Input/output functionality in the C programming language
The
C programming language
provides many
standard library
functions
for
file
input and output
. These functions make up the bulk of the
C standard library
header
<
stdio.h
>
.
[1]
The functionality descends from a "portable I/O package" written by
Mike Lesk
at
Bell Labs
in the early 1970s,
[2]
and officially became part of the
Unix
operating system in
Version 7
.
[3]
The I/O functionality of C is fairly low-level by modern standards; C abstracts all file operations into operations on
streams
of
bytes
, which may be "input streams" or "output streams". Unlike some earlier programming languages, C has no direct support for
random-access
data files; to read from a record in the middle of a file, the programmer must create a stream,
seek
to the middle of the file, and then read bytes in sequence from the stream.
The stream model of file I/O was popularized by Unix, which was developed concurrently with the C programming language itself. The vast majority of modern operating systems have inherited streams from Unix, and many languages in the
C programming language family
have inherited C's file I/O interface with few if any changes (for example,
PHP
).
Overview
[
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]
This library uses what are called streams to operate with physical devices such as keyboards, printers, terminals or with any other type of files supported by the system. Streams are an abstraction to interact with these in a uniform way. All streams have similar properties independent of the individual characteristics of the physical media they are associated with.
[4]
Functions
[
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]
Most of the C file input/output functions are defined in
<stdio.h>
(or in the
C++
header
cstdio
, which contains the standard C functionality but in the
std
namespace
).
|
Byte
character
|
Wide
character
|
Description
|
File access
|
fopen
|
Opens a file (with a non-Unicode filename on Windows and possible UTF-8 filename on Linux)
|
freopen
|
Opens a different file with an existing stream
|
fflush
|
Synchronizes an output stream with the actual file
|
fclose
|
Closes a file
|
setbuf
|
Sets the buffer for a file stream
|
setvbuf
|
Sets the buffer and its size for a file stream
|
fwide
|
Switches a file stream between wide-character I/O and narrow-character I/O
|
Direct
input/output
|
fread
|
Reads from a file
|
fwrite
|
Writes to a file
|
Unformatted
input/output
|
fgetc
getc
|
fgetwc
getwc
|
Reads a byte/
wchar_t
from a file stream
|
fgets
|
fgetws
|
Reads a byte/
wchar_t
line from a file stream
|
fputc
putc
|
fputwc
putwc
|
Writes a byte/
wchar_t
to a file stream
|
fputs
|
fputws
|
Writes a byte/
wchar_t
string to a file stream
|
getchar
|
getwchar
|
Reads a byte/
wchar_t
from stdin
|
gets
|
?
|
Reads a byte string from stdin until a newline or end of file is encountered (deprecated in C99, removed from C11)
|
putchar
|
putwchar
|
Writes a byte/
wchar_t
to stdout
|
puts
|
?
|
Writes a byte string to stdout
|
ungetc
|
ungetwc
|
Puts a byte/
wchar_t
back into a file stream
|
Formatted
input/output
|
scanf
fscanf
sscanf
|
wscanf
fwscanf
swscanf
|
Reads formatted byte/
wchar_t
input from stdin,
a file stream or a buffer
|
vscanf
vfscanf
vsscanf
|
vwscanf
vfwscanf
vswscanf
|
Reads formatted input byte/
wchar_t
from stdin,
a file stream or a buffer using variable argument list
|
printf
fprintf
sprintf
snprintf
|
wprintf
fwprintf
swprintf
|
Prints formatted byte/
wchar_t
output to stdout,
a file stream or a buffer
|
vprintf
vfprintf
vsprintf
vsnprintf
|
vwprintf
vfwprintf
vswprintf
|
Prints formatted byte/
wchar_t
output to stdout,
a file stream, or a buffer using variable argument list
|
perror
|
?
|
Writes a description of the
current error
to stderr
|
File positioning
|
ftell
ftello
|
Returns the current file position indicator
|
fseek
fseeko
|
Moves the file position indicator to a specific location in a file
|
fgetpos
|
Gets the file position indicator
|
fsetpos
|
Moves the file position indicator to a specific location in a file
|
rewind
|
Moves the file position indicator to the beginning in a file
|
Error
handling
|
clearerr
|
Clears errors
|
feof
|
Checks for the end-of-file
|
ferror
|
Checks for a file error
|
Operations
on files
|
remove
|
Erases a file
|
rename
|
Renames
a file
|
tmpfile
|
Returns a pointer to a temporary file
|
tmpnam
|
Returns a unique filename
|
Constants
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]
Constants defined in the
<stdio.h>
header include:
Name
|
Notes
|
EOF
|
A negative integer of type
int
used to indicate end-of-file conditions
|
BUFSIZ
|
An integer which is the size of the buffer used by the
setbuf()
function
|
FILENAME_MAX
|
The size of a
char
array which is large enough to store the name of any file that can be opened
|
FOPEN_MAX
|
The number of files that may be open simultaneously; will be at least eight
|
_IOFBF
|
An abbreviation for "input/output fully buffered"; it is an integer which may be passed to the
setvbuf()
function to request
block buffered
input and output for an open stream
|
_IOLBF
|
An abbreviation for "input/output line buffered"; it is an integer which may be passed to the
setvbuf()
function to request
line buffered
input and output for an open stream
|
_IONBF
|
An abbreviation for "input/output not buffered"; it is an integer which may be passed to the
setvbuf()
function to request
unbuffered
input and output for an open stream
|
L_tmpnam
|
The size of a
char
array which is large enough to store a temporary filename generated by the
tmpnam()
function
|
NULL
|
A macro expanding to the
null pointer
constant; that is, a constant representing a pointer value which is guaranteed
not
to be a valid address of an object in memory
|
SEEK_CUR
|
An integer which may be passed to the
fseek()
function to request positioning relative to the current file position
|
SEEK_END
|
An integer which may be passed to the
fseek()
function to request positioning relative to the end of the file
|
SEEK_SET
|
An integer which may be passed to the
fseek()
function to request positioning relative to the beginning of the file
|
TMP_MAX
|
The maximum number of unique filenames generable by the
tmpnam()
function; will be at least 25
|
Variables
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]
Variables defined in the
<stdio.h>
header include:
Name
|
Notes
|
stdin
|
A pointer to a
FILE
which refers to the standard input stream, usually a keyboard.
