C file input/output

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).

Constants

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

Variables

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

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

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 20050201.^[8]

See also

• printf format string
• scanf format string

References

External links

• Media related to C file input/output at Wikimedia Commons