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

# 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 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
asprintf | formatted output conversion with allocation
asprintf_l | formatted output conversion with allocation
clearerr | check and reset stream status
dprintf | formatted output conversion
dprintf_l | 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
fileno | check and reset stream status
flockfile | lock a stream
fmemopen | open a stream that points to a memory buffer
fopen | stream open functions
fprintf | formatted output conversion
fprintf_l | 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
fscanf_l | input format conversion
fseek | reposition a stream
fseeko | reposition a stream
fsetpos | reposition a stream
ftell | reposition a stream
ftello | reposition a stream
ftrylockfile | lock a stream (non-blocking)
funlockfile | unlock a stream
funopen | open a stream
funopen2 | open a stream, with flush support
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
open_memstream | open memory as a stream
popen | open a program as a stream
popenve | open a program as a stream
pclose | close an opened program stream
perror | system error messages
printf | formatted output conversion
printf_l | 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
scanf_l | input format conversion
setbuf | stream buffering operations
setbuffer | stream buffering operations
setlinebuf | stream buffering operations
setvbuf | stream buffering operations
snprintf | formatted output conversion
snprintf_l | formatted output conversion
sprintf | formatted output conversion
sscanf | input format conversion
sscanf_l | 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
vasprintf | formatted output conversion with allocation
vasprintf_l | formatted output conversion with allocation
vdprintf | formatted output conversion
vdprintf_l | formatted output conversion
vfprintf | formatted output conversion
vfprintf_l | formatted output conversion
vfscanf | input format conversion
vfscanf_l | input format conversion
vprintf | formatted output conversion
vprintf_l | formatted output conversion
vscanf | input format conversion
vscanf_l | input format conversion
vsnprintf | formatted output conversion
vsnprintf_l | formatted output conversion
vsprintf | formatted output conversion
vsprintf_l | formatted output conversion
vsscanf | input format conversion
vsscanf_l | 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).

February 22, 2018 | NetBSD 9.4