usr/include/a.out.h

struct  exec {  /* a.out header */
        int             a_magic;        /* magic number */
        unsigned        a_text;         /* size of text segment */
        unsigned        a_data;         /* size of initialized data */
        unsigned        a_bss;          /* size of unitialized data */
        unsigned        a_syms;         /* size of symbol table */
        unsigned        a_entry;        /* entry point */
        unsigned        a_unused;       /* not used */
        unsigned        a_flag;         /* relocation info stripped */
};

#define A_MAGIC1        0407            /* normal */
#define A_MAGIC2        0410            /* read-only text */
#define A_MAGIC3        0411            /* separated I&D */
#define A_MAGIC4        0405            /* overlay */

struct  nlist { /* symbol table entry */
        char            n_name[8];      /* symbol name */
        int             n_type;         /* type flag */
        unsigned        n_value;        /* value */
};

                /* values for type flag */
#define N_UNDF  0       /* undefined */
#define N_ABS   01      /* absolute */
#define N_TEXT  02      /* text symbol */
#define N_DATA  03      /* data symbol */
#define N_BSS   04      /* bss symbol */
#define N_TYPE  037
#define N_REG   024     /* register name */
#define N_FN    037     /* file name symbol */
#define N_EXT   040     /* external bit, or'ed in */
#define FORMAT  "%06o"  /* to print a value */