Ticket #31158: loader.h

/*
 * Copyright (c) 1999-2010 Apple Inc.  All Rights Reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * 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.
 */
#include <stdint.h>

/*
 * <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 32-bit mach header appears at the very beginning of the object file for
 * 32-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      /* NXSwapInt(MH_MAGIC) */

/*
 * The 64-bit mach header appears at the very beginning of object files for
 * 64-bit architectures.
 */
struct mach_header_64 {
        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 */
        uint32_t        reserved;       /* reserved */
};

/* 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 /* NXSwapInt(MH_MAGIC_64) */

/*
 * 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 its size.
 *
 * The file type MH_PRELOAD is an executable format intended for things that
 * are 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             /* dynamically bound shared library */
#define MH_DYLINKER     0x7             /* dynamic link editor */
#define MH_BUNDLE       0x8             /* dynamically bound bundle file */
#define MH_DYLIB_STUB   0x9             /* shared library stub for static */
                                        /*  linking only, no section contents */
#define MH_DSYM         0xa             /* companion file with only debug */
                                        /*  sections */
#define MH_KEXT_BUNDLE  0xb             /* x86_64 kexts */

/* Constants for the flags field of the mach_header */
#define MH_NOUNDEFS     0x1             /* the object file has no undefined
                                           references */
#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 its dynamic undefined
                                           references prebound. */
#define MH_SPLIT_SEGS   0x20            /* the file has its read-only and
                                           read-write segments split */
#define MH_LAZY_INIT    0x40            /* the shared library init routine is
                                           to be run lazily via catching memory
                                           faults to its writeable segments
                                           (obsolete) */
#define MH_TWOLEVEL     0x80            /* the image is using two-level name
                                           space bindings */
#define MH_FORCE_FLAT   0x100           /* the executable is forcing all images
                                           to use flat name space bindings */
#define MH_NOMULTIDEFS  0x200           /* this umbrella guarantees no multiple
                                           defintions of symbols in its
                                           sub-images so the two-level namespace
                                           hints can always be used. */
#define MH_NOFIXPREBINDING 0x400        /* do not have dyld notify the
                                           prebinding agent about this
                                           executable */
#define MH_PREBINDABLE  0x800           /* the binary is not prebound but can
                                           have its prebinding redone. only used
                                           when MH_PREBOUND is not set. */
#define MH_ALLMODSBOUND 0x1000          /* indicates that this binary binds to
                                           all two-level namespace modules of
                                           its dependent libraries. only used
                                           when MH_PREBINDABLE and MH_TWOLEVEL
                                           are both set. */ 
#define MH_SUBSECTIONS_VIA_SYMBOLS 0x2000/* safe to divide up the sections into
                                            sub-sections via symbols for dead
                                            code stripping */
#define MH_CANONICAL    0x4000          /* the binary has been canonicalized
                                           via the unprebind operation */
#define MH_WEAK_DEFINES 0x8000          /* the final linked image contains
                                           external weak symbols */
#define MH_BINDS_TO_WEAK 0x10000        /* the final linked image uses
                                           weak symbols */

#define MH_ALLOW_STACK_EXECUTION 0x20000/* When this bit is set, all stacks 
                                           in the task will be given stack
                                           execution privilege.  Only used in
                                           MH_EXECUTE filetypes. */
#define MH_ROOT_SAFE 0x40000           /* When this bit is set, the binary 
                                          declares it is safe for use in
                                          processes with uid zero */
                                         
#define MH_SETUID_SAFE 0x80000         /* When this bit is set, the binary 
                                          declares it is safe for use in
                                          processes when issetugid() is true */

#define MH_NO_REEXPORTED_DYLIBS 0x100000 /* When this bit is set on a dylib, 
                                          the static linker does not need to
                                          examine dependent dylibs to see
                                          if any are re-exported */
#define MH_PIE 0x200000                 /* When this bit is set, the OS will
                                           load the main executable at a
                                           random address.  Only used in
                                           MH_EXECUTE filetypes. */
#define MH_DEAD_STRIPPABLE_DYLIB 0x400000 /* Only for use on dylibs.  When
                                             linking against a dylib that
                                             has this bit set, the static linker
                                             will automatically not create a
                                             LC_LOAD_DYLIB load command to the
                                             dylib if no symbols are being
                                             referenced from the dylib. */
#define MH_HAS_TLV_DESCRIPTORS 0x800000 /* Contains a section of type 
                                            S_THREAD_LOCAL_VARIABLES */

