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).

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]

Variables

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

More information Name, Notes ...

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.

Close

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;
}

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]

• printf format string
• scanf format string