The end of Chapter 4

• lseek - go to a position in a file.
  • Needs sys/types.h and unistd.h
  • off_t lseek(int fd, off_t offset, int whence);
  • whence is one of three values
    • SEEK_SET seek offset bytes from the beginning of the file.
    • offset must be positive.
    • SEEK_CUR seek offset bytes from the current position.
    • offset can be positive or negative, but the final position must not be less than 0.
    • SEEK_END seek offset bytes from the end of the file.
    • offset can be positive or negative, but the final position must not be less than 0.
  • If a write occurs past the end of the file, a hole will be created in the file.
    • Bytes in a hole are reported as 0.
    • Disk blocks are not allocated.
    • Some file systems do not support file holes.
  • Newer versions of linux support the following whence values
    • SEEK_DATA : adjust the offset to the next location in the file containing data.
    • SEEK_HOLE : adjust the offset to the next hole.
    • Both return -1 if they fail.
  • The return value is the location measured in bytes on success or -1 on failure.
  • lseek will not work on some file types.