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

     /*
      * Copyright (c) 2000 Apple Computer, Inc. All rights reserved.
      *
      * @APPLE_LICENSE_HEADER_START@
      * 
      * 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
     * compliance with the License. Please obtain a copy of the License at
     * http://www.opensource.apple.com/apsl/ and read it before using this
     * file.
     * 
     * The Original Code and all software distributed under the License are
     * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
     * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
     * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
     * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
     * Please see the License for the specific language governing rights and
     * limitations under the License.
     * 
     * @APPLE_LICENSE_HEADER_END@
     */
    #ifndef _MACHO_LOADER_H_
    #define _MACHO_LOADER_H_
    
    /*
     * This file describes the format of mach object files.
     */
    
    /*
     * <mach/machine.h> is needed here for the cpu_type_t and cpu_subtype_t types
     * and contains the constants for the possible values of these types.
     */
    #include <mach/machine.h>
    
    /*
     * <mach/vm_prot.h> is needed here for the vm_prot_t type and contains the 
     * constants that are or'ed together for the possible values of this type.
     */
    #include <mach/vm_prot.h>
    
    /*
     * <machine/thread_status.h> is expected to define the flavors of the thread
     * states and the structures of those flavors for each machine.
     */
    #include <mach/machine/thread_status.h>
    #include <architecture/byte_order.h>
    
    /*
     * The mach header appears at the very beginning of the object file.
     */
    struct mach_header {
            unsigned long   magic;          /* mach magic number identifier */
            cpu_type_t      cputype;        /* cpu specifier */
            cpu_subtype_t   cpusubtype;     /* machine specifier */
            unsigned long   filetype;       /* type of file */
            unsigned long   ncmds;          /* number of load commands */
            unsigned long   sizeofcmds;     /* the size of all the load commands */
            unsigned long   flags;          /* flags */
    };
    
    /* Constant for the magic field of the mach_header */
    #define MH_MAGIC        0xfeedface      /* the mach magic number */
    #define MH_CIGAM        NXSwapInt(MH_MAGIC)
    
    /*
     * The layout of the file depends on the filetype.  For all but the MH_OBJECT
     * file type the segments are padded out and aligned on a segment alignment
     * boundary for efficient demand pageing.  The MH_EXECUTE, MH_FVMLIB, MH_DYLIB,
     * MH_DYLINKER and MH_BUNDLE file types also have the headers included as part
     * of their first segment.
     * 
     * The file type MH_OBJECT is a compact format intended as output of the
     * assembler and input (and possibly output) of the link editor (the .o
     * format).  All sections are in one unnamed segment with no segment padding. 
     * This format is used as an executable format when the file is so small the
     * segment padding greatly increases it's size.
     *
     * The file type MH_PRELOAD is an executable format intended for things that
     * not executed under the kernel (proms, stand alones, kernels, etc).  The
     * format can be executed under the kernel but may demand paged it and not
     * preload it before execution.
     *
     * A core file is in MH_CORE format and can be any in an arbritray legal
     * Mach-O file.
     *
     * Constants for the filetype field of the mach_header
     */
    #define MH_OBJECT       0x1             /* relocatable object file */
    #define MH_EXECUTE      0x2             /* demand paged executable file */
    #define MH_FVMLIB       0x3             /* fixed VM shared library file */
    #define MH_CORE         0x4             /* core file */
    #define MH_PRELOAD      0x5             /* preloaded executable file */
    #define MH_DYLIB        0x6             /* dynamicly bound shared library file*/
    #define MH_DYLINKER     0x7             /* dynamic link editor */
    #define MH_BUNDLE       0x8             /* dynamicly bound bundle file */
    
   /* Constants for the flags field of the mach_header */
   #define MH_NOUNDEFS     0x1             /* the object file has no undefined
                                              references, can be executed */
   #define MH_INCRLINK     0x2             /* the object file is the output of an
                                              incremental link against a base file
                                              and can't be link edited again */
   #define MH_DYLDLINK     0x4             /* the object file is input for the
                                              dynamic linker and can't be staticly
                                              link edited again */
   #define MH_BINDATLOAD   0x8             /* the object file's undefined
                                              references are bound by the dynamic
                                              linker when loaded. */
   #define MH_PREBOUND     0x10            /* the file has it's dynamic undefined
                                              references prebound. */
   
