x86-64我对 Mac OS X 平台上的汇编编程很感兴趣。我遇到了这个关于创建 248B Mach-O 程序的页面，这让我找到了Apple 自己的 Mach-O 格式参考。之后我想我会在 Xcode 中制作同样简单的 C 程序并检查生成的程序集。
这是代码：
int main(int argc, const char * argv[])
{
return 42;
}
但是生成的程序集是 334 行，包含（基于 248B 模型）很多多余的内容。
首先，为什么 C 可执行文件的发布版本中包含这么多 DWARF 调试信息？其次，我注意到 Mach-O 标头数据包含 4 次（在不同的 DWARF-related 中sections）。为什么这是必要的？最后，Xcode 程序集包括：
.private_extern _main
.globl _main
_main:
.cfi_startproc
但在 248B 程序中，这些都无处可见——程序反而从_start. 如果根据定义所有程序都开始于 ，那怎么可能main？
完整的 Xcode 组装：
# Assembly output for main.c
# Generated at 4:04:08 PM on Sunday, January 20, 2013
# Using Release configuration, x86_64 architecture for Tiny target of Tiny project
.section __TEXT,__text,regular,pure_instructions
.file 1 "/Users/####/Desktop/Tiny/Tiny/main.c"
.section __DWARF,__debug_info,regular,debug
Lsection_info:
.section __DWARF,__debug_abbrev,regular,debug
Lsection_abbrev:
.section __DWARF,__debug_aranges,regular,debug
.section __DWARF,__debug_macinfo,regular,debug
.section __DWARF,__debug_line,regular,debug
Lsection_line:
.section __DWARF,__debug_loc,regular,debug
.section __DWARF,__debug_pubtypes,regular,debug
.section __DWARF,__debug_str,regular,debug
Lsection_str:
.section __DWARF,__debug_ranges,regular,debug
Ldebug_range:
.section __DWARF,__debug_loc,regular,debug
Lsection_debug_loc:
.section __TEXT,__text,regular,pure_instructions
Ltext_begin:
.section __DATA,__data
.section __TEXT,__text,regular,pure_instructions
.private_extern _main
.globl _main
_main: ## @main
.cfi_startproc
Lfunc_begin0:
.loc 1 12 0 ## /Users/####/Desktop/Tiny/Tiny/main.c:12:0
## BB#0:
pushq %rbp
Ltmp2:
.cfi_def_cfa_offset 16
Ltmp3:
.cfi_offset %rbp, -16
movq %rsp, %rbp
Ltmp4:
.cfi_def_cfa_register %rbp
##DEBUG_VALUE: main:argc <- EDI+0
##DEBUG_VALUE: main:argv <- RSI+0
movl $42, %eax
.loc 1 15 5 prologue_end ## /Users/####/Desktop/Tiny/Tiny/main.c:15:5
Ltmp5:
popq %rbp
ret
Ltmp6:
Lfunc_end0:
.cfi_endproc
Ltext_end:
.section __DATA,__data
Ldata_end:
.section __TEXT,__text,regular,pure_instructions
Lsection_end1:
.section __DWARF,__debug_info,regular,debug
Linfo_begin1:
.long 127 ## Length of Compilation Unit Info
.short 2 ## DWARF version number
Lset0 = Labbrev_begin-Lsection_abbrev ## Offset Into Abbrev. Section
.long Lset0
.byte 8 ## Address Size (in bytes)
.byte 1 ## Abbrev [1] 0xb:0x78 DW_TAG_compile_unit
Lset1 = Lstring0-Lsection_str ## DW_AT_producer
.long Lset1
.short 12 ## DW_AT_language
Lset2 = Lstring1-Lsection_str ## DW_AT_name
.long Lset2
.quad 0 ## DW_AT_entry_pc
.long 0 ## DW_AT_stmt_list
