FreeBSD/Linux Kernel Cross Reference
sys/EXTERNAL_HEADERS/mach-o/reloc.h

     /*
      * Copyright (c) 1999 Apple Computer, Inc. All rights reserved.
      *
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      * Copyright (c) 1999-2003 Apple Computer, Inc.  All Rights Reserved.
      * 
      * This file contains Original Code and/or Modifications of Original Code
      * as defined in and that are subject to the Apple Public Source License
     * Version 2.0 (the 'License'). You may not use this file except in
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     * http://www.opensource.apple.com/apsl/ and read it before using this
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    /*      $NetBSD: exec.h,v 1.6 1994/10/27 04:16:05 cgd Exp $     */
    
    /*
     * Copyright (c) 1993 Christopher G. Demetriou
     * All rights reserved.
     *
     * Redistribution and use in source and binary forms, with or without
     * modification, are permitted provided that the following conditions
     * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     * 3. The name of the author may not be used to endorse or promote products
     *    derived from this software without specific prior written permission
     *
     * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
     * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
     * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
     * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
     * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
     * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
     * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
     * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
     * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
     * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     */
    
    #ifndef _MACHO_RELOC_H_
    #define _MACHO_RELOC_H_
    
    /*
     * Format of a relocation entry of a Mach-O file.  Modified from the 4.3BSD
     * format.  The modifications from the original format were changing the value
     * of the r_symbolnum field for "local" (r_extern == 0) relocation entries.
     * This modification is required to support symbols in an arbitrary number of
     * sections not just the three sections (text, data and bss) in a 4.3BSD file.
     * Also the last 4 bits have had the r_type tag added to them.
     */
    struct relocation_info {
       long         r_address;      /* offset in the section to what is being
                                       relocated */
       unsigned int r_symbolnum:24, /* symbol index if r_extern == 1 or section
                                       ordinal if r_extern == 0 */
                    r_pcrel:1,      /* was relocated pc relative already */
                    r_length:2,     /* 0=byte, 1=word, 2=long */
                    r_extern:1,     /* does not include value of sym referenced */
                    r_type:4;       /* if not 0, machine specific relocation type */
    };
    #define R_ABS   0               /* absolute relocation type for Mach-O files */
    
    /*
     * The r_address is not really the address as it's name indicates but an offset.
     * In 4.3BSD a.out objects this offset is from the start of the "segment" for
     * which relocation entry is for (text or data).  For Mach-O object files it is
     * also an offset but from the start of the "section" for which the relocation
     * entry is for.  See comments in <mach-o/loader.h> about the r_address feild
     * in images for used with the dynamic linker.
     * 
     * In 4.3BSD a.out objects if r_extern is zero then r_symbolnum is an ordinal
     * for the segment the symbol being relocated is in.  These ordinals are the
     * symbol types N_TEXT, N_DATA, N_BSS or N_ABS.  In Mach-O object files these
     * ordinals refer to the sections in the object file in the order their section
     * structures appear in the headers of the object file they are in.  The first
     * section has the ordinal 1, the second 2, and so on.  This means that the
     * same ordinal in two different object files could refer to two different
     * sections.  And further could have still different ordinals when combined
     * by the link-editor.  The value R_ABS is used for relocation entries for
     * absolute symbols which need no further relocation.
     */
    
    /*
     * For RISC machines some of the references are split across two instructions
     * and the instruction does not contain the complete value of the reference.
    * In these cases a second, or paired relocation entry, follows each of these
    * relocation entries, using a PAIR r_type, which contains the other part of the
    * reference not contained in the instruction.  This other part is stored in the
    * pair's r_address field.  The exact number of bits of the other part of the
    * reference store in the r_address field is dependent on the particular
    * relocation type for the particular architecture.
    */
   