#define MH_NO_HEAP_EXECUTION 0x1000000  /* When this bit is set, the OS will
                                           run the main executable with
                                           a non-executable heap even on
                                           platforms (e.g. i386) that don't
                                           require it. Only used in MH_EXECUTE
                                           filetypes. */

/*
 * 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).
 * 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 {
        uint32_t cmd;           /* type of load command */
        uint32_t cmdsize;       /* total size of command in bytes */
};

/*
 * After MacOS X 10.1 when a new load command is added that is required to be
 * understood by the dynamic linker for the image to execute properly the
 * LC_REQ_DYLD bit will be or'ed into the load command constant.  If the dynamic
 * linker sees such a load command it it does not understand will issue a
 * "unknown load command required for execution" error and refuse to use the
 * image.  Other load commands without this bit that are not understood will
 * simply be ignored.
 */
#define LC_REQ_DYLD 0x80000000

/* 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 dynamically linked shared library */
#define LC_ID_DYLIB     0xd     /* dynamically linked shared lib ident */
#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 dynamically */
                                /*  linked shared library */
#define LC_ROUTINES     0x11    /* image routines */
#define LC_SUB_FRAMEWORK 0x12   /* sub framework */
#define LC_SUB_UMBRELLA 0x13    /* sub umbrella */
#define LC_SUB_CLIENT   0x14    /* sub client */
#define LC_SUB_LIBRARY  0x15    /* sub library */
#define LC_TWOLEVEL_HINTS 0x16  /* two-level namespace lookup hints */
#define LC_PREBIND_CKSUM  0x17  /* prebind checksum */

/*
 * load a dynamically linked shared library that is allowed to be missing
 * (all symbols are weak imported).
 */
#define LC_LOAD_WEAK_DYLIB (0x18 | LC_REQ_DYLD)

#define LC_SEGMENT_64   0x19    /* 64-bit segment of this file to be
                                   mapped */
#define LC_ROUTINES_64  0x1a    /* 64-bit image routines */
#define LC_UUID         0x1b    /* the uuid */
#define LC_RPATH       (0x1c | LC_REQ_DYLD)    /* runpath additions */
#define LC_CODE_SIGNATURE 0x1d  /* local of code signature */
#define LC_SEGMENT_SPLIT_INFO 0x1e /* local of info to split segments */
#define LC_REEXPORT_DYLIB (0x1f | LC_REQ_DYLD) /* load and re-export dylib */
#define LC_LAZY_LOAD_DYLIB 0x20 /* delay load of dylib until first use */
#define LC_ENCRYPTION_INFO 0x21 /* encrypted segment information */
#define LC_DYLD_INFO    0x22    /* compressed dyld information */
#define LC_DYLD_INFO_ONLY (0x22|LC_REQ_DYLD)    /* compressed dyld information only */
#define LC_LOAD_UPWARD_DYLIB (0x23 | LC_REQ_DYLD) /* load upward dylib */
#define LC_VERSION_MIN_MACOSX 0x24   /* build for MacOSX min OS version */
#define LC_VERSION_MIN_IPHONEOS 0x25 /* build for iPhoneOS min OS version */
#define LC_FUNCTION_STARTS 0x26 /* compressed table of function start addresses */

/*
 * 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 4 bytes must be zero.
 */
union lc_str {
        uint32_t        offset; /* offset to the string */
#ifndef __LP64__
        char            *ptr;   /* pointer to the string */
#endif 
};

/*
 * 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 */
        uint32_t        cmd;            /* LC_SEGMENT */
        uint32_t        cmdsize;        /* includes sizeof section structs */
        char            segname[16];    /* segment name */
        uint32_t        vmaddr;         /* memory address of this segment */
        uint32_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 */
};

/*
 * 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 */
        uint64_t        fileoff;        /* file offset of this segment */
        uint64_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*/
#define SG_PROTECTED_VERSION_1  0x8 /* This segment is protected.  If the
                                       segment starts at file offset 0, the
                                       first page of the segment is not
                                       protected.  All other pages of the
                                       segment are protected. */

/*
 * 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 its 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 its 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 */
        uint32_t        addr;           /* memory address of this section */
        uint32_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) */
};

