.A

Introduction

A .a file is a static library on Linux, macOS and other Unix-like systems: a bundle of object files (.o, in ELF or Mach-O format) packed with the ar archiver. When a program is linked against it, the linker copies in only the object files that define something the program uses. Library names follow the pattern libname.a, which the option -lname finds.

The ar archive format is older than static libraries and is used for other things too: a Debian package is an ar archive, and Windows static libraries (.lib) use a variant of it.

Structure

The file starts with the eight bytes !<arch>\n. Each member file follows, with a 60-byte header of plain ASCII fields padded with spaces: the name, the modification time, owner and group IDs, the file mode in octal, the size in decimal, and the two bytes `\n. The member’s contents come next, padded to an even length.

For a library, the first member is a symbol index that maps each global symbol to the member defining it, so the linker need not read every object file. In the GNU and System V variant it is named /, and a member named // holds file names longer than 15 characters; the BSD and macOS variant names the index __.SYMDEF and stores long names in the member itself. The ranlib tool, or ar s, rebuilds the index.

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