mach-o标题中的标志是指什么？

Question

我正在尝试这个,我注意到mach-o标题的标志部分,我想知道它做了什么.我试着谷歌搜索它,发现很多引用它,但没有解释标志是什么.

如果我在编译的基本hello世界中使用otool,clang hello.c那么运行otool它会给我这些标志:

$ otool a.out -h
Mach header
      magic cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777223          3  0x80           2    14        664 0x00200085

对于Chess.app

Mach header
      magic cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777223          3  0x80           2    32       4576 0x00210085

这里有什么标志？我在哪里可以找到这样的答案？

Answer 1

他们定义并记录在苹果的源代码在<Mach-O的/ loader.h>

/* 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. */

#define MH_APP_EXTENSION_SAFE 0x02000000 /* The code was linked for use in an
                        application extension. */

#define MH_NLIST_OUTOFSYNC_WITH_DYLDINFO 0x04000000
                    /* The external symbols listed in the nlist
                       symbol table do not include all the symbols
                       listed in the dyld info. */

#define MH_SIM_SUPPORT 0x08000000   /* Allow LC_MIN_VERSION_MACOS and
                       LC_BUILD_VERSION load commands with
                       the platforms macOS, iOSMac,
                       iOSSimulator, tvOSSimulator and
                       watchOSSimulator. */