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*/
#define S_MOD_TERM_FUNC_POINTERS        0xa     /* section with only function
                                                   pointers for termination */
#define S_COALESCED                     0xb     /* section contains symbols that
                                                   are to be coalesced */
#define S_GB_ZEROFILL                   0xc     /* zero fill on demand section
                                                   (that can be larger than 4
                                                   gigabytes) */
#define S_INTERPOSING                   0xd     /* section with only pairs of
                                                   function pointers for
                                                   interposing */
#define S_16BYTE_LITERALS               0xe     /* section with only 16 byte
                                                   literals */
#define S_DTRACE_DOF                    0xf     /* section contains 
                                                   DTrace Object Format */
#define S_LAZY_DYLIB_SYMBOL_POINTERS    0x10    /* section with only lazy
                                                   symbol pointers to lazy
                                                   loaded dylibs */
/*
 * Section types to support thread local variables
 */
#define S_THREAD_LOCAL_REGULAR                   0x11  /* template of initial 
                                                          values for TLVs */
#define S_THREAD_LOCAL_ZEROFILL                  0x12  /* template of initial 
                                                          values for TLVs */
#define S_THREAD_LOCAL_VARIABLES                 0x13  /* TLV descriptors */
#define S_THREAD_LOCAL_VARIABLE_POINTERS         0x14  /* pointers to TLV 
                                                          descriptors */
#define S_THREAD_LOCAL_INIT_FUNCTION_POINTERS    0x15  /* functions to call
                                                          to initialize TLV
                                                          values */

/*
 * 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 S_ATTR_NO_TOC            0x40000000     /* section contains coalesced
                                                   symbols that are not to be
                                                   in a ranlib table of
                                                   contents */
#define S_ATTR_STRIP_STATIC_SYMS 0x20000000     /* ok to strip static symbols
                                                   in this section in files
                                                   with the MH_DYLDLINK flag */
#define S_ATTR_NO_DEAD_STRIP     0x10000000     /* no dead stripping */
#define S_ATTR_LIVE_SUPPORT      0x08000000     /* blocks are live if they
                                                   reference live blocks */
#define S_ATTR_SELF_MODIFYING_CODE 0x04000000   /* Used with i386 code stubs
                                                   written on by dyld */
/*
 * If a segment contains any sections marked with S_ATTR_DEBUG then all
 * sections in that segment must have this attribute.  No section other than
 * a section marked with this attribute may reference the contents of this
 * section.  A section with this attribute may contain no symbols and must have
 * a section type S_REGULAR.  The static linker will not copy section contents
 * from sections with this attribute into its output file.  These sections
 * generally contain DWARF debugging info.
 */ 
#define S_ATTR_DEBUG             0x02000000     /* a debug section */
#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 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 */

#define SEG_IMPORT      "__IMPORT"      /* the segment for the self (dyld) */
                                        /* modifing code stubs that has read, */
                                        /* write and execute permissions */

/*
 * 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. (THIS IS OBSOLETE and no longer supported).
 */
struct fvmlib {
        union lc_str    name;           /* library's target pathname */
        uint32_t        minor_version;  /* library's minor version number */
        uint32_t        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.
 * (THIS IS OBSOLETE and no longer supported).
 */
struct fvmlib_command {
        uint32_t        cmd;            /* LC_IDFVMLIB or LC_LOADFVMLIB */
        uint32_t        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 */
    uint32_t timestamp;                 /* library's build time stamp */
    uint32_t current_version;           /* library's current version number */
    uint32_t compatibility_version;     /* library's compatibility vers number*/
};

/*
 * A dynamically 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 dynamically linked shared library also contains a
 * dylib_command (cmd == LC_LOAD_DYLIB, LC_LOAD_WEAK_DYLIB, or
 * LC_REEXPORT_DYLIB) for each library it uses.
 */
struct dylib_command {
        uint32_t        cmd;            /* LC_ID_DYLIB, LC_LOAD_{,WEAK_}DYLIB,
                                           LC_REEXPORT_DYLIB */
        uint32_t        cmdsize;        /* includes pathname string */
        struct dylib    dylib;          /* the library identification */
};

/*
 * A dynamically linked shared library may be a subframework of an umbrella
 * framework.  If so it will be linked with "-umbrella umbrella_name" where
 * Where "umbrella_name" is the name of the umbrella framework. A subframework
 * can only be linked against by its umbrella framework or other subframeworks
 * that are part of the same umbrella framework.  Otherwise the static link
 * editor produces an error and states to link against the umbrella framework.
 * The name of the umbrella framework for subframeworks is recorded in the
 * following structure.
 */
struct sub_framework_command {
        uint32_t        cmd;            /* LC_SUB_FRAMEWORK */
        uint32_t        cmdsize;        /* includes umbrella string */
        union lc_str    umbrella;       /* the umbrella framework name */
};

