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Annotation of researchv10dc/man/man5/a.out.5, revision 1.1

1.1     ! root        1: .TH A.OUT 5
        !             2: .CT 1 lib_obj
        !             3: .SH NAME
        !             4: a.out \- object file format
        !             5: .SH SYNOPSIS
        !             6: .B #include <a.out.h>
        !             7: .SH DESCRIPTION
        !             8: .I A.out
        !             9: is the default name of the output file of the assembler
        !            10: .IR as (1)
        !            11: or the link editor
        !            12: .IR ld (1).
        !            13: Both programs make
        !            14: .I a.out
        !            15: executable if there were no
        !            16: errors and no unresolved external references.
        !            17: An object file has five sections:
        !            18: a header, the program text and data,
        !            19: relocation information, a symbol table and a string table (in that order).
        !            20: The last three may be absent; see
        !            21: .IR strip (1)
        !            22: and option
        !            23: .B -s
        !            24: of
        !            25: .IR ld (1).
        !            26: The header format, given in 
        !            27: .BR <a.out.h> ,
        !            28: is
        !            29: .PP
        !            30: .EX
        !            31: .ta \w'#define  'u +\w'unsigned  'u +\w'a_dirsize  'u +4n
        !            32: struct exec {
        !            33:        long    a_magic;        /* magic number */
        !            34:        unsigned        a_text; /* size of text segment */
        !            35:        unsigned        a_data; /* size of initialized data */
        !            36:        unsigned        a_bss;  /* size of uninitialized data */
        !            37:        unsigned        a_syms; /* size of symbol table */
        !            38:        unsigned        a_entry;        /* entry point */
        !            39:        unsigned        a_trsize;       /* size of text relocation */
        !            40:        unsigned        a_drsize;       /* size of data relocation */
        !            41: };
        !            42: #define        OMAGIC  0407    /* old impure format */
        !            43: #define        NMAGIC  0410    /* read-only text */
        !            44: #define        ZMAGIC  0413    /* demand load format */
        !            45: .EE
        !            46: .PP
        !            47: Macros which take
        !            48: .L exec
        !            49: structures as arguments and tell whether
        !            50: the file has a reasonable magic number or return offsets:
        !            51: .EX
        !            52: #define        N_BADMAG(x) (((x).a_magic)!=OMAGIC && \e
        !            53:        ((x).a_magic)!=NMAGIC && ((x).a_magic)!=ZMAGIC)
        !            54: #define        N_TXTOFF(x) \e
        !            55:        ((x).a_magic==ZMAGIC ? 1024 : sizeof (struct exec))
        !            56: #define N_SYMOFF(x) (N_TXTOFF(x) + (x).a_text+(x).a_data + \e
        !            57:        (x).a_trsize+(x).a_drsize)
        !            58: #define        N_STROFF(x) (N_SYMOFF(x) + (x).a_syms)
        !            59: .EE
        !            60: .DT
        !            61: .PP
        !            62: Sizes are expressed in bytes.
        !            63: The size of the header is not included in any of the other sizes.
        !            64: .PP
        !            65: When an
        !            66: .F a.out
        !            67: file is executed, a memory image of three segments is
        !            68: set up: the text segment, the data segment,
        !            69: and a stack.
        !            70: The text segment begins at virtual address 0.
        !            71: Following the text segment is the data segment, in which
        !            72: explicitly initialized data come first, then
        !            73: other data, initialized to 0.
        !            74: The stack occupies the highest possible locations
        !            75: in the core image, automatically growing downwards from
        !            76: .lg 0
        !            77: .BR 0x7ffff400 
        !            78: .lg 1
        !            79: as needed.
        !            80: The data segment may be extended by
        !            81: .IR brk (2).
        !            82: .PP
        !            83: If the magic number in the header is
        !            84: .BR OMAGIC ,
        !            85: the text
        !            86: segment is neither write-protected nor shared and
        !            87: the data segment is immediately contiguous
        !            88: with the text segment.
        !            89: This kind of executable program is rarely used.
        !            90: If the magic number is
        !            91: .BR NMAGIC 
        !            92: or
        !            93: .BR ZMAGIC ,
        !            94: the data segment is loaded at the first 0 mod 1024 address
        !            95: following the text segment,
        !            96: and the text segment is write-protected and shared among
        !            97: all processes that are executing the same file.
        !            98: .BR ZMAGIC
        !            99: format, which
        !           100: .IR ld (1)
        !           101: produces by default, supports demand paging: the
        !           102: text and data segments are multiples of
        !           103: 1024 bytes long and begin at byte offsets of 0 mod 1024
        !           104: in the
        !           105: .F a.out
        !           106: file.
        !           107: .PP
        !           108: Macros are provided to compute the absolute offset of various
        !           109: parts of the file:
        !           110: .TF N_TXTOFF
        !           111: .TP 
        !           112: .B N_TXTOFF
        !           113: Text segment.
