.a File Extension

What kind of file is a .a?

A .a file is usually a static library used by Unix-like compiler toolchains. It normally uses the Unix ar container and holds compiled object files plus a symbol index that helps a linker find the code it needs. An ar container can hold other kinds of files, so the suffix describes a strong convention rather than a guarantee.¹

Most conventional files start with the eight ASCII bytes !<arch>\n, or 21 3C 61 72 63 68 3E 0A in hexadecimal. Check that signature instead of trusting the filename. A GNU thin archive starts with !<thin>\n and refers to files elsewhere rather than embedding their contents.¹

The de facto MIME type is application/x-unix-archive. It is not registered by IANA, and some tools instead use application/x-archive.²

How do I open a .a on my computer?

A .a library is meant for development tools, not a document viewer.

• On Linux and other Unix systems, use GNU ar, nm, or the equivalents supplied by the compiler toolchain.
• On macOS, use Apple's command-line developer tools, including ar, nm, and libtool.
• On Windows, use llvm-ar from LLVM or a Unix-compatible development environment. Microsoft-native static libraries more commonly use .lib.

List members without extracting them:

ar t library.a
nm -s library.a

LLVM documents support for several archive styles, including GNU, BSD, Darwin, SVR4, and AIX big archives.³ Recognizing the outer archive does not guarantee that your linker can use its members. CPU architecture, object format, compiler ABI, and target operating system must also match.

Poly support for .a

Poly stores, syncs, shares, versions, and downloads .a files. They remain searchable by filename and ordinary file properties.

Poly does not infer the ar media type from the .a suffix, open an archive browser, extract members, index symbols or embedded object code, or pass those members to the Poly agent. Download the file and inspect it with a trusted compiler toolchain. Treat libraries from unknown sources as executable supply-chain inputs, even though the archive itself is not launched when you view it.

History of .a

The ar command and archive format appeared in the first edition of the Unix Programmer's Manual, dated November 3, 1971.⁴ Static libraries adopted the short .a convention, with “a” standing for archive.

The basic container survived because linkers can use its symbol table to select only the members needed by a program. Modern archivers also offer deterministic output, replacing variable timestamps and ownership fields so repeated builds can produce identical archives.³

Working with .a and alternatives

Choose the next step based on the file's purpose:

TAR preserves directory-oriented bundles and Unix metadata. ZIP offers broad desktop interchange and built-in compression. A static library needs ar structure, compatible object files, and usually a symbol index, so repackaging it as TAR or ZIP will stop it from working as a library.