/*
 * For dynamically linked shared libraries that are subframework of an umbrella
 * framework they can allow clients other than the umbrella framework or other
 * subframeworks in the same umbrella framework.  To do this the subframework
 * is built with "-allowable_client client_name" and an LC_SUB_CLIENT load
 * command is created for each -allowable_client flag.  The client_name is
 * usually a framework name.  It can also be a name used for bundles clients
 * where the bundle is built with "-client_name client_name".
 */
struct sub_client_command {
        uint32_t        cmd;            /* LC_SUB_CLIENT */
        uint32_t        cmdsize;        /* includes client string */
        union lc_str    client;         /* the client name */
};

/*
 * A dynamically linked shared library may be a sub_umbrella of an umbrella
 * framework.  If so it will be linked with "-sub_umbrella umbrella_name" where
 * Where "umbrella_name" is the name of the sub_umbrella framework.  When
 * staticly linking when -twolevel_namespace is in effect a twolevel namespace 
 * umbrella framework will only cause its subframeworks and those frameworks
 * listed as sub_umbrella frameworks to be implicited linked in.  Any other
 * dependent dynamic libraries will not be linked it when -twolevel_namespace
 * is in effect.  The primary library recorded by the static linker when
 * resolving a symbol in these libraries will be the umbrella framework.
 * Zero or more sub_umbrella frameworks may be use by an umbrella framework.
 * The name of a sub_umbrella framework is recorded in the following structure.
 */
struct sub_umbrella_command {
        uint32_t        cmd;            /* LC_SUB_UMBRELLA */
        uint32_t        cmdsize;        /* includes sub_umbrella string */
        union lc_str    sub_umbrella;   /* the sub_umbrella framework name */
};

/*
 * A dynamically linked shared library may be a sub_library of another shared
 * library.  If so it will be linked with "-sub_library library_name" where
 * Where "library_name" is the name of the sub_library shared library.  When
 * staticly linking when -twolevel_namespace is in effect a twolevel namespace 
 * shared library will only cause its subframeworks and those frameworks
 * listed as sub_umbrella frameworks and libraries listed as sub_libraries to
 * be implicited linked in.  Any other dependent dynamic libraries will not be
 * linked it when -twolevel_namespace is in effect.  The primary library
 * recorded by the static linker when resolving a symbol in these libraries
 * will be the umbrella framework (or dynamic library). Zero or more sub_library
 * shared libraries may be use by an umbrella framework or (or dynamic library).
 * The name of a sub_library framework is recorded in the following structure.
 * For example /usr/lib/libobjc_profile.A.dylib would be recorded as "libobjc".
 */
struct sub_library_command {
        uint32_t        cmd;            /* LC_SUB_LIBRARY */
        uint32_t        cmdsize;        /* includes sub_library string */
        union lc_str    sub_library;    /* the sub_library name */
};

/*
 * A program (filetype == MH_EXECUTE) that is
 * prebound to its 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 {
        uint32_t        cmd;            /* LC_PREBOUND_DYLIB */
        uint32_t        cmdsize;        /* includes strings */
        union lc_str    name;           /* library's path name */
        uint32_t        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 {
        uint32_t        cmd;            /* LC_ID_DYLINKER or LC_LOAD_DYLINKER */
        uint32_t        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 uint32_t
 * 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
 * 4 bytes  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 {
        uint32_t        cmd;            /* LC_THREAD or  LC_UNIXTHREAD */
        uint32_t        cmdsize;        /* total size of this command */
        /* uint32_t flavor                 flavor of thread state */
        /* uint32_t count                  count of longs in thread state */
        /* struct XXX_thread_state state   thread state for this flavor */
        /* ... */
};

/*
 * The routines command contains the address of the dynamic shared library 
 * initialization routine and an index into the module table for the module
 * that defines the routine.  Before any modules are used from the library the
 * dynamic linker fully binds the module that defines the initialization routine
 * and then calls it.  This gets called before any module initialization
 * routines (used for C++ static constructors) in the library.
 */
struct routines_command { /* for 32-bit architectures */
        uint32_t        cmd;            /* LC_ROUTINES */
        uint32_t        cmdsize;        /* total size of this command */
        uint32_t        init_address;   /* address of initialization routine */
        uint32_t        init_module;    /* index into the module table that */
                                        /*  the init routine is defined in */
        uint32_t        reserved1;
        uint32_t        reserved2;
        uint32_t        reserved3;
        uint32_t        reserved4;
        uint32_t        reserved5;
        uint32_t        reserved6;
};

