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Tar (tape archive) File Manipulation
Learning Objectives
Upon completion of this assignment, you should be able:
1. To read and understand directory entries and file attributes;
2. To do complex file input/output and file manipulation operations.
The mechanisms you will practice using include:
• Buffered I/O: fopen(), fclose(), fread(), fwrite(), fseek()
• Directory entries: opendir(), readdir(), closedir()
• File metadata: stat(), chmod(), utimes()
• Hard links: link()
Program Specification
NAME
mytar – create and manipulate tape archives
SYNOPSIS
mytar [cxtf:] [file]filename
DESCRIPTION
mytar creates an archive from of a directory tree, extracts files from an archive, or prints
the contents and details of an archive file.
Options followed by a ‘:’ expect a subsequent parameter. The options1 are as follows:
-c
create an archive of the given directory tree. A directory name must be specified.
-x
extract the directory tree contained in the specified archive
-t
print the contents of the specified archive
-f:
the subsequent argument is the name of the archive file (to create, extract or print).
This option must always be specified.
EXIT STATUS
mytar exits 0 on success and -1 on failure.
ERRORS
Upon error, mytar exits after printing to the standard error stream an appropriate message
using one of the format strings below:
"Error: No tarfile specified.\n"
1 Options followed by a ‘:’ expect a subsequent parameter.
2
"Error: Specified target (\"%s\") does not exist.\n"
"Error: Specified target (\"%s\") is not a directory.\n"
"Error: No directory target specified.\n"
"Error: Bad magic number (%d), should be: %d.\n"
Additionally, if a library or system call fails, mytar calls perror() with the name of the
failing routine then exits.
EXIT STATUS
mytar exits with 0 on success and -1 on failure.
EXAMPLES
mytar -c -f a.tar a
create an archive, a.tar, containing all files in the directory tree, a
mytar -x -f a.tar
extract the files in the archive, a.tar
mytar -t -f a.tar
print the details of all files in the archive, a.tar
mytar -c a
Error: No tarfile specified.
Implementation Details
mytar File Format
mytar archives can be read or printed by any other program that observes the proper format2.
The mytar format specification follows: first, a magic number that identifies mytar archives.
Then for each file in the archive, the archive contains in order: inode, filename length, filename,
mode, modification time, and for regular files, file size and file content.
Size Content Notes
4 bytes 0x7261746D magic number, once per archive
8 bytes file inode number
4 bytes file name length
n bytes file name n is file name length
4 bytes file mode
8 bytes file modification time in seconds
8 bytes file size only for regular (not directory) files
n bytes file content n is file size;
only for regular (not directory) files
2 When transferring tar files between heterogeneous platforms, endianness may be an issue.
3
(As discussed below, hard links to files already in the archive only have an inode number, file
name length and file name.)
Printing
In print mode, mytar uses the following formatted print statements:
For directories: "inode: %llu, mode: %d, mtime: %llu, name: %s/\n"
For non-executable files: "inode: %llu, mode: %d, mtime: %llu, size: %llu, name: %s\n"
For executable files: "inode: %llu, mode: %d, mtime: %llu, size: %llu, name: %s*\n"
Requirements and Constraints
1. In the archive, directories should appear before any contained files or directories.
2. Your archive must specify file and directory paths relative to the specified directory: you
should not use absolute paths.
3. Close files immediately when you are finished reading or writing them.
4. Call closedir() immediately when you are finished using a directory stream opened
with opendir().
5. Contents should appear in the archive in the order returned by readdir().
6. Directories should be traversed in a depth-first manner.
7. You must track file inodes to exclude redundant information when encountering hard
links that reference the same inode. For redundant hard links, only include the filename
(and length) and inode number in the archive.
8. mytar must re-establish the original mode and modification times for extracted files.
Notes
1. mytar should ignore symbolic links, ‘.’ and ‘..’ directory entries
2. When you extract files and directories from an archive, there is no (portable) way to
retain the original inode values.

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