2010-02-03 |  Thiyagarajanarun  | Viewed: 1041  |    0

Programming Random Access File I/O in C

The basic file operations are

• fopen - open a file- specify how its opened (read/write) and type (binary/text)
• fclose - close an opened file
• fread - read from a file
• fwrite - write to a file
• fseek/fsetpos - move a file pointer to somewhere in a file.
• ftell/fgetpos - tell you where the file pointer is located.

There are two fundamental types of file: text and binary. Of these two, binary are generally the simpler to deal with. As doing random access on a text file isn't something you need to do too often, we'll stick with binary files for the rest of this lesson. The first four operations listed above are for both text and random access files. The last two just for random access.

Random access means we can move to any part of a file and read or write data from it without having to read through the entire file. Back thirty years ago, much data was stored on large reels of computer tape. The only way to get to a point on the tape was by reading all the way through the tape. Then disks came along and now we can read any part of a file directly.

Programming With Binary Files

A binary file is a file of any length that holds bytes with values in the range 0 to 0xff. (0 to 255). These bytes have no other meaning unlike in a text file where a value of 13 means carriage return, 10 means line feed, 26 means end of file and software reading text files has to deal with these.

Download Example 1.

This shows a simple binary file being opened for writing, with a text string (char *) being written into it. Normally you'd use a text file for that but I wanted to show that you can write text to a binary file.

// ex1.c
#include <stdio.h>
#include <string.h>

int main(int argc, char * argv[])
{
  const char * filename="test.txt";
  const char * mytext="Once upon a time there were three bears.";
  int byteswritten=0;
  FILE * ft= fopen(filename, "wb") ;
  if (ft) {
    fwrite(mytext,sizeof(char),strlen(mytext), ft) ;
    fclose( ft ) ;
  }
  printf("len of mytext = %i ",strlen(mytext)) ;
  return 0;
}