/*
 * The 64-bit routines command.  Same use as above.
 */
struct routines_command_64 { /* for 64-bit architectures */
        uint32_t        cmd;            /* LC_ROUTINES_64 */
        uint32_t        cmdsize;        /* total size of this command */
        uint64_t        init_address;   /* address of initialization routine */
        uint64_t        init_module;    /* index into the module table that */
                                        /*  the init routine is defined in */
        uint64_t        reserved1;
        uint64_t        reserved2;
        uint64_t        reserved3;
        uint64_t        reserved4;
        uint64_t        reserved5;
        uint64_t        reserved6;
};

/*
 * 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 {
        uint32_t        cmd;            /* LC_SYMTAB */
        uint32_t        cmdsize;        /* sizeof(struct symtab_command) */
        uint32_t        symoff;         /* symbol table offset */
        uint32_t        nsyms;          /* number of symbol table entries */
        uint32_t        stroff;         /* string table offset */
        uint32_t        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 dynamically 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 if MH_BINDATLOAD is not set,
 *                                  and in order the were seen by the static
 *                                  linker if MH_BINDATLOAD is set)
 * 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 dynamically 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 dynamically 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 {
    uint32_t cmd;       /* LC_DYSYMTAB */
    uint32_t 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 dynamically linked shared library file).
     */
    uint32_t ilocalsym; /* index to local symbols */
    uint32_t nlocalsym; /* number of local symbols */

    uint32_t iextdefsym;/* index to externally defined symbols */
    uint32_t nextdefsym;/* number of externally defined symbols */

    uint32_t iundefsym; /* index to undefined symbols */
    uint32_t 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 dynamically 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.
     */
    uint32_t tocoff;    /* file offset to table of contents */
    uint32_t 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 dynamically linked
     * shared library file.  For executable and object modules the file only
     * contains one module so everything in the file belongs to the module.
     */
    uint32_t modtaboff; /* file offset to module table */
    uint32_t 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 dynamically linked shared library file.  For
     * executable and object modules the defined external symbols and the
     * undefined external symbols indicates the external references.
     */
    uint32_t extrefsymoff;      /* offset to referenced symbol table */
    uint32_t 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.
     */
    uint32_t indirectsymoff; /* file offset to the indirect symbol table */
    uint32_t 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.
     * For MH_SPLIT_SEGS images r_address is set to the the offset from the
     * vmaddr of the first read-write 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).
     */
    uint32_t extreloff; /* offset to external relocation entries */
    uint32_t 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).
     */
    uint32_t locreloff; /* offset to local relocation entries */
    uint32_t 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 {
    uint32_t symbol_index;      /* the defined external symbol
                                   (index into the symbol table) */
    uint32_t module_index;      /* index into the module table this symbol
                                   is defined in */
};      

/* a module table entry */
struct dylib_module {
    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 address of the start of */
        objc_module_info_addr;  /*  the (__OBJC,__module_info) section */
    uint32_t                    /* 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 */
        objc_module_info_size;  /*  the (__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 {
    uint32_t isym:24,           /* index into the symbol table */
                  flags:8;      /* flags to indicate the type of reference */
};

/*
 * The twolevel_hints_command contains the offset and number of hints in the
 * two-level namespace lookup hints table.
 */
struct twolevel_hints_command {
    uint32_t cmd;       /* LC_TWOLEVEL_HINTS */
    uint32_t cmdsize;   /* sizeof(struct twolevel_hints_command) */
    uint32_t offset;    /* offset to the hint table */
    uint32_t nhints;    /* number of hints in the hint table */
};