Lset3 = Lstring2-Lsection_str ## DW_AT_comp_dir
.long Lset3
.byte 1 ## DW_AT_APPLE_optimized
.byte 2 ## Abbrev [2] 0x27:0x3e DW_TAG_subprogram
Lset4 = Lstring3-Lsection_str ## DW_AT_name
.long Lset4
.byte 1 ## DW_AT_decl_file
.byte 11 ## DW_AT_decl_line
.byte 1 ## DW_AT_prototyped
.long 101 ## DW_AT_type
.byte 1 ## DW_AT_external
.quad Lfunc_begin0 ## DW_AT_low_pc
.quad Lfunc_end0 ## DW_AT_high_pc
.byte 1 ## DW_AT_frame_base
.byte 86
.byte 3 ## Abbrev [3] 0x46:0xf DW_TAG_formal_parameter
Lset5 = Lstring5-Lsection_str ## DW_AT_name
.long Lset5
.byte 1 ## DW_AT_decl_file
.byte 11 ## DW_AT_decl_line
.long 101 ## DW_AT_type
Lset6 = Ldebug_loc0-Lsection_debug_loc ## DW_AT_location
.long Lset6
.byte 3 ## Abbrev [3] 0x55:0xf DW_TAG_formal_parameter
Lset7 = Lstring6-Lsection_str ## DW_AT_name
.long Lset7
.byte 1 ## DW_AT_decl_file
.byte 11 ## DW_AT_decl_line
.long 125 ## DW_AT_type
Lset8 = Ldebug_loc2-Lsection_debug_loc ## DW_AT_location
.long Lset8
.byte 0 ## End Of Children Mark
.byte 4 ## Abbrev [4] 0x65:0x7 DW_TAG_base_type
Lset9 = Lstring4-Lsection_str ## DW_AT_name
.long Lset9
.byte 5 ## DW_AT_encoding
.byte 4 ## DW_AT_byte_size
.byte 4 ## Abbrev [4] 0x6c:0x7 DW_TAG_base_type
Lset10 = Lstring7-Lsection_str ## DW_AT_name
.long Lset10
.byte 6 ## DW_AT_encoding
.byte 1 ## DW_AT_byte_size
.byte 5 ## Abbrev [5] 0x73:0x5 DW_TAG_const_type
.long 108 ## DW_AT_type
.byte 6 ## Abbrev [6] 0x78:0x5 DW_TAG_pointer_type
.long 115 ## DW_AT_type
.byte 6 ## Abbrev [6] 0x7d:0x5 DW_TAG_pointer_type
.long 120 ## DW_AT_type
.byte 0 ## End Of Children Mark
Linfo_end1:
.section __DWARF,__debug_abbrev,regular,debug
Labbrev_begin:
.byte 1 ## Abbreviation Code
.byte 17 ## DW_TAG_compile_unit
.byte 1 ## DW_CHILDREN_yes
.byte 37 ## DW_AT_producer
.byte 14 ## DW_FORM_strp
.byte 19 ## DW_AT_language
.byte 5 ## DW_FORM_data2
.byte 3 ## DW_AT_name
.byte 14 ## DW_FORM_strp
.byte 82 ## DW_AT_entry_pc
.byte 1 ## DW_FORM_addr
.byte 16 ## DW_AT_stmt_list
.byte 6 ## DW_FORM_data4
.byte 27 ## DW_AT_comp_dir
.byte 14 ## DW_FORM_strp
.ascii "\341\177" ## DW_AT_APPLE_optimized
.byte 12 ## DW_FORM_flag
.byte 0 ## EOM(1)
.byte 0 ## EOM(2)
.byte 2 ## Abbreviation Code
.byte 46 ## DW_TAG_subprogram
.byte 1 ## DW_CHILDREN_yes
.byte 3 ## DW_AT_name
.byte 14 ## DW_FORM_strp
.byte 58 ## DW_AT_decl_file
.byte 11 ## DW_FORM_data1
.byte 59 ## DW_AT_decl_line
.byte 11 ## DW_FORM_data1
.byte 39 ## DW_AT_prototyped
.byte 12 ## DW_FORM_flag
.byte 73 ## DW_AT_type
.byte 19 ## DW_FORM_ref4
.byte 63 ## DW_AT_external
.byte 12 ## DW_FORM_flag