   /*
    * The load commands directly follow the mach_header.  The total size of all
    * of the commands is given by the sizeofcmds field in the mach_header.  All
    * load commands must have as their first two fields cmd and cmdsize.  The cmd
    * field is filled in with a constant for that command type.  Each command type
    * has a structure specifically for it.  The cmdsize field is the size in bytes
    * of the particular load command structure plus anything that follows it that
    * is a part of the load command (i.e. section structures, strings, etc.).  To
    * advance to the next load command the cmdsize can be added to the offset or
    * pointer of the current load command.  The cmdsize MUST be a multiple of
    * sizeof(long) (this is forever the maximum alignment of any load commands).
    * The padded bytes must be zero.  All tables in the object file must also
    * follow these rules so the file can be memory mapped.  Otherwise the pointers
    * to these tables will not work well or at all on some machines.  With all
    * padding zeroed like objects will compare byte for byte.
    */
   struct load_command {
           unsigned long cmd;              /* type of load command */
           unsigned long cmdsize;          /* total size of command in bytes */
   };
   
   /* Constants for the cmd field of all load commands, the type */
   #define LC_SEGMENT      0x1     /* segment of this file to be mapped */
   #define LC_SYMTAB       0x2     /* link-edit stab symbol table info */
   #define LC_SYMSEG       0x3     /* link-edit gdb symbol table info (obsolete) */
   #define LC_THREAD       0x4     /* thread */
   #define LC_UNIXTHREAD   0x5     /* unix thread (includes a stack) */
   #define LC_LOADFVMLIB   0x6     /* load a specified fixed VM shared library */
   #define LC_IDFVMLIB     0x7     /* fixed VM shared library identification */
   #define LC_IDENT        0x8     /* object identification info (obsolete) */
   #define LC_FVMFILE      0x9     /* fixed VM file inclusion (internal use) */
   #define LC_PREPAGE      0xa     /* prepage command (internal use) */
   #define LC_DYSYMTAB     0xb     /* dynamic link-edit symbol table info */
   #define LC_LOAD_DYLIB   0xc     /* load a dynamicly linked shared library */
   #define LC_ID_DYLIB     0xd     /* dynamicly linked shared lib identification */
   #define LC_LOAD_DYLINKER 0xe    /* load a dynamic linker */
   #define LC_ID_DYLINKER  0xf     /* dynamic linker identification */
   #define LC_PREBOUND_DYLIB 0x10  /* modules prebound for a dynamicly */
                                   /*  linked shared library */
   
   /*
    * A variable length string in a load command is represented by an lc_str
    * union.  The strings are stored just after the load command structure and
    * the offset is from the start of the load command structure.  The size
    * of the string is reflected in the cmdsize field of the load command.
    * Once again any padded bytes to bring the cmdsize field to a multiple
    * of sizeof(long) must be zero.
    */
   union lc_str {
           unsigned long   offset; /* offset to the string */
           char            *ptr;   /* pointer to the string */
   };
   
   /*
    * The segment load command indicates that a part of this file is to be
    * mapped into the task's address space.  The size of this segment in memory,
    * vmsize, maybe equal to or larger than the amount to map from this file,
    * filesize.  The file is mapped starting at fileoff to the beginning of
    * the segment in memory, vmaddr.  The rest of the memory of the segment,
    * if any, is allocated zero fill on demand.  The segment's maximum virtual
    * memory protection and initial virtual memory protection are specified
    * by the maxprot and initprot fields.  If the segment has sections then the
    * section structures directly follow the segment command and their size is
    * reflected in cmdsize.
    */
   struct segment_command {
           unsigned long   cmd;            /* LC_SEGMENT */
           unsigned long   cmdsize;        /* includes sizeof section structs */
           char            segname[16];    /* segment name */
           unsigned long   vmaddr;         /* memory address of this segment */
           unsigned long   vmsize;         /* memory size of this segment */
           unsigned long   fileoff;        /* file offset of this segment */
           unsigned long   filesize;       /* amount to map from the file */
           vm_prot_t       maxprot;        /* maximum VM protection */
           vm_prot_t       initprot;       /* initial VM protection */
           unsigned long   nsects;         /* number of sections in segment */
           unsigned long   flags;          /* flags */
   };
   
