# 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 <stdio.h>. 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 (for example, PHP).

## 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 <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 | fopen | Opens a file (with a non-Unicode filename on Windows and possible UTF-8 filename on Linux) |
| File access | popen | popen | opens a process by creating a pipe, forking, and invoking the shell |
| File access | freopen | freopen | Opens a different file with an existing stream |
| File access | fflush | fflush | Synchronizes an output stream with the actual file |
| File access | fclose | fclose | Closes a file |
| File access | pclose | pclose | closes a stream |
| File access | setbuf | setbuf | Sets the buffer for a file stream |
| File access | setvbuf | setvbuf | Sets the buffer and its size for a file stream |
| File access | fwide | fwide | Switches a file stream between wide-character I/O and narrow-character I/O |
| Direct input/output | fread | fread | Reads from a file |
| Direct input/output | fwrite | fwrite | Writes to a file |
| Unformatted input/output | fgetc getc | fgetwc getwc | Reads a byte/wchar_t from a file stream |
| Unformatted input/output | fgets | fgetws | Reads a byte/wchar_t line from a file stream |
| Unformatted input/output | fputc putc | fputwc putwc | Writes a byte/wchar_t to a file stream |
| Unformatted input/output | fputs | fputws | Writes a byte/wchar_t string to a file stream |
| Unformatted input/output | getchar | getwchar | Reads a byte/wchar_t from stdin |
| Unformatted input/output | gets | — | Reads a byte string from stdin until a newline or end of file is encountered (deprecated in C99, removed from C11) |
| Unformatted input/output | putchar | putwchar | Writes a byte/wchar_t to stdout |
| Unformatted input/output | puts | — | Writes a byte string to stdout |
| Unformatted input/output | 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 |
| Formatted input/output | vscanf vfscanf vsscanf | vwscanf vfwscanf vswscanf | Reads formatted input byte/wchar_t from stdin, a file stream or a buffer using variable argument list |
| Formatted input/output | printf fprintf sprintf snprintf | wprintf fwprintf swprintf | Prints formatted byte/wchar_t output to stdout, a file stream or a buffer |
| Formatted input/output | 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 |
| Formatted input/output | perror | — | Writes a description of the current error to stderr |
| File positioning | ftell ftello | ftell ftello | Returns the current file position indicator |
| File positioning | fseek fseeko | fseek fseeko | Moves the file position indicator to a specific location in a file |
| File positioning | fgetpos | fgetpos | Gets the file position indicator |
| File positioning | fsetpos | fsetpos | Moves the file position indicator to a specific location in a file |
| File positioning | rewind | rewind | Moves the file position indicator to the beginning in a file |
| Error handling | clearerr | clearerr | Clears errors |
| Error handling | feof | feof | Checks for the end-of-file |
| Error handling | ferror | ferror | Checks for a file error |
| Operations on files | remove | remove | Erases a file |
| Operations on files | rename | rename | Renames a file |
| Operations on files | tmpfile | tmpfile | Returns a pointer to a temporary file |
| Operations on files | tmpnam | tmpnam | Returns a unique filename |

### Constants

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

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

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

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.

## Example

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

## Alternatives to stdio

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 (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. It was released to the outside world in 1997, and the last release was 1 February 2005.