.byte 17 ## DW_AT_low_pc
.byte 1 ## DW_FORM_addr
.byte 18 ## DW_AT_high_pc
.byte 1 ## DW_FORM_addr
.byte 64 ## DW_AT_frame_base
.byte 10 ## DW_FORM_block1
.byte 0 ## EOM(1)
.byte 0 ## EOM(2)
.byte 3 ## Abbreviation Code
.byte 5 ## DW_TAG_formal_parameter
.byte 0 ## DW_CHILDREN_no
.byte 3 ## DW_AT_name
.byte 14 ## DW_FORM_strp
.byte 58 ## DW_AT_decl_file
.byte 11 ## DW_FORM_data1
.byte 59 ## DW_AT_decl_line
.byte 11 ## DW_FORM_data1
.byte 73 ## DW_AT_type
.byte 19 ## DW_FORM_ref4
.byte 2 ## DW_AT_location
.byte 6 ## DW_FORM_data4
.byte 0 ## EOM(1)
.byte 0 ## EOM(2)
.byte 4 ## Abbreviation Code
.byte 36 ## DW_TAG_base_type
.byte 0 ## DW_CHILDREN_no
.byte 3 ## DW_AT_name
.byte 14 ## DW_FORM_strp
.byte 62 ## DW_AT_encoding
.byte 11 ## DW_FORM_data1
.byte 11 ## DW_AT_byte_size
.byte 11 ## DW_FORM_data1
.byte 0 ## EOM(1)
.byte 0 ## EOM(2)
.byte 5 ## Abbreviation Code
.byte 38 ## DW_TAG_const_type
.byte 0 ## DW_CHILDREN_no
.byte 73 ## DW_AT_type
.byte 19 ## DW_FORM_ref4
.byte 0 ## EOM(1)
.byte 0 ## EOM(2)
.byte 6 ## Abbreviation Code
.byte 15 ## DW_TAG_pointer_type
.byte 0 ## DW_CHILDREN_no
.byte 73 ## DW_AT_type
.byte 19 ## DW_FORM_ref4
.byte 0 ## EOM(1)
.byte 0 ## EOM(2)
.byte 0 ## EOM(3)
Labbrev_end:
.section __DWARF,__apple_names,regular,debug
Lnames_begin:
.long 1212240712 ## Header Magic
.short 1 ## Header Version
.short 0 ## Header Hash Function
.long 1 ## Header Bucket Count
.long 1 ## Header Hash Count
.long 12 ## Header Data Length
.long 0 ## HeaderData Die Offset Base
.long 1 ## HeaderData Atom Count
.short 1 ## eAtomTypeDIEOffset
.short 6 ## DW_FORM_data4
.long 0 ## Bucket 0
.long 2090499946 ## Hash in Bucket 0
.long LNames0-Lnames_begin ## Offset in Bucket 0
LNames0:
Lset11 = Lstring3-Lsection_str ## main
.long Lset11
.long 1 ## Num DIEs
.long 39
.long 0
.section __DWARF,__apple_objc,regular,debug
Lobjc_begin:
.long 1212240712 ## Header Magic
.short 1 ## Header Version
.short 0 ## Header Hash Function
.long 1 ## Header Bucket Count
.long 0 ## Header Hash Count
.long 12 ## Header Data Length
.long 0 ## HeaderData Die Offset Base
.long 1 ## HeaderData Atom Count
.short 1 ## eAtomTypeDIEOffset
.short 6 ## DW_FORM_data4
.long -1 ## Bucket 0
.section __DWARF,__apple_namespac,regular,debug
Lnamespac_begin:
.long 1212240712 ## Header Magic
.short 1 ## Header Version
.short 0 ## Header Hash Function
.long 1 ## Header Bucket Count
.long 0 ## Header Hash Count
.long 12 ## Header Data Length
.long 0 ## HeaderData Die Offset Base
.long 1 ## HeaderData Atom Count