|
stdout
|
A pointer to a
FILE
which refers to the standard output stream, usually a display terminal.
|
stderr
|
A pointer to a
FILE
which refers to the standard error stream, often a display terminal.
|
Member types
[
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]
Data types defined in the
<stdio.h>
header include:
- FILE
? also known as a
file
handle
or a
FILE pointer
, this is an
opaque pointer
containing the information about a file or text stream needed to perform input or output operations on it, including:
- platform-specific identifier of the associated I/O device, such as a
file descriptor
- the buffer
- stream orientation indicator (unset, narrow, or wide)
- stream buffering state indicator (unbuffered, line buffered, fully buffered)
- I/O mode indicator (input stream, output stream, or update stream)
- binary/text mode indicator
- end-of-file indicator
- error indicator
- the current stream position and multibyte conversion state (an object of type mbstate_t)
- reentrant lock (required as of
C11
)
- fpos_t
? a non-array type capable of uniquely identifying the position of every byte in a file and every conversion state that can occur in all supported multibyte character encodings
- size_t
? an
unsigned integer
type which is the type of the result of the
sizeof
operator.
Extensions
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]
The
POSIX
standard defines several extensions to
stdio
in its Base Definitions, among which are a
readline
function that allocates memory, the
fileno
and
fdopen
functions that establish the link between
FILE
objects and
file descriptors
, and a group of functions for creating
FILE
objects that refer to in-memory buffers.
[5]
Example
[
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]
The following C program opens a binary file called
myfile
, reads five bytes from it, and then closes the file.
#include
<stdio.h>
#include
<stdlib.h>
int
main
(
void
)
{
char
buffer
[
5
];
FILE
*
fp
=
fopen
(
"myfile"
,
"rb"
);
if
(
fp
==
NULL
)
{
perror
(
"Failed to open file
\"
myfile
\"
"
);
return
EXIT_FAILURE
;
}
if
(
fread
(
buffer
,
1
,
5
,
fp
)
<
5
)
{
fputs
(
"An error occurred while reading the file.
\n
"
,
stderr
);
return
EXIT_FAILURE
;
}
fclose
(
fp
);
printf
(
"The bytes read were: "
);
for
(
int
i
=
0
;
i
<
5
;
++
i
)
{
printf
(
"%02X "
,
buffer
[
i
]);
}
putchar
(
'\n'
);
return
EXIT_SUCCESS
;
}
Alternatives to stdio
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]
Several alternatives to
stdio
have been developed. Among these is the
C++
iostream
library, part of the
ISO C++ standard
. ISO C++ still requires the
stdio
functionality.
Other alternatives include the Sfio
[6]
(A Safe/Fast I/O Library) library from
AT&T Bell Laboratories
. This library, introduced in 1991, aimed to avoid inconsistencies, unsafe practices and inefficiencies in the design of
stdio
. Among its features is the possibility to insert
callback functions
into a stream to customize the handling of data read from or written to the stream.
[7]
It was released to the outside world in 1997, and the last release was 1 February 2005.
[8]
See also
[
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]
References
[
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]
- ^
ISO/IEC 9899:1999 specification
. p. 274, § 7.19.
- ^
Kernighan, Brian
;
Pike, Rob
(1984).
The UNIX Programming Environment
.
Englewood Cliffs
:
Prentice Hall
. p. 200.
- ^
McIlroy, M. D.
(1987).
A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971?1986
(PDF)
(Technical report). CSTR. Bell Labs. 139.
- ^
"(stdio.h) - C++ Reference"
.
C++
. Retrieved
25 July
2021
.
- ^
stdio.h
? Base Definitions Reference,
The Single UNIX Specification
, Version 4 from
The Open Group
- ^
"Sfio: A Safe/Fast I/O Library"
. Archived from the original on 11 February 2006
. Retrieved
16 March
2021
.
{{
cite web
}}
: CS1 maint: bot: original URL status unknown (
link
)
- ^
Korn, David G.
; Vo, Kiem-Phong (1991).
SFIO: Safe/Fast String/File IO
. Proc. Summer USENIX Conf.
CiteSeerX
10.1.1.51.6574
.
- ^
Fowler, Glenn S.; Korn, David G.; Vo, Kiem-Phong (2000).
Extended Formatting with Sfio
. Proc. Summer USENIX Conf.
External links
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]