[apple/xnu.git] / libkern / mach-o / loader.h

/*
 * Copyright (c) 2000 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * The contents of this file constitute Original Code as defined in and
 * are subject to the Apple Public Source License Version 1.1 (the
 * "License").  You may not use this file except in compliance with the
 * License.  Please obtain a copy of the License at
 * http://www.apple.com/publicsource and read it before using this file.
 * 
 * This 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 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>

/*
 * The mach header appears at the very beginning of the object file; it
 * is the same for both 32-bit and 64-bit architectures.
 */
struct mach_header {
        uint32_t        magic;          /* mach magic number identifier */
        cpu_type_t      cputype;        /* cpu specifier */
        cpu_subtype_t   cpusubtype;     /* machine specifier */
        uint32_t        filetype;       /* type of file */
        uint32_t        ncmds;          /* number of load commands */
        uint32_t        sizeofcmds;     /* the size of all the load commands */
        uint32_t        flags;          /* flags */
};

/* Constant for the magic field of the mach_header (32-bit architectures) */
#define MH_MAGIC        0xfeedface      /* the mach magic number */
#define MH_CIGAM        0xcefaedfe      

/* Constant for the magic field of the mach_header_64 (64-bit architectures) */
#define MH_MAGIC_64     0xfeedfacf      /* the 64-bit mach magic number */
#define MH_CIGAM_64     0xcffaedfe      

/* Constants for the cmd field of new load commands, the type */
#define LC_SEGMENT_64   0x19    /* 64-bit segment of this file to be mapped */
#define LC_ROUTINES_64  0x1a    /* 64-bit image routines */


/*
 * 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 for 32-bit architectures
 * MUST be a multiple of 4 bytes and for 64-bit architectures MUST be a multiple
 * of 8 bytes (these are forever the maximum alignment of any load commands).
 * 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 {        /* for 32-bit architectures */
        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 */
};

/*
 * The 64-bit segment load command indicates that a part of this file is to be
 * mapped into a 64-bit task's address space.  If the 64-bit segment has
 * sections then section_64 structures directly follow the 64-bit segment
 * command and their size is reflected in cmdsize.
 */
struct segment_command_64 {     /* for 64-bit architectures */
        uint32_t        cmd;            /* LC_SEGMENT_64 */
        uint32_t        cmdsize;        /* includes sizeof section_64 structs */
        char            segname[16];    /* segment name */
        uint64_t        vmaddr;         /* memory address of this segment */
        uint64_t        vmsize;         /* memory size of this segment */
        uint32_t        fileoff;        /* file offset of this segment */
        uint32_t        filesize;       /* amount to map from the file */
        vm_prot_t       maxprot;        /* maximum VM protection */
        vm_prot_t       initprot;       /* initial VM protection */
        uint32_t        nsects;         /* number of sections in segment */
        uint32_t        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 gigabyte zero fill sections, those with the section
 * type S_GB_ZEROFILL, can only be in a segment with sections of this type.
 * These segments are then placed after all other segments.
 *
 * 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 {                /* for 32-bit architectures */
        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 */
};

struct section_64 { /* for 64-bit architectures */
        char            sectname[16];   /* name of this section */
        char            segname[16];    /* segment this section goes in */
        uint64_t        addr;           /* memory address of this section */
        uint64_t        size;           /* size in bytes of this section */
        uint32_t        offset;         /* file offset of this section */
        uint32_t        align;          /* section alignment (power of 2) */
        uint32_t        reloff;         /* file offset of relocation entries */
        uint32_t        nreloc;         /* number of relocation entries */
        uint32_t        flags;          /* flags (section type and attributes)*/
        uint32_t        reserved1;      /* reserved (for offset or index) */
        uint32_t        reserved2;      /* reserved (for count or sizeof) */
        uint32_t        reserved3;      /* 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 */
};      

/* a 64-bit module table entry */
struct dylib_module_64 {
        uint32_t module_name;   /* the module name (index into string table) */

        uint32_t iextdefsym;    /* index into externally defined symbols */
        uint32_t nextdefsym;    /* number of externally defined symbols */
        uint32_t irefsym;       /* index into reference symbol table */
        uint32_t nrefsym;       /* number of reference symbol table entries */
        uint32_t ilocalsym;     /* index into symbols for local symbols */
        uint32_t nlocalsym;     /* number of local symbols */

        uint32_t iextrel;       /* index into external relocation entries */
        uint32_t nextrel;       /* number of external relocation entries */

        uint32_t iinit_iterm;   /* low 16 bits are the index into the init
                                   section, high 16 bits are the index into
                                   the term section */
        uint32_t ninit_nterm;   /* low 16 bits are the number of init section
                                   entries, high 16 bits are the number of
                                   term section entries */

        uint32_t                /* for this module size of the */
                objc_module_info_size;  /* (__OBJC,__module_info) section */
        uint64_t                /* for this module address of the start of */
                objc_module_info_addr;  /* 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_*/