Unix ar Archive MIME Type

What does application/x-unix-archive mean?

The application/x-unix-archive media type identifies an archive in the Unix ar family. An ar file holds a sequence of members, usually compiled object files. Build systems commonly use that container as a static library with a name such as libexample.a, although an ar archive can contain any kind of file.¹

The label is useful but not formally registered. application/x-unix-archive does not appear in IANA's application media-type registry, and other software uses application/x-archive for the same family.²³ That means a MIME label alone is not enough to select a particular ar variant.

Where did the Unix ar format come from?

The format reaches back to early Unix at Bell Laboratories. The first edition of the Unix Programmer's Manual, dated November 3, 1971, documented the ar command and archive-related tooling. This page therefore uses 1971 as the origin year.⁴

Ar remains relevant because linkers consume static libraries built from object files. The archive can carry a symbol table so a linker can locate the member defining a needed symbol without scanning every member in full.¹ Modern tools also use deterministic archives, which replace variable timestamps and ownership fields with fixed values to make repeated builds reproducible.⁵

How do you identify an ar archive?

A conventional ar archive begins with the eight ASCII bytes !<arch>\n, represented in hexadecimal as 21 3C 61 72 63 68 3E 0A. Members follow in even-length records. Each member starts with a fixed-width ASCII header describing its name, modification time, owner, group, mode, and size.⁶

The beginning therefore looks like:

21 3C 61 72 63 68 3E 0A ...
!  <  a  r  c  h  >  \n

That signature is stronger evidence than an extension. The .a suffix is the usual convention for a static library, while .ar makes the container itself explicit.³⁷

Several incompatible details exist behind the shared outer structure. GNU, BSD, Darwin, and System V variants differ in areas such as long member names and symbol-table conventions. LLVM's current llvm-ar reads SVR4, GNU, BSD, AIX big archives, and Darwin archives, and it can write several of those styles.⁵

How do you open an application/x-unix-archive file?

Browsers do not provide a native member browser for ar archives. Download the file and use a build tool or archive utility that understands its variant.

GNU Binutils and LLVM both provide compatible command-line tools. These commands only list and inspect the archive:

ar t library.a
ar tv library.a
nm -s library.a

To extract members into a new, empty directory, change to that directory and run:

ar x ../library.a

Listing first matters because extraction writes member names to disk. Treat archives from unknown sources as untrusted input, and do not link extracted object files into software you plan to run unless you trust their origin.

Support in Poly

Poly recognizes .ar filenames as application/x-unix-archive and classifies them as archives. It can store, sync, share, version, download, and find the outer file by name.

Poly treats the payload as opaque:

• It does not list or extract members.
• It does not parse the archive symbol table or embedded object files.
• It does not generate a thumbnail or in-app archive preview.
• It does not index member names or contents, and the Poly agent cannot read them.

The common .a extension is an important limitation. Poly's filename map currently recognizes .ar, but not .a, as this MIME type. A .a file can still be stored, but filename inference will not identify it as application/x-unix-archive.

Is an ar archive the same as a static library?

Not exactly. Ar is the container. A static library is a common use of that container, usually holding relocatable object files plus a symbol index. Ar can also hold arbitrary files, and not every .a library uses identical object formats or archive conventions.¹⁵

Apple likewise describes static libraries as archives of object files and uses the .a extension. A library built for one operating system or processor is not automatically usable on another, even when both tools recognize the outer ar container.⁷

How does ar compare with TAR and ZIP?

Choose TAR when directory structure and filesystem metadata are central. Choose ZIP when recipients need a broadly supported compressed package. Choose ar when a compiler toolchain or another specification explicitly expects it.

Debian's .deb format is a useful example of ar used as a higher-level container. A Debian binary package is an ar archive whose members include debian-binary, a control TAR archive, and a data TAR archive.⁸ Use dpkg-deb for package work because it understands those required members and package semantics. Raw ar only exposes the outer container.

Can you convert an ar archive?

Archive conversion means extracting the members and creating a new container. It does not convert object code inside a static library.

• Use TAR when you want a Unix-oriented file bundle rather than a linkable library.
• Use ZIP when convenient desktop interchange matters more than Unix metadata.
• Rebuild a static library with the target platform's compiler and archiver when architecture, object format, or symbol-table compatibility matters.

Do not convert a .a library to ZIP and expect a linker to accept it. The linker expects ar structure, compatible object files, and often a usable symbol index.