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]

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). The C++ standard library reflects the "stream" concept in its syntax, see iostream for details.

Overview of functions

Most of the C file input/output functions are defined in stdio.h (cstdio header in C++).

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:
  • a file descriptor
  • the current stream position
  • an end-of-file indicator
  • an error indicator
  • a pointer to the stream's buffer, if applicable
• fpos_t - a non-array type capable of uniquely identifying the position of every byte in a file.
• size_t - an unsigned integer type which is the type of the result of the sizeof operator.

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] = {0};  /* Initialized to zeroes */
    int i;
    FILE *fp = fopen("myfile", "rb");
 
    if (fp == NULL) {
        perror("Failed to open file \"myfile\"");
        return EXIT_FAILURE;
    }
 
    /* Be sure to never read more than five characters */
    for (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;
}

See also

• printf format string
• scanf format string

References

External links

The Wikibook C Programming has a page on the topic of: C Programming/C Reference