.short 1 ## eAtomTypeDIEOffset
.short 6 ## DW_FORM_data4
.long -1 ## Bucket 0
.section __DWARF,__apple_types,regular,debug
Ltypes_begin:
.long 1212240712 ## Header Magic
.short 1 ## Header Version
.short 0 ## Header Hash Function
.long 2 ## Header Bucket Count
.long 2 ## Header Hash Count
.long 20 ## Header Data Length
.long 0 ## HeaderData Die Offset Base
.long 3 ## HeaderData Atom Count
.short 1 ## eAtomTypeDIEOffset
.short 6 ## DW_FORM_data4
.short 3 ## eAtomTypeTag
.short 5 ## DW_FORM_data2
.short 5 ## eAtomTypeTypeFlags
.short 11 ## DW_FORM_data1
.long 0 ## Bucket 0
.long 1 ## Bucket 1
.long 193495088 ## Hash in Bucket 0
.long 2090147939 ## Hash in Bucket 1
.long Ltypes0-Ltypes_begin ## Offset in Bucket 0
.long Ltypes1-Ltypes_begin ## Offset in Bucket 1
Ltypes0:
Lset12 = Lstring4-Lsection_str ## int
.long Lset12
.long 1 ## Num DIEs
.long 101
.short 36
.byte 0
.long 0
Ltypes1:
Lset13 = Lstring7-Lsection_str ## char
.long Lset13
.long 1 ## Num DIEs
.long 108
.short 36
.byte 0
.long 0
.section __DWARF,__debug_pubtypes,regular,debug
Lset14 = Lpubtypes_end1-Lpubtypes_begin1 ## Length of Public Types Info
.long Lset14
Lpubtypes_begin1:
.short 2 ## DWARF Version
Lset15 = Linfo_begin1-Lsection_info ## Offset of Compilation Unit Info
.long Lset15
Lset16 = Linfo_end1-Linfo_begin1 ## Compilation Unit Length
.long Lset16
.long 0 ## End Mark
Lpubtypes_end1:
.section __DWARF,__debug_loc,regular,debug
Ldebug_loc0:
.quad Lfunc_begin0
.quad Ltmp6
Lset17 = Ltmp8-Ltmp7 ## Loc expr size
.short Lset17
Ltmp7:
.byte 85 ## DW_OP_reg5
Ltmp8:
.quad 0
.quad 0
Ldebug_loc2:
.quad Lfunc_begin0
.quad Ltmp6
Lset18 = Ltmp10-Ltmp9 ## Loc expr size
.short Lset18
Ltmp9:
.byte 84 ## DW_OP_reg4
Ltmp10:
.quad 0
.quad 0
Ldebug_loc4:
.section __DWARF,__debug_aranges,regular,debug
.section __DWARF,__debug_ranges,regular,debug
.section __DWARF,__debug_macinfo,regular,debug
.section __DWARF,__debug_inlined,regular,debug
Lset19 = Ldebug_inlined_end1-Ldebug_inlined_begin1 ## Length of Debug Inlined Information Entry
.long Lset19
Ldebug_inlined_begin1:
.short 2 ## Dwarf Version
.byte 8 ## Address Size (in bytes)
Ldebug_inlined_end1:
.section __DWARF,__debug_str,regular,debug
Lstring0:
.asciz "Apple clang version 4.1 (tags/Apple/clang-421.11.66) (based on LLVM 3.1svn)"
Lstring1:
.asciz "/Users/####/Desktop/Tiny/Tiny/main.c"
Lstring2:
.asciz "/Users/####/Desktop/Tiny"
Lstring3:
.asciz "main"
Lstring4:
.asciz "int"
Lstring5:
.asciz "argc"
Lstring6:
.asciz "argv"
Lstring7:
.asciz "char"
.subsections_via_symbols