   /* Constants for the flags field of the segment_command */
   #define SG_HIGHVM       0x1     /* the file contents for this segment is for
                                      the high part of the VM space, the low part
                                      is zero filled (for stacks in core files) */
   #define SG_FVMLIB       0x2     /* this segment is the VM that is allocated by
                                      a fixed VM library, for overlap checking in
                                      the link editor */
   #define SG_NORELOC      0x4     /* this segment has nothing that was relocated
                                      in it and nothing relocated to it, that is
                                      it maybe safely replaced without relocation*/
   
   /*
    * A segment is made up of zero or more sections.  Non-MH_OBJECT files have
    * all of their segments with the proper sections in each, and padded to the
    * specified segment alignment when produced by the link editor.  The first
    * segment of a MH_EXECUTE and MH_FVMLIB format file contains the mach_header
    * and load commands of the object file before it's first section.  The zero
    * fill sections are always last in their segment (in all formats).  This
    * allows the zeroed segment padding to be mapped into memory where zero fill
    * sections might be.
    *
    * The MH_OBJECT format has all of it's sections in one segment for
    * compactness.  There is no padding to a specified segment boundary and the
    * mach_header and load commands are not part of the segment.
    *
    * Sections with the same section name, sectname, going into the same segment,
    * segname, are combined by the link editor.  The resulting section is aligned
    * to the maximum alignment of the combined sections and is the new section's
    * alignment.  The combined sections are aligned to their original alignment in
    * the combined section.  Any padded bytes to get the specified alignment are
    * zeroed.
    *
    * The format of the relocation entries referenced by the reloff and nreloc
    * fields of the section structure for mach object files is described in the
    * header file <reloc.h>.
    */
   struct section {
           char            sectname[16];   /* name of this section */
           char            segname[16];    /* segment this section goes in */
           unsigned long   addr;           /* memory address of this section */
           unsigned long   size;           /* size in bytes of this section */
           unsigned long   offset;         /* file offset of this section */
           unsigned long   align;          /* section alignment (power of 2) */
           unsigned long   reloff;         /* file offset of relocation entries */
           unsigned long   nreloc;         /* number of relocation entries */
           unsigned long   flags;          /* flags (section type and attributes)*/
           unsigned long   reserved1;      /* reserved */
           unsigned long   reserved2;      /* reserved */
   };
   
   /*
    * The flags field of a section structure is separated into two parts a section
    * type and section attributes.  The section types are mutually exclusive (it
    * can only have one type) but the section attributes are not (it may have more
    * than one attribute).
    */
   #define SECTION_TYPE             0x000000ff     /* 256 section types */
   #define SECTION_ATTRIBUTES       0xffffff00     /*  24 section attributes */
   