/*
 * The entries in the two-level namespace lookup hints table are twolevel_hint
 * structs.  These provide hints to the dynamic link editor where to start
 * looking for an undefined symbol in a two-level namespace image.  The
 * isub_image field is an index into the sub-images (sub-frameworks and
 * sub-umbrellas list) that made up the two-level image that the undefined
 * symbol was found in when it was built by the static link editor.  If
 * isub-image is 0 the the symbol is expected to be defined in library and not
 * in the sub-images.  If isub-image is non-zero it is an index into the array
 * of sub-images for the umbrella with the first index in the sub-images being
 * 1. The array of sub-images is the ordered list of sub-images of the umbrella
 * that would be searched for a symbol that has the umbrella recorded as its
 * primary library.  The table of contents index is an index into the
 * library's table of contents.  This is used as the starting point of the
 * binary search or a directed linear search.
 */
struct twolevel_hint {
    uint32_t 
        isub_image:8,   /* index into the sub images */
        itoc:24;        /* index into the table of contents */
};

/*
 * The prebind_cksum_command contains the value of the original check sum for
 * prebound files or zero.  When a prebound file is first created or modified
 * for other than updating its prebinding information the value of the check sum
 * is set to zero.  When the file has it prebinding re-done and if the value of
 * the check sum is zero the original check sum is calculated and stored in
 * cksum field of this load command in the output file.  If when the prebinding
 * is re-done and the cksum field is non-zero it is left unchanged from the
 * input file.
 */
struct prebind_cksum_command {
    uint32_t cmd;       /* LC_PREBIND_CKSUM */
    uint32_t cmdsize;   /* sizeof(struct prebind_cksum_command) */
    uint32_t cksum;     /* the check sum or zero */
};

/*
 * The uuid load command contains a single 128-bit unique random number that
 * identifies an object produced by the static link editor.
 */
struct uuid_command {
    uint32_t    cmd;            /* LC_UUID */
    uint32_t    cmdsize;        /* sizeof(struct uuid_command) */
    uint8_t     uuid[16];       /* the 128-bit uuid */
};

/*
 * The rpath_command contains a path which at runtime should be added to
 * the current run path used to find @rpath prefixed dylibs.
 */
struct rpath_command {
    uint32_t     cmd;           /* LC_RPATH */
    uint32_t     cmdsize;       /* includes string */
    union lc_str path;          /* path to add to run path */
};

/*
 * The linkedit_data_command contains the offsets and sizes of a blob
 * of data in the __LINKEDIT segment.  
 */
struct linkedit_data_command {
    uint32_t    cmd;            /* LC_CODE_SIGNATURE, LC_SEGMENT_SPLIT_INFO,
                                   or LC_FUNCTION_STARTS */
    uint32_t    cmdsize;        /* sizeof(struct linkedit_data_command) */
    uint32_t    dataoff;        /* file offset of data in __LINKEDIT segment */
    uint32_t    datasize;       /* file size of data in __LINKEDIT segment  */
};

/*
 * The encryption_info_command contains the file offset and size of an
 * of an encrypted segment.
 */
struct encryption_info_command {
   uint32_t     cmd;            /* LC_ENCRYPTION_INFO */
   uint32_t     cmdsize;        /* sizeof(struct encryption_info_command) */
   uint32_t     cryptoff;       /* file offset of encrypted range */
   uint32_t     cryptsize;      /* file size of encrypted range */
   uint32_t     cryptid;        /* which enryption system,
                                   0 means not-encrypted yet */
};

/*
 * The version_min_command contains the min OS version on which this 
 * binary was built to run.
 */
struct version_min_command {
    uint32_t    cmd;            /* LC_VERSION_MIN_MACOSX or
                                   LC_VERSION_MIN_IPHONEOS  */
    uint32_t    cmdsize;        /* sizeof(struct min_version_command) */
    uint32_t    version;        /* X.Y.Z is encoded in nibbles xxxx.yy.zz */
    uint32_t    reserved;       /* zero */
};

/*
 * The dyld_info_command contains the file offsets and sizes of 
 * the new compressed form of the information dyld needs to 
 * load the image.  This information is used by dyld on Mac OS X
 * 10.6 and later.  All information pointed to by this command
 * is encoded using byte streams, so no endian swapping is needed
 * to interpret it. 
 */
struct dyld_info_command {
   uint32_t   cmd;              /* LC_DYLD_INFO or LC_DYLD_INFO_ONLY */
   uint32_t   cmdsize;          /* sizeof(struct dyld_info_command) */

    /*
     * Dyld rebases an image whenever dyld loads it at an address different
     * from its preferred address.  The rebase information is a stream
     * of byte sized opcodes whose symbolic names start with REBASE_OPCODE_.
     * Conceptually the rebase information is a table of tuples:
     *    <seg-index, seg-offset, type>
     * The opcodes are a compressed way to encode the table by only
     * encoding when a column changes.  In addition simple patterns
     * like "every n'th offset for m times" can be encoded in a few
     * bytes.
     */
    uint32_t   rebase_off;      /* file offset to rebase info  */
    uint32_t   rebase_size;     /* size of rebase info   */
    
