushed by [abort(3)](/abort/3.md) 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)](/isatty/3.md) 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)](/fflush/3.md) the standard output before going off and computing so that the output will appear. Alternatively, these defaults may be modified via the [setvbuf(3)](/setvbuf/3.md) function.

The **stdio** 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 **#undef**: BUFSIZ, EOF, FILENAME\_MAX, FOPEN\_MAX, L\_ctermid, L\_cuserid, L\_tmpnam, NULL, P\_tmpdir, SEEK\_CUR, SEEK\_END, SEEK\_SET, TMP\_MAX, clearerr, clearerr\_unlocked, feof, feof\_unlocked, ferror, ferror\_unlocked, fileno, fileno\_unlocked, fropen, fwopen, getc, getc\_unlocked, getchar, getchar\_unlocked, putc, putc\_unlocked, putchar, putchar\_unlocked, stderr, stdin and stdout. Function versions of the macro functions clearerr, clearerr\_unlocked, feof, feof\_unlocked, ferror, ferror\_unlocked, fileno, fileno\_unlocked, getc, getc\_unlocked, getchar, getchar\_unlocked, putc, putc\_unlocked, putchar, and putchar\_unlocked exist and will be used if the macro definitions are explicitly removed.

## SEE ALSO

[close(2)](/close/2.md), [open(2)](/open/2.md), [read(2)](/read/2.md), [write(2)](/write/2.md)

## STANDARDS

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

## LIST OF FUNCTIONS

| | |
| --- | --- |
| **Function** | Description |
| asprintf | formatted output conversion |
| 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 | map a stream pointer to a 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 | get a line from a stream |
| getline | 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 |
| 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 |
| 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 |

## BUGS

The standard buffered functions do not interact well with certain other library and system functions, especially [vfork(2)](/vfork/2.md).