   /*
    * To make scattered loading by the link editor work correctly "local"
    * relocation entries can't be used when the item to be relocated is the value
    * of a symbol plus an offset (where the resulting expresion is outside the
    * block the link editor is moving, a blocks are divided at symbol addresses).
    * In this case. where the item is a symbol value plus offset, the link editor
    * needs to know more than just the section the symbol was defined.  What is
    * needed is the actual value of the symbol without the offset so it can do the
    * relocation correctly based on where the value of the symbol got relocated to
    * not the value of the expression (with the offset added to the symbol value).
    * So for the NeXT 2.0 release no "local" relocation entries are ever used when
    * there is a non-zero offset added to a symbol.  The "external" and "local"
    * relocation entries remain unchanged.
    *
    * The implemention is quite messy given the compatibility with the existing
    * relocation entry format.  The ASSUMPTION is that a section will never be
    * bigger than 2**24 - 1 (0x00ffffff or 16,777,215) bytes.  This assumption
    * allows the r_address (which is really an offset) to fit in 24 bits and high
    * bit of the r_address field in the relocation_info structure to indicate
    * it is really a scattered_relocation_info structure.  Since these are only
    * used in places where "local" relocation entries are used and not where
    * "external" relocation entries are used the r_extern field has been removed.
    *
    * For scattered loading to work on a RISC machine where some of the references
    * are split across two instructions the link editor needs to be assured that
    * each reference has a unique 32 bit reference (that more than one reference is
    * NOT sharing the same high 16 bits for example) so it move each referenced
    * item independent of each other.  Some compilers guarantees this but the
    * compilers don't so scattered loading can be done on those that do guarantee
    * this.
    */
   #if defined(__BIG_ENDIAN__) || defined(__LITTLE_ENDIAN__)
   /*
    * The reason for the ifdef's of __BIG_ENDIAN__ and __LITTLE_ENDIAN__ are that
    * when stattered relocation entries were added the mistake of using a mask
    * against a structure that is made up of bit fields was used.  To make this
    * design work this structure must be laid out in memory the same way so the
    * mask can be applied can check the same bit each time (r_scattered).
    */
   #endif /* defined(__BIG_ENDIAN__) || defined(__LITTLE_ENDIAN__) */
   #define R_SCATTERED 0x80000000  /* mask to be applied to the r_address field 
                                      of a relocation_info structure to tell that
                                      is is really a scattered_relocation_info
                                      stucture */
   struct scattered_relocation_info {
   #ifdef __BIG_ENDIAN__
      unsigned int r_scattered:1,  /* 1=scattered, 0=non-scattered (see above) */
                   r_pcrel:1,      /* was relocated pc relative already */
                   r_length:2,     /* 0=byte, 1=word, 2=long */
                   r_type:4,       /* if not 0, machine specific relocation type */
                   r_address:24;   /* offset in the section to what is being
                                      relocated */
      long         r_value;        /* the value the item to be relocated is
                                      refering to (without any offset added) */
   #endif /* __BIG_ENDIAN__ */
   #ifdef __LITTLE_ENDIAN__
      unsigned int
                   r_address:24,   /* offset in the section to what is being
                                      relocated */
                   r_type:4,       /* if not 0, machine specific relocation type */
                   r_length:2,     /* 0=byte, 1=word, 2=long */
                   r_pcrel:1,      /* was relocated pc relative already */
                   r_scattered:1;  /* 1=scattered, 0=non-scattered (see above) */
      long         r_value;        /* the value the item to be relocated is
                                      refering to (without any offset added) */
   #endif /* __LITTLE_ENDIAN__ */
   };
   
   /*
    * Relocation types used in a generic implementation.  Relocation entries for
    * nornal things use the generic relocation as discribed above and their r_type
    * is GENERIC_RELOC_VANILLA (a value of zero).
    *
    * Another type of generic relocation, GENERIC_RELOC_SECTDIFF, is to support
    * the difference of two symbols defined in different sections.  That is the
    * expression "symbol1 - symbol2 + constant" is a relocatable expression when
    * both symbols are defined in some section.  For this type of relocation the
    * both relocations entries are scattered relocation entries.  The value of
    * symbol1 is stored in the first relocation entry's r_value field and the
    * value of symbol2 is stored in the pair's r_value field.
    *
    * A special case for a prebound lazy pointer is needed to beable to set the
    * value of the lazy pointer back to its non-prebound state.  This is done
    * using the GENERIC_RELOC_PB_LA_PTR r_type.  This is a scattered relocation
    * entry where the r_value feild is the value of the lazy pointer not prebound.
    */
   enum reloc_type_generic
   {
       GENERIC_RELOC_VANILLA,      /* generic relocation as discribed above */
       GENERIC_RELOC_PAIR,         /* Only follows a GENRIC_RELOC_SECTDIFF */
       GENERIC_RELOC_SECTDIFF,
       GENERIC_RELOC_PB_LA_PTR     /* prebound lazy pointer */
   };
   
   #endif /* _MACHO_RELOC_H_ */

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