21_seek_dir.md

name: main

.aim[Systems: Directory Listings]

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template: main

???

• lseek - <unistd.h>
  • Set the current position in an open file
  • lseek( file_descriptor, offset, whence )
    • offset
      • Number of bytes to move the position by, Can be negative.
    • whence
      • Where to measure the offset from
      • SEEK_SET: offset is evaluated from the beginning of the file
      • SEEK_CUR: offset is relative to the current position in the file
      • SEEK_END: offset is evaluated from the end of the file
  • Returns the number of bytes the current position is from the beginning of the file, or -1 (errno)

---

template: main

File functions

• stat - <sys/stat.h>
  • Get information about a file (metadata)

--

• stat( path, stat_buffer )
  • stat_buffer
    • Must be a pointer to a struct stat
    • All the file information gets put into the stat buffer.

--

• Some of the fields in struct stat:
  • st_size: file size in bytes
  • st_uid, st_gid: user id, group id
  • st_mode: file permissions
  • st_atime, st_mtime: last access, last modification

-- - These are time_t variables. We can use functions in time.h to make sense of them - ctime( time ) - Returns the time as a string - time is type time_t *

---

template: main

Directories

• A *nix directory is a file containing the names of the files within the directory along with basic information, like file type.

--

• Moving files into/out of a directory means changing the directory file, not actually moving any data.

--

• opendir - <dirent.h>
  • Open a directory file
  • This will not change the current working directory (cwd), it only allows your program to read the contents of the directory file

--

• opendir( path )
  • Returns a pointer to a directory stream (DIR *)

--

• closedir - <dirent.h>
  • Closes the directory stream and frees the pointer associated with it.
  • closedir( dir_stream )

---

Directories

• readdir - <dirent.h>
  • readdir( dir_stream )
  • Returns a pointer to the next entry in a directory stream, or NULL if all entries have already been returned.

--

• struct dirent - <sys/types.h>
  • Directory struct that contains the information stored in a directory file. Some of the data members
  • d_name: Name of a file
  • d_type: File type as an int

--

• rewinddir - <dirent.h>
  • rewinddir(d)
    • d must be a DIR * returned from opendir
    • Resets the directory stream of d to the beginning.

???

• Example usage:

  DIR * d;
  d = opendir( "somedir" );
  struct dirent *entry;
  entry = readdir( d );
  closedir(d);