A file produced by ar has a magic string at the start, followed by the constituent files, each preceded by a file header. The magic number and header layout as described in the include file are:

#define ARMAG   "!<arch>\n"
#define SARMAG  8

#define ARFMAG  "`\n"

struct ar_hdr {
    char    name[16];
    char    date[12];
    char    uid[6];
    char    gid[6];
    char    mode[8];
    char    size[10];
    char    fmag[2];
};
#define SAR_HDR 60

The name is a blank-padded string. The fmag field contains ARFMAG to help verify the presence of a header. The other fields are left-adjusted, blank-padded numbers. They are decimal except for mode, which is octal. The date is the modification date of the file (see sys-stat(2)) at the time of its insertion into the archive. The mode is the low 9 bits of the file permission mode. The length of the header is SAR_HDR. Because the ar_hdr structure is padded in an architecture-dependent manner, the structure should never be read or written as a unit; instead, each field should be read or written independently.

Each file begins on an even (0 mod 2) boundary; a newline is inserted between files if necessary. Nevertheless size reflects the actual size of the file exclusive of padding.

When all members of an archive are object files of the same architecture, ar automatically adds an extra file, named __.SYMDEF, as the first member of the archive. This file contains an index used by the loaders to locate all externally defined text and data symbols in the archive.

There is no provision for empty areas in an archive file.