   /* Constants for the type of a section */
   #define S_REGULAR               0x0     /* regular section */
   #define S_ZEROFILL              0x1     /* zero fill on demand section */
   #define S_CSTRING_LITERALS      0x2     /* section with only literal C strings*/
   #define S_4BYTE_LITERALS        0x3     /* section with only 4 byte literals */
   #define S_8BYTE_LITERALS        0x4     /* section with only 8 byte literals */
   #define S_LITERAL_POINTERS      0x5     /* section with only pointers to */
                                           /*  literals */
   /*
    * For the two types of symbol pointers sections and the symbol stubs section
    * they have indirect symbol table entries.  For each of the entries in the
    * section the indirect symbol table entries, in corresponding order in the
    * indirect symbol table, start at the index stored in the reserved1 field
    * of the section structure.  Since the indirect symbol table entries
    * correspond to the entries in the section the number of indirect symbol table
    * entries is inferred from the size of the section divided by the size of the
    * entries in the section.  For symbol pointers sections the size of the entries
    * in the section is 4 bytes and for symbol stubs sections the byte size of the
    * stubs is stored in the reserved2 field of the section structure.
    */
   #define S_NON_LAZY_SYMBOL_POINTERS      0x6     /* section with only non-lazy
                                                      symbol pointers */
   #define S_LAZY_SYMBOL_POINTERS          0x7     /* section with only lazy symbol
                                                      pointers */
   #define S_SYMBOL_STUBS                  0x8     /* section with only symbol
                                                      stubs, byte size of stub in
                                                      the reserved2 field */
   #define S_MOD_INIT_FUNC_POINTERS        0x9     /* section with only function
                                                      pointers for initialization*/
   /*
    * Constants for the section attributes part of the flags field of a section
    * structure.
    */
   #define SECTION_ATTRIBUTES_USR   0xff000000     /* User setable attributes */
   #define S_ATTR_PURE_INSTRUCTIONS 0x80000000     /* section contains only true
                                                      machine instructions */
   #define SECTION_ATTRIBUTES_SYS   0x00ffff00     /* system setable attributes */
   #define S_ATTR_SOME_INSTRUCTIONS 0x00000400     /* section contains some
                                                      machine instructions */
   #define S_ATTR_EXT_RELOC         0x00000200     /* section has external
                                                      relocation entries */
   #define S_ATTR_LOC_RELOC         0x00000100     /* section has local
                                                      relocation entries */
   
   
   /*
    * The names of segments and sections in them are mostly meaningless to the
    * link-editor.  But there are few things to support traditional UNIX
    * executables that require the link-editor and assembler to use some names
    * agreed upon by convention.
    *
    * The initial protection of the "__TEXT" segment has write protection turned
    * off (not writeable).
    *
    * The link-editor will allocate common symbols at the end of the "__common"
    * section in the "__DATA" segment.  It will create the section and segment
    * if needed.
    */
   
   /* The currently known segment names and the section names in those segments */
   
   #define SEG_PAGEZERO    "__PAGEZERO"    /* the pagezero segment which has no */
                                           /* protections and catches NULL */
                                           /* references for MH_EXECUTE files */
   
   
   #define SEG_TEXT        "__TEXT"        /* the tradition UNIX text segment */
   #define SECT_TEXT       "__text"        /* the real text part of the text */
                                           /* section no headers, and no padding */
   #define SECT_FVMLIB_INIT0 "__fvmlib_init0"      /* the fvmlib initialization */
                                                   /*  section */
   #define SECT_FVMLIB_INIT1 "__fvmlib_init1"      /* the section following the */
                                                   /*  fvmlib initialization */
                                                   /*  section */
   
   #define SEG_DATA        "__DATA"        /* the tradition UNIX data segment */
   #define SECT_DATA       "__data"        /* the real initialized data section */
                                           /* no padding, no bss overlap */
   #define SECT_BSS        "__bss"         /* the real uninitialized data section*/
                                           /* no padding */
   #define SECT_COMMON     "__common"      /* the section common symbols are */
                                           /* allocated in by the link editor */
   
   #define SEG_OBJC        "__OBJC"        /* objective-C runtime segment */
   #define SECT_OBJC_SYMBOLS "__symbol_table"      /* symbol table */
   #define SECT_OBJC_MODULES "__module_info"       /* module information */
   #define SECT_OBJC_STRINGS "__selector_strs"     /* string table */
   #define SECT_OBJC_REFS "__selector_refs"        /* string table */
   
   #define SEG_ICON         "__ICON"       /* the NeXT icon segment */
   #define SECT_ICON_HEADER "__header"     /* the icon headers */
   #define SECT_ICON_TIFF   "__tiff"       /* the icons in tiff format */
   
   #define SEG_LINKEDIT    "__LINKEDIT"    /* the segment containing all structs */
                                           /* created and maintained by the link */
                                           /* editor.  Created with -seglinkedit */
                                           /* option to ld(1) for MH_EXECUTE and */
                                           /* FVMLIB file types only */
   