    /*
     * Dyld binds an image during the loading process, if the image
     * requires any pointers to be initialized to symbols in other images.  
     * The bind information is a stream of byte sized 
     * opcodes whose symbolic names start with BIND_OPCODE_.
     * Conceptually the bind information is a table of tuples:
     *    <seg-index, seg-offset, type, symbol-library-ordinal, symbol-name, addend>
     * The opcodes are a compressed way to encode the table by only
     * encoding when a column changes.  In addition simple patterns
     * like for runs of pointers initialzed to the same value can be 
     * encoded in a few bytes.
     */
    uint32_t   bind_off;        /* file offset to binding info   */
    uint32_t   bind_size;       /* size of binding info  */
        
    /*
     * Some C++ programs require dyld to unique symbols so that all
     * images in the process use the same copy of some code/data.
     * This step is done after binding. The content of the weak_bind
     * info is an opcode stream like the bind_info.  But it is sorted
     * alphabetically by symbol name.  This enable dyld to walk 
     * all images with weak binding information in order and look
     * for collisions.  If there are no collisions, dyld does
     * no updating.  That means that some fixups are also encoded
     * in the bind_info.  For instance, all calls to "operator new"
     * are first bound to libstdc++.dylib using the information
     * in bind_info.  Then if some image overrides operator new
     * that is detected when the weak_bind information is processed
     * and the call to operator new is then rebound.
     */
    uint32_t   weak_bind_off;   /* file offset to weak binding info   */
    uint32_t   weak_bind_size;  /* size of weak binding info  */
    
    /*
     * Some uses of external symbols do not need to be bound immediately.
     * Instead they can be lazily bound on first use.  The lazy_bind
     * are contains a stream of BIND opcodes to bind all lazy symbols.
     * Normal use is that dyld ignores the lazy_bind section when
     * loading an image.  Instead the static linker arranged for the
     * lazy pointer to initially point to a helper function which 
     * pushes the offset into the lazy_bind area for the symbol
     * needing to be bound, then jumps to dyld which simply adds
     * the offset to lazy_bind_off to get the information on what 
     * to bind.  
     */
    uint32_t   lazy_bind_off;   /* file offset to lazy binding info */
    uint32_t   lazy_bind_size;  /* size of lazy binding infs */
    
    /*
     * The symbols exported by a dylib are encoded in a trie.  This
     * is a compact representation that factors out common prefixes.
     * It also reduces LINKEDIT pages in RAM because it encodes all  
     * information (name, address, flags) in one small, contiguous range.
     * The export area is a stream of nodes.  The first node sequentially
     * is the start node for the trie.  
     *
     * Nodes for a symbol start with a uleb128 that is the length of
     * the exported symbol information for the string so far.
     * If there is no exported symbol, the node starts with a zero byte. 
     * If there is exported info, it follows the length.  First is
     * a uleb128 containing flags.  Normally, it is followed by a
     * uleb128 encoded offset which is location of the content named
     * by the symbol from the mach_header for the image.  If the flags
     * is EXPORT_SYMBOL_FLAGS_REEXPORT, then following the flags is
     * a uleb128 encoded library ordinal, then a zero terminated
     * UTF8 string.  If the string is zero length, then the symbol
     * is re-export from the specified dylib with the same name.
     *
     * After the optional exported symbol information is a byte of
     * how many edges (0-255) that this node has leaving it, 
     * followed by each edge.
     * Each edge is a zero terminated UTF8 of the addition chars
     * in the symbol, followed by a uleb128 offset for the node that
     * edge points to.
     *  
     */
    uint32_t   export_off;      /* file offset to lazy binding info */
    uint32_t   export_size;     /* size of lazy binding infs */
};

/*
 * The following are used to encode rebasing information
 */
#define REBASE_TYPE_POINTER                                     1
#define REBASE_TYPE_TEXT_ABSOLUTE32                             2
#define REBASE_TYPE_TEXT_PCREL32                                3

