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Archive Headers

An archive header describes the file attributes for the archive member that follows it. The ar format only supports a limited number of attributes: the file name, the file creation time stamp, the uid and gid of the creator, the file mode and the file size.

Archive headers are placed at an even byte offset in the archive file. If the data for an archive member ends at an odd byte offset, then a padding byte with value 0x0A is used to position the next archive header on an even byte offset.

An archive header comprises the following fixed sized fields:

ar_name

(16 bytes) The file name of the archive member. The format of this field varies between the BSD and SVR4/GNU formats and is described in more detail in the section Representing File Names below.

ar_date

(12 bytes) The file modification time for the member in seconds since the epoch, encoded as a decimal number.

ar_uid

(6 bytes) The uid associated with the archive member, encoded as a decimal number.

ar_gid

(6 bytes) The gid associated with the archive member, encoded as a decimal number.

ar_mode

(8 bytes) The file mode for the archive member, encoded as an octal number.

ar_size

(10 bytes) In the SVR4/GNU archive format this field holds the size in bytes of the archive member, encoded as a decimal number. In the BSD archive format, for short file names, this field holds the size in bytes of the archive member, encoded as a decimal number. For long file names (see Representing File Names below), the field contains the combined size of the archive member and its file name, encoded as a decimal number.

ar_fmag

(2 bytes) This field holds 2 bytes with values 0x96 and 0x0A respectively, marking the end of the header.

Unused bytes in the fields of an archive header are set to the value 0x20.

Representing File Names

The BSD and SVR4/GNU variants use different schemes for encoding file names for members.

BSD

File names that are up to 16 bytes long and which do not contain embedded spaces are stored directly in the ar_name field of the archive header. File names that are either longer than 16 bytes or which contain embedded spaces are stored immediately after the archive header and the ar_name field of the archive header is set to the string “#1/” followed by a decimal representation of the number of bytes needed for the file name. In addition, the ar_size field of the archive header is set to the decimal representation of the combined sizes of the archive member and the file name. The file contents of the member follows the file name without further padding.

As an example, if the file name for a member was “A B” and its contents was the string “C D”, then the ar_name field of the header would contain “#1/3”, the ar_size field of the header would contain “6”, and the bytes immediately following the header would be 0x41, 0x20, 0x42, 0x43, 0x20 and 0x44 (ASCII “A BC D”).

SVR4/GNU

File names that are up to 15 characters long are stored directly in the ar_name field of the header, terminated by a “/” character.

If the file name is larger than would fit in space for the ar_name field, then the actual file name is kept in the archive string table (see Archive String Tables below), and the decimal offset of the file name in the string table is stored in the ar_name field, prefixed by a “/” character.

As an example, if the real file name has been stored at offset 768 in the archive string table, the ar_name field of the header will contain the string “/768”.

Special Archive Members

The following archive members are special.

“/”

In the SVR4/GNU variant of the archive format, the archive member with name “/” denotes an archive symbol table. If present, this member will be the very first member in the archive.

“//”

In the SVR4/GNU variant of the archive format, the archive member with name “//” denotes the archive string table. This special member is used to hold filenames that do not fit in the file name field of the header (see Representing File Names above). If present, this member immediately follows the archive symbol table if an archive symbol table is present, or is the first member otherwise.

“__.SYMDEF”

This special member contains the archive symbol table in the BSD variant of the archive format. If present, this member will be the very first member in the archive.

Archive String Tables

An archive string table is used in the SVR4/GNU archive format to hold file names that are too large to fit into the constraints of the ar_name field of the archive header. An archive string table contains a sequence of file names. Each file name in the archive string table is terminated by the byte sequence 0x2F, 0x0A (the ASCII string “/\n”). No padding is used to separate adjacent file names.

Archive Symbol Tables

Archive symbol tables are used to speed up link editing by providing a mapping between the program symbols defined in the archive and the corresponding archive members. Archive symbol tables are managed by the ranlib(1) utility.

The format of archive symbol tables is as follows:

BSD

In the BSD archive format, the archive symbol table comprises of two parts: a part containing an array of struct ranlib descriptors, followed by a part containing a symbol string table. The sizes and layout of the structures that make up a BSD format archive symbol table are machine dependent.

The part containing struct ranlib descriptors begins with a field containing the size in bytes of the array of struct ranlib descriptors encoded as a C long value.

The array of struct ranlib descriptors follows the size field. Each struct ranlib descriptor describes one symbol.

A struct ranlib descriptor comprises two fields:

ran_strx

(C long) This field contains the zero-based offset of the symbol name in the symbol string table.

ran_off

(C long) This field is the file offset to the archive header for the archive member defining the symbol.

The part containing the symbol string table begins with a field containing the size in bytes of the string table, encoded as a C long value. This string table follows the size field, and contains NUL-terminated strings for the symbols in the symbol table.

SVR4/GNU

In the SVR4/GNU archive format, the archive symbol table starts with a 4-byte binary value containing the number of entries contained in the archive symbol table. This count of entries is stored most significant byte first.

Next, there are count 4-byte numbers, each stored most significant byte first. Each number is a binary offset to the archive header for the member in the archive file for the corresponding symbol table entry.

After the binary offset values, there are count NUL-terminated strings in sequence, holding the symbol names for the corresponding symbol table entries.