   #define SEG_UNIXSTACK   "__UNIXSTACK"   /* the unix stack segment */
   
   /*
    * Fixed virtual memory shared libraries are identified by two things.  The
    * target pathname (the name of the library as found for execution), and the
    * minor version number.  The address of where the headers are loaded is in
    * header_addr.
    */
   struct fvmlib {
           union lc_str    name;           /* library's target pathname */
           unsigned long   minor_version;  /* library's minor version number */
           unsigned long   header_addr;    /* library's header address */
   };
   
   /*
    * A fixed virtual shared library (filetype == MH_FVMLIB in the mach header)
    * contains a fvmlib_command (cmd == LC_IDFVMLIB) to identify the library.
    * An object that uses a fixed virtual shared library also contains a
    * fvmlib_command (cmd == LC_LOADFVMLIB) for each library it uses.
    */
   struct fvmlib_command {
           unsigned long   cmd;            /* LC_IDFVMLIB or LC_LOADFVMLIB */
           unsigned long   cmdsize;        /* includes pathname string */
           struct fvmlib   fvmlib;         /* the library identification */
   };
   
   /*
    * Dynamicly linked shared libraries are identified by two things.  The
    * pathname (the name of the library as found for execution), and the
    * compatibility version number.  The pathname must match and the compatibility
    * number in the user of the library must be greater than or equal to the
    * library being used.  The time stamp is used to record the time a library was
    * built and copied into user so it can be use to determined if the library used
    * at runtime is exactly the same as used to built the program.
    */
   struct dylib {
       union lc_str  name;                 /* library's path name */
       unsigned long timestamp;            /* library's build time stamp */
       unsigned long current_version;      /* library's current version number */
       unsigned long compatibility_version;/* library's compatibility vers number*/
   };
   
   /*
    * A dynamicly linked shared library (filetype == MH_DYLIB in the mach header)
    * contains a dylib_command (cmd == LC_ID_DYLIB) to identify the library.
    * An object that uses a dynamicly linked shared library also contains a
    * dylib_command (cmd == LC_LOAD_DYLIB) for each library it uses.
    */
   struct dylib_command {
           unsigned long   cmd;            /* LC_ID_DYLIB or LC_LOAD_DYLIB */
           unsigned long   cmdsize;        /* includes pathname string */
           struct dylib    dylib;          /* the library identification */
   };
   
   /*
    * A program (filetype == MH_EXECUTE) or bundle (filetype == MH_BUNDLE) that is
    * prebound to it's dynamic libraries has one of these for each library that
    * the static linker used in prebinding.  It contains a bit vector for the
    * modules in the library.  The bits indicate which modules are bound (1) and
    * which are not (0) from the library.  The bit for module 0 is the low bit
    * of the first byte.  So the bit for the Nth module is:
    * (linked_modules[N/8] >> N%8) & 1
    */
   struct prebound_dylib_command {
           unsigned long   cmd;            /* LC_PREBOUND_DYLIB */
           unsigned long   cmdsize;        /* includes strings */
           union lc_str    name;           /* library's path name */
           unsigned long   nmodules;       /* number of modules in library */
           union lc_str    linked_modules; /* bit vector of linked modules */
   };
   
   /*
    * A program that uses a dynamic linker contains a dylinker_command to identify
    * the name of the dynamic linker (LC_LOAD_DYLINKER).  And a dynamic linker
    * contains a dylinker_command to identify the dynamic linker (LC_ID_DYLINKER).
    * A file can have at most one of these.
    */
   struct dylinker_command {
           unsigned long   cmd;            /* LC_ID_DYLINKER or LC_LOAD_DYLINKER */
           unsigned long   cmdsize;        /* includes pathname string */
           union lc_str    name;           /* dynamic linker's path name */
   };
   
