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>.[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]

C standard library
General topics
Miscellaneous headers

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.[4]

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 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 N/A 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 N/A 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 N/A 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

Constants defined in the <stdio.h> header include:

NameNotes
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:

NameNotes
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, this is an opaque type 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.[5]

Example

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;
    }

    for (int i = 0; i < 5; i++) {
        int rc = getc(fp);
        if (rc == EOF) {
            fputs("An error occurred while reading the file.\n", stderr);
            return EXIT_FAILURE;
        }
        
        buffer[i] = rc;
    }

    fclose(fp);

    printf("The bytes read were... %x %x %x %x %x\n", buffer[0], buffer[1],
        buffer[2], buffer[3], buffer[4]);

    return EXIT_SUCCESS;
}

Alternatives to stdio
"Sfio" redirects here. For the defunct French political party, see French Section of the Workers' International.

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 (safe-fast / string-file I/O) 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.[6] It was released to the outside world in 1997.[7]

See also

References
  1. ISO/IEC 9899:1999 specification (PDF). p. 274, § 7.19.
  2. Kernighan, Brian; Pike, Rob (1984). The UNIX Programming Environment. Englewood Cliffs: Prentice Hall. p. 200.
  3. McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. 139.
  4. http://www.cplusplus.com/reference/cstdio/
  5. stdio.h – Base Definitions Reference, The Single UNIX Specification, Issue 7 from The Open Group
  6. Korn, David G.; Vo, Kiem-Phong (1991). SFIO: Safe/Fast String/File IO. Proc. Summer USENIX Conf. CiteSeerX 10.1.1.51.6574.
  7. Fowler, Glenn S.; Korn, David G.; Vo, Kiem-Phong (2000). Extended Formatting with Sfio. Proc. Summer USENIX Conf.

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