#define REBASE_OPCODE_MASK                                      0xF0
#define REBASE_IMMEDIATE_MASK                                   0x0F
#define REBASE_OPCODE_DONE                                      0x00
#define REBASE_OPCODE_SET_TYPE_IMM                              0x10
#define REBASE_OPCODE_SET_SEGMENT_AND_OFFSET_ULEB               0x20
#define REBASE_OPCODE_ADD_ADDR_ULEB                             0x30
#define REBASE_OPCODE_ADD_ADDR_IMM_SCALED                       0x40
#define REBASE_OPCODE_DO_REBASE_IMM_TIMES                       0x50
#define REBASE_OPCODE_DO_REBASE_ULEB_TIMES                      0x60
#define REBASE_OPCODE_DO_REBASE_ADD_ADDR_ULEB                   0x70
#define REBASE_OPCODE_DO_REBASE_ULEB_TIMES_SKIPPING_ULEB        0x80


/*
 * The following are used to encode binding information
 */
#define BIND_TYPE_POINTER                                       1
#define BIND_TYPE_TEXT_ABSOLUTE32                               2
#define BIND_TYPE_TEXT_PCREL32                                  3

#define BIND_SPECIAL_DYLIB_SELF                                  0
#define BIND_SPECIAL_DYLIB_MAIN_EXECUTABLE                      -1
#define BIND_SPECIAL_DYLIB_FLAT_LOOKUP                          -2

#define BIND_SYMBOL_FLAGS_WEAK_IMPORT                           0x1
#define BIND_SYMBOL_FLAGS_NON_WEAK_DEFINITION                   0x8

#define BIND_OPCODE_MASK                                        0xF0
#define BIND_IMMEDIATE_MASK                                     0x0F
#define BIND_OPCODE_DONE                                        0x00
#define BIND_OPCODE_SET_DYLIB_ORDINAL_IMM                       0x10
#define BIND_OPCODE_SET_DYLIB_ORDINAL_ULEB                      0x20
#define BIND_OPCODE_SET_DYLIB_SPECIAL_IMM                       0x30
#define BIND_OPCODE_SET_SYMBOL_TRAILING_FLAGS_IMM               0x40
#define BIND_OPCODE_SET_TYPE_IMM                                0x50
#define BIND_OPCODE_SET_ADDEND_SLEB                             0x60
#define BIND_OPCODE_SET_SEGMENT_AND_OFFSET_ULEB                 0x70
#define BIND_OPCODE_ADD_ADDR_ULEB                               0x80
#define BIND_OPCODE_DO_BIND                                     0x90
#define BIND_OPCODE_DO_BIND_ADD_ADDR_ULEB                       0xA0
#define BIND_OPCODE_DO_BIND_ADD_ADDR_IMM_SCALED                 0xB0
#define BIND_OPCODE_DO_BIND_ULEB_TIMES_SKIPPING_ULEB            0xC0


/*
 * The following are used on the flags byte of a terminal node
 * in the export information.
 */
#define EXPORT_SYMBOL_FLAGS_KIND_MASK                           0x03
#define EXPORT_SYMBOL_FLAGS_KIND_REGULAR                        0x00
#define EXPORT_SYMBOL_FLAGS_KIND_THREAD_LOCAL                   0x01
#define EXPORT_SYMBOL_FLAGS_WEAK_DEFINITION                     0x04
#define EXPORT_SYMBOL_FLAGS_REEXPORT                            0x08
#define EXPORT_SYMBOL_FLAGS_STUB_AND_RESOLVER                   0x10

/*
 * 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 4 bytes 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 {
        uint32_t        cmd;            /* LC_SYMSEG */
        uint32_t        cmdsize;        /* sizeof(struct symseg_command) */
        uint32_t        offset;         /* symbol segment offset */
        uint32_t        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 4 bytes/
 * (THIS IS OBSOLETE and no longer supported).
 */
struct ident_command {
        uint32_t cmd;           /* LC_IDENT */
        uint32_t 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
 * internal use.  The kernel ignores this command when loading a program into
 * memory).
 */
struct fvmfile_command {
        uint32_t cmd;                   /* LC_FVMFILE */
        uint32_t cmdsize;               /* includes pathname string */
        union lc_str    name;           /* files pathname */
        uint32_t        header_addr;    /* files virtual address */
};

/*
 * Sections of type S_THREAD_LOCAL_VARIABLES contain an array 
 * of tlv_descriptor structures.
 */
struct tlv_descriptor
{
        void*           (*thunk)(struct tlv_descriptor*);
        unsigned long   key;
        unsigned long   offset;
};

#endif /* _MACHO_LOADER_H_ */