   /*
    * Thread commands contain machine-specific data structures suitable for
    * use in the thread state primitives.  The machine specific data structures
    * follow the struct thread_command as follows.
    * Each flavor of machine specific data structure is preceded by an unsigned
    * long constant for the flavor of that data structure, an unsigned long
    * that is the count of longs of the size of the state data structure and then
    * the state data structure follows.  This triple may be repeated for many
    * flavors.  The constants for the flavors, counts and state data structure
    * definitions are expected to be in the header file <machine/thread_status.h>.
    * These machine specific data structures sizes must be multiples of
    * sizeof(long).  The cmdsize reflects the total size of the thread_command
    * and all of the sizes of the constants for the flavors, counts and state
    * data structures.
    *
    * For executable objects that are unix processes there will be one
    * thread_command (cmd == LC_UNIXTHREAD) created for it by the link-editor.
    * This is the same as a LC_THREAD, except that a stack is automatically
    * created (based on the shell's limit for the stack size).  Command arguments
    * and environment variables are copied onto that stack.
    */
   struct thread_command {
           unsigned long   cmd;            /* LC_THREAD or  LC_UNIXTHREAD */
           unsigned long   cmdsize;        /* total size of this command */
           /* unsigned long flavor            flavor of thread state */
           /* unsigned long count             count of longs in thread state */
           /* struct XXX_thread_state state   thread state for this flavor */
           /* ... */
   };
   
   /*
    * The symtab_command contains the offsets and sizes of the link-edit 4.3BSD
    * "stab" style symbol table information as described in the header files
    * <nlist.h> and <stab.h>.
    */
   struct symtab_command {
           unsigned long   cmd;            /* LC_SYMTAB */
           unsigned long   cmdsize;        /* sizeof(struct symtab_command) */
           unsigned long   symoff;         /* symbol table offset */
           unsigned long   nsyms;          /* number of symbol table entries */
           unsigned long   stroff;         /* string table offset */
           unsigned long   strsize;        /* string table size in bytes */
   };
   
   /*
    * This is the second set of the symbolic information which is used to support
    * the data structures for the dynamicly link editor.
    *
    * The original set of symbolic information in the symtab_command which contains
    * the symbol and string tables must also be present when this load command is
    * present.  When this load command is present the symbol table is organized
    * into three groups of symbols:
    *      local symbols (static and debugging symbols) - grouped by module
    *      defined external symbols - grouped by module (sorted by name if not lib)
    *      undefined external symbols (sorted by name)
    * In this load command there are offsets and counts to each of the three groups
    * of symbols.
    *
    * This load command contains a the offsets and sizes of the following new
    * symbolic information tables:
    *      table of contents
    *      module table
    *      reference symbol table
    *      indirect symbol table
    * The first three tables above (the table of contents, module table and
    * reference symbol table) are only present if the file is a dynamicly linked
    * shared library.  For executable and object modules, which are files
    * containing only one module, the information that would be in these three
    * tables is determined as follows:
    *      table of contents - the defined external symbols are sorted by name
    *      module table - the file contains only one module so everything in the
    *                     file is part of the module.
    *      reference symbol table - is the defined and undefined external symbols
    *
    * For dynamicly linked shared library files this load command also contains
    * offsets and sizes to the pool of relocation entries for all sections
    * separated into two groups:
    *      external relocation entries
    *      local relocation entries
    * For executable and object modules the relocation entries continue to hang
    * off the section structures.
    */
   struct dysymtab_command {
       unsigned long cmd;          /* LC_DYSYMTAB */
       unsigned long cmdsize;      /* sizeof(struct dysymtab_command) */
   
       /*
        * The symbols indicated by symoff and nsyms of the LC_SYMTAB load command
        * are grouped into the following three groups:
        *    local symbols (further grouped by the module they are from)
        *    defined external symbols (further grouped by the module they are from)
        *    undefined symbols
        *
        * The local symbols are used only for debugging.  The dynamic binding
        * process may have to use them to indicate to the debugger the local
        * symbols for a module that is being bound.
        *
        * The last two groups are used by the dynamic binding process to do the
        * binding (indirectly through the module table and the reference symbol
        * table when this is a dynamicly linked shared library file).
        */
       unsigned long ilocalsym;    /* index to local symbols */
       unsigned long nlocalsym;    /* number of local symbols */
   
       unsigned long iextdefsym;   /* index to externally defined symbols */
       unsigned long nextdefsym;   /* number of externally defined symbols */
   
       unsigned long iundefsym;    /* index to undefined symbols */
       unsigned long nundefsym;    /* number of undefined symbols */
   