        !           114: .TP
        !           115: .B N_SYMOFF
        !           116: Symbol table.
        !           117: .TP
        !           118: .B N_STROFF
        !           119: String table.
        !           120: .PD
        !           121: .PP
        !           122: The offsets of the data segment, text relocation information,
        !           123: and data relocation information are obtained by successively
        !           124: adding to 
        !           125: .B N_TXTOFF
        !           126: the size fields
        !           127: .BR a_text ,
        !           128: .BR a_data ,
        !           129: and
        !           130: .BR a_trsize .
        !           131: The first 4 bytes of the string table contain it size, including
        !           132: the 4 bytes.
        !           133: .PP
        !           134: The layout of a symbol table entry, as given in
        !           135: .BR <a.out.h> ,
        !           136: is
        !           137: .PP
        !           138: .EX
        !           139: .ta \w'#define  'u +\w'char'u-1u +\w'unsigned  'u+1u +\w'*n_name 'u
        !           140: struct nlist {
        !           141:        union {
        !           142:                char    *n_name;        /* for use when in-core */
        !           143:                long    n_strx; /* index into file string table */
        !           144:        } n_un;
        !           145:        unsigned char   n_type; /* type flag; see below */
        !           146:        char    n_other;
        !           147:        short   n_desc; /* see \fIstab\fR(5)\f5 */
        !           148:        unsigned        n_value;        /* value of this symbol (or struct offset) */
        !           149: };
        !           150: .EE
        !           151: .PP
        !           152: Basic values for
        !           153: .BR n_type :
        !           154: .PP
        !           155: .EX
        !           156: #define        N_UNDF  0x0     /* undefined */
        !           157: #define        N_ABS   0x2     /* absolute */
        !           158: #define        N_TEXT  0x4     /* text */
        !           159: #define        N_DATA  0x6     /* data */
        !           160: #define        N_BSS   0x8     /* bss */
        !           161: #define        N_COMM  0x12    /* common (internal to ld) */
        !           162: #define        N_FN    0x1f    /* file name symbol */
        !           163: #define        N_EXT   01      /* external bit, or'ed in */
        !           164: #define        N_TYPE  0x1e    /* mask for all the type bits */
        !           165: .EE
        !           166: .PP
        !           167: Other permanent symbol table entries have some
        !           168: .B N_STAB
        !           169: bits set.
        !           170: These are given in
        !           171: .LR <stab.h> :
        !           172: .EX
        !           173: #define        N_STAB  0xe0    /* if any of these bits set, keep */
        !           174: .EE
        !           175: .DT
        !           176: .PP
        !           177: The field
        !           178: .B n_un.n_strx
        !           179: gives an index into the
        !           180: string table; 0
        !           181: indicates that no name is associated
        !           182: with the entry.
        !           183: The field
        !           184: .B n_un.n_name
        !           185: can be used
        !           186: to refer to the symbol name only if the program sets this up using
        !           187: .B n_strx
        !           188: and appropriate data from the string table.
        !           189: .PP
        !           190: A symbol of type undefined external with nonzero value field
        !           191: names a common region; the value specifies its size.
        !           192: .PP
        !           193: Relocation
        !           194: information occupies eight bytes per
        !           195: relocatable datum:
        !           196: .PP
        !           197: .EX
        !           198: .ta \w'#define  'u +\w'unsigned  'u +\w'r_symbolnum:24,  'u +4n
        !           199: struct relocation_info {
        !           200:        int     r_address;      /* address of datum to be relocated */
        !           201:        unsigned        r_symbolnum:24, /* local symbol ordinal */
        !           202:                r_pcrel:1,      /* is referenced relative to pc */
        !           203:                r_length:2,     /* 0=byte, 1=word, 2=long */
        !           204:                r_extern:1,     /* symbol value unknown */
        !           205:                :4;     /* nothing, yet */
        !           206: };
        !           207: .EE
        !           208: .DT
        !           209: .PP
        !           210: If
        !           211: .B r_extern 
        !           212: is 1, the datum designated by
        !           213: .B r_address 
        !           214: and
        !           215: .B r_length
        !           216: will be relocated by adding to it the value of the associated
        !           217: external symbol.
        !           218: If
        !           219: .B r_extern
        !           220: is 0, 
        !           221: .BR r_symbolnum
        !           222: is encoded in the style of
        !           223: .B n_type
        !           224: and the value will be relocated by adding the relocated base of
        !           225: the designated area (text, initialized data, or common data).
        !           226: .SH "SEE ALSO"
        !           227: .IR adb (1), 
        !           228: .IR as (1), 
        !           229: .IR ld (1), 
        !           230: .IR nm (1), 
        !           231: .IR stab (5), 
        !           232: .IR strip (1)

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