C file input/output

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. The functionality descends from a "portable I/O package" written by Mike Lesk at Bell Labs in the early 1970s, and officially became part of the Unix operating system in Version 7.
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.

Overview

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.

Functions

Most of the C file input/output functions are defined in .

Constants

Constants defined in the header include:

Name	Notes
EOF	A negative integer of type used to indicate end-of-file conditions
	An integer which is the size of the buffer used by the function
FILENAME_MAX	The size of a 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 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 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 function to request unbuffered input and output for an open stream
L_tmpnam	The size of a array which is large enough to store a temporary filename generated by the 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 function to request positioning relative to the current file position
SEEK_END	An integer which may be passed to the function to request positioning relative to the end of the file
SEEK_SET	An integer which may be passed to the function to request positioning relative to the beginning of the file
TMP_MAX	The maximum number of unique filenames generable by the function; will be at least 25

Variables

Variables defined in the header include:

Name	Notes
stdin	A pointer to a which refers to the standard input stream, usually a keyboard.
stdout	A pointer to a which refers to the standard output stream, usually a display terminal.
stderr	A pointer to a which refers to the standard error stream, often a display terminal.

Member types

Data types defined in the header include:

– also known as a file handle or a , 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
*stream buffering state indicator
*I/O mode indicator
*binary/text mode indicator
*end-of-file indicator
*error indicator
*the current stream position and multibyte conversion state
*reentrant lock
– 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
– an unsigned integer type which is the type of the result of the operator.

Extensions

The POSIX standard defines several extensions to in its Base Definitions, among which are a function that allocates memory, the and functions that establish the link between objects and file descriptors, and a group of functions for creating objects that refer to in-memory buffers.

Example

The following C program opens a binary file called, reads five bytes from it, and then closes the file.

include
include

int main

Alternatives to stdio

Several alternatives to have been developed. Among these are C++ I/O headers and , part of the C++ Standard Library. ISO C++ still requires the functionality, and it is found under header .
Other alternatives include the Sfio library from AT&T Bell Laboratories. This library, introduced in 1991, aimed to avoid inconsistencies, unsafe practices and inefficiencies in the design of. 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. It was released to the outside world in 1997, and the last official stable release was ''2005-02-01''