       /*
        * For the for the dynamic binding process to find which module a symbol
        * is defined in the table of contents is used (analogous to the ranlib
        * structure in an archive) which maps defined external symbols to modules
        * they are defined in.  This exists only in a dynamicly linked shared
        * library file.  For executable and object modules the defined external
        * symbols are sorted by name and is use as the table of contents.
        */
       unsigned long tocoff;       /* file offset to table of contents */
       unsigned long ntoc;         /* number of entries in table of contents */
   
       /*
        * To support dynamic binding of "modules" (whole object files) the symbol
        * table must reflect the modules that the file was created from.  This is
        * done by having a module table that has indexes and counts into the merged
        * tables for each module.  The module structure that these two entries
        * refer to is described below.  This exists only in a dynamicly linked
        * shared library file.  For executable and object modules the file only
        * contains one module so everything in the file belongs to the module.
        */
       unsigned long modtaboff;    /* file offset to module table */
       unsigned long nmodtab;      /* number of module table entries */
   
       /*
        * To support dynamic module binding the module structure for each module
        * indicates the external references (defined and undefined) each module
        * makes.  For each module there is an offset and a count into the
        * reference symbol table for the symbols that the module references.
        * This exists only in a dynamicly linked shared library file.  For
        * executable and object modules the defined external symbols and the
        * undefined external symbols indicates the external references.
        */
       unsigned long extrefsymoff;  /* offset to referenced symbol table */
       unsigned long nextrefsyms;   /* number of referenced symbol table entries */
   
       /*
        * The sections that contain "symbol pointers" and "routine stubs" have
        * indexes and (implied counts based on the size of the section and fixed
        * size of the entry) into the "indirect symbol" table for each pointer
        * and stub.  For every section of these two types the index into the
        * indirect symbol table is stored in the section header in the field
        * reserved1.  An indirect symbol table entry is simply a 32bit index into
        * the symbol table to the symbol that the pointer or stub is referring to.
        * The indirect symbol table is ordered to match the entries in the section.
        */
       unsigned long indirectsymoff; /* file offset to the indirect symbol table */
       unsigned long nindirectsyms;  /* number of indirect symbol table entries */
   
       /*
        * To support relocating an individual module in a library file quickly the
        * external relocation entries for each module in the library need to be
        * accessed efficiently.  Since the relocation entries can't be accessed
        * through the section headers for a library file they are separated into
        * groups of local and external entries further grouped by module.  In this
        * case the presents of this load command who's extreloff, nextrel,
        * locreloff and nlocrel fields are non-zero indicates that the relocation
        * entries of non-merged sections are not referenced through the section
        * structures (and the reloff and nreloc fields in the section headers are
        * set to zero).
        *
        * Since the relocation entries are not accessed through the section headers
        * this requires the r_address field to be something other than a section
        * offset to identify the item to be relocated.  In this case r_address is
        * set to the offset from the vmaddr of the first LC_SEGMENT command.
        *
        * The relocation entries are grouped by module and the module table
        * entries have indexes and counts into them for the group of external
        * relocation entries for that the module.
        *
        * For sections that are merged across modules there must not be any
        * remaining external relocation entries for them (for merged sections
        * remaining relocation entries must be local).
        */
       unsigned long extreloff;    /* offset to external relocation entries */
       unsigned long nextrel;      /* number of external relocation entries */
   
       /*
        * All the local relocation entries are grouped together (they are not
        * grouped by their module since they are only used if the object is moved
        * from it staticly link edited address).
        */
       unsigned long locreloff;    /* offset to local relocation entries */
       unsigned long nlocrel;      /* number of local relocation entries */
   
   };      
   
