Types of Files

• So far we have assumed that our files were sequential: sucessive reads or writes read or write from sucessive bytes in the file. While this is a very good model of some files (the keyboard or a file on a tape drive), it is a limited model of others (files stored on a disk drive or cd rom). These other files support random access: they can quickly and easily be read or written starting from any byte in the file.

• The following routines are defined in stdio.h and may be used on random access files:
  • void rewind(FILE *); is easy. It rewinds the file to the beginning -- all further reads or writes take place starting at the beginning of the file.

  • long int ftell(FILE *); returns the current byte location of the "read-write head" in the current file. For example, if you have read 123 bytes from the file testfile, ftell(testfile) returns 123.

  • int fseek(FILE *, long int offset, int whence); has three different behaviours, depending on whether whence is 0, 1, or 2. If whence is 0, then fseek moves the read-write head to position offset. If whence is 1, then fseek moves the read-write head to the current position plus offset. If whence is 2, then fseek moves the read-write head to the position of the end of the file plus offset. (offset is definitely allowed to be negative.)