Lseek

Objectives

1. Gain a basic understaing of the lseek system call.

Notes

• off_t lseek(int fd, off_t offset, int whence);
  • Requires unistd.h
  • Takes a file descriptor to an open file.
  • An ammount to move by
  • A type of movement to do whence
    • SEEK_SET set to this byte from the begiining
    • SEEK_CUR Move by this amount relative to the current position
    • SEEK_END move to the end of the file + offset
    • But in mordern linux
      • SEEK_DATA seeks the next byte of data in the file
      • SEEK_HOLE seeks the enxt hole in the file.
  • Returns
    • The position in a file on success
    • -1 on error.
• File holes: By seeking past the end of the file then writing a hole is produced in the file.
  • The data is 0
• From the book:
  • lseek(fd,0, SEEK_SET) will go to the first byte
  • lseek(fd,0, SEEK_END) will go to one byte past the end of the file.
  • lseek(fd, -1, SEEK_END) will go to the last byte in the file.
  • lseek(fd, -10, SEEK_CUR) will back up 10 bytes, but you can not seek before the beginning of a file.
• dice.cpp is a bad example of how to use this.
• seeker.cpp is a different example.