   /*
    * An indirect symbol table entry is simply a 32bit index into the symbol table 
    * to the symbol that the pointer or stub is refering to.  Unless it is for a
    * non-lazy symbol pointer section for a defined symbol which strip(1) as 
    * removed.  In which case it has the value INDIRECT_SYMBOL_LOCAL.  If the
    * symbol was also absolute INDIRECT_SYMBOL_ABS is or'ed with that.
    */
   #define INDIRECT_SYMBOL_LOCAL   0x80000000
   #define INDIRECT_SYMBOL_ABS     0x40000000
   
   
   /* a table of contents entry */
   struct dylib_table_of_contents {
       unsigned long symbol_index; /* the defined external symbol
                                      (index into the symbol table) */
       unsigned long module_index; /* index into the module table this symbol
                                      is defined in */
   };      
   
   /* a module table entry */
   struct dylib_module {
       unsigned long module_name;  /* the module name (index into string table) */
   
       unsigned long iextdefsym;   /* index into externally defined symbols */
       unsigned long nextdefsym;   /* number of externally defined symbols */
       unsigned long irefsym;              /* index into reference symbol table */
       unsigned long nrefsym;      /* number of reference symbol table entries */
       unsigned long ilocalsym;    /* index into symbols for local symbols */
       unsigned long nlocalsym;    /* number of local symbols */
   
       unsigned long iextrel;      /* index into external relocation entries */
       unsigned long nextrel;      /* number of external relocation entries */
   
       unsigned long iinit;        /* index into the init section */
       unsigned long ninit;        /* number of init section entries */
   
       unsigned long               /* for this module address of the start of */
           objc_module_info_addr;  /*  the (__OBJC,__module_info) section */
       unsigned long               /* for this module size of */
           objc_module_info_size;  /*  the (__OBJC,__module_info) section */
   };      
   
   /* 
    * The entries in the reference symbol table are used when loading the module
    * (both by the static and dynamic link editors) and if the module is unloaded
    * or replaced.  Therefore all external symbols (defined and undefined) are
    * listed in the module's reference table.  The flags describe the type of
    * reference that is being made.  The constants for the flags are defined in
    * <mach-o/nlist.h> as they are also used for symbol table entries.
    */
   struct dylib_reference {
       unsigned long isym:24,      /* index into the symbol table */
                     flags:8;      /* flags to indicate the type of reference */
   };
   
   /*
    * The symseg_command contains the offset and size of the GNU style
    * symbol table information as described in the header file <symseg.h>.
    * The symbol roots of the symbol segments must also be aligned properly
    * in the file.  So the requirement of keeping the offsets aligned to a
    * multiple of a sizeof(long) translates to the length field of the symbol
    * roots also being a multiple of a long.  Also the padding must again be
    * zeroed. (THIS IS OBSOLETE and no longer supported).
    */
   struct symseg_command {
           unsigned long   cmd;            /* LC_SYMSEG */
           unsigned long   cmdsize;        /* sizeof(struct symseg_command) */
           unsigned long   offset;         /* symbol segment offset */
           unsigned long   size;           /* symbol segment size in bytes */
   };
   
   /*
    * The ident_command contains a free format string table following the
    * ident_command structure.  The strings are null terminated and the size of
    * the command is padded out with zero bytes to a multiple of sizeof(long).
    * (THIS IS OBSOLETE and no longer supported).
    */
   struct ident_command {
           unsigned long cmd;              /* LC_IDENT */
           unsigned long cmdsize;          /* strings that follow this command */
   };
   
   /*
    * The fvmfile_command contains a reference to a file to be loaded at the
    * specified virtual address.  (Presently, this command is reserved for NeXT
    * internal use.  The kernel ignores this command when loading a program into
    * memory).
    */
   struct fvmfile_command {
           unsigned long cmd;              /* LC_FVMFILE */
           unsigned long cmdsize;          /* includes pathname string */
           union lc_str    name;           /* files pathname */
           unsigned long   header_addr;    /* files virtual address */
   };
   
   #endif /* _MACHO_LOADER_H_ */

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