在u-boot下，定义变量，
编译，编译完后 使用arm-linux-nm arm 没有去头的二进制可执行文件
都在BSS段，均为初始化。
打印之后会出算随机值。
目前还处于uboot阶段，如果在根文件系统的话，加载器把二进制可执行文件加载到内存，并将BSS段清零。 放在BSS段的变量所对应的空间，在二进制可执行文件中不占空间。放在Data段才分配空间。
ld --help
默认链接脚本：arm-linux-ld -verbose
1 GNU ld (GNU Binutils) 2.20.1.20100303 2 Supported emulations: 3 armelf_linux_eabi 4 armelfb_linux_eabi 5 using internal linker script: 6 ================================================== 7 /* Script for -z combreloc: combine and sort reloc sections */ 8 OUTPUT_FORMAT("elf32-littlearm", "elf32-bigarm", 9 "elf32-littlearm") //小端存储的arm 10 OUTPUT_ARCH(arm) 11 ENTRY(_start) 12 SEARCH_DIR("=/usr/local/lib"); SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib"); 13 SECTIONS 14 { 15 /* Read-only sections, merged into text segment: */ 16 PROVIDE (__executable_start = SEGMENT_START("text-segment", 0x00008000)); . = SEGMENT_START("text-segment", 0x00008000) + SIZEOF_HEADERS; 17 .interp : { *(.interp) } 18 .note.gnu.build-id : { *(.note.gnu.build-id) } 19 .hash : { *(.hash) } 20 .gnu.hash : { *(.gnu.hash) } 21 .dynsym : { *(.dynsym) } 22 .dynstr : { *(.dynstr) } 23 .gnu.version : { *(.gnu.version) } 24 .gnu.version_d : { *(.gnu.version_d) } 25 .gnu.version_r : { *(.gnu.version_r) } 26 .rel.dyn : 27 { 28 *(.rel.init) 29 *(.rel.text .rel.text.* .rel.gnu.linkonce.t.*) 30 *(.rel.fini) 31 *(.rel.rodata .rel.rodata.* .rel.gnu.linkonce.r.*) 32 *(.rel.data.rel.ro* .rel.gnu.linkonce.d.rel.ro.*) 33 *(.rel.data .rel.data.* .rel.gnu.linkonce.d.*) 34 *(.rel.tdata .rel.tdata.* .rel.gnu.linkonce.td.*) 35 *(.rel.tbss .rel.tbss.* .rel.gnu.linkonce.tb.*) 36 *(.rel.ctors) 37 *(.rel.dtors) 38 *(.rel.got) 39 *(.rel.bss .rel.bss.* .rel.gnu.linkonce.b.*) 40 PROVIDE_HIDDEN (__rel_iplt_start = .); 41 *(.rel.iplt) 42 PROVIDE_HIDDEN (__rel_iplt_end = .); 43 PROVIDE_HIDDEN (__rela_iplt_start = .); 44 PROVIDE_HIDDEN (__rela_iplt_end = .); 45 } 46 .rela.dyn : 47 { 48 *(.rela.init) 49 *(.rela.text .rela.text.* .rela.gnu.linkonce.t.*) 50 *(.rela.fini) 51 *(.rela.rodata .rela.rodata.* .rela.gnu.linkonce.r.*) 52 *(.rela.data .rela.data.* .rela.gnu.linkonce.d.*) 53 *(.rela.tdata .rela.tdata.* .rela.gnu.linkonce.td.*) 54 *(.rela.tbss .rela.tbss.* .rela.gnu.linkonce.tb.*) 55 *(.rela.ctors) 56 *(.rela.dtors) 57 *(.rela.got) 58 *(.rela.bss .rela.bss.* .rela.gnu.linkonce.b.*) 59 PROVIDE_HIDDEN (__rel_iplt_start = .); 60 PROVIDE_HIDDEN (__rel_iplt_end = .); 61 PROVIDE_HIDDEN (__rela_iplt_start = .); 62 *(.rela.iplt) 63 PROVIDE_HIDDEN (__rela_iplt_end = .); 64 } 65 .rel.plt : 66 { 67 *(.rel.plt) 68 } 69 .rela.plt : 70 { 71 *(.rela.plt) 72 } 73 .init : 74 { 75 KEEP (*(.init)) 76 } =0 77 .plt : { *(.plt) } 78 .iplt : { *(.iplt) } 79 .text : 80 { 81 *(.text.unlikely .text.*_unlikely) 82 *(.text .stub .text.* .gnu.linkonce.t.*) 83 /* .gnu.warning sections are handled specially by elf32.em. */ 84 *(.gnu.warning) 85 *(.glue_7t) *(.glue_7) *(.vfp11_veneer) *(.v4_bx) 86 } =0 87 .fini : 88 { 89 KEEP (*(.fini)) 90 } =0 91 PROVIDE (__etext = .); 92 PROVIDE (_etext = .); 93 PROVIDE (etext = .); 94 .rodata : { *(.rodata .rodata.* .gnu.linkonce.r.*) } 95 .rodata1 : { *(.rodata1) } 96 .ARM.extab : { *(.ARM.extab* .gnu.linkonce.armextab.*) } 97 __exidx_start = .; 98 .ARM.exidx : { *(.ARM.exidx* .gnu.linkonce.armexidx.*) } 99 __exidx_end = .;100 .eh_frame_hdr : { *(.eh_frame_hdr) }101 .eh_frame : ONLY_IF_RO { KEEP (*(.eh_frame)) }102 .gcc_except_table : ONLY_IF_RO { *(.gcc_except_table .gcc_except_table.*) }103 /* Adjust the address for the data segment. We want to adjust up to104 the same address within the page on the next page up. */105 . = ALIGN (CONSTANT (MAXPAGESIZE)) - ((CONSTANT (MAXPAGESIZE) - .) & (CONSTANT (MAXPAGESIZE) - 1)); . = DATA_SEGMENT_ALIGN (CONSTANT (MAXPAGESIZE), CONSTANT (COMMONPAGESIZE));106 /* Exception handling */107 .eh_frame : ONLY_IF_RW { KEEP (*(.eh_frame)) }108 .gcc_except_table : ONLY_IF_RW { *(.gcc_except_table .gcc_except_table.*) }109 /* Thread Local Storage sections */110 .tdata : { *(.tdata .tdata.* .gnu.linkonce.td.*) }111 .tbss : { *(.tbss .tbss.* .gnu.linkonce.tb.*) *(.tcommon) }112 .preinit_array :113 {114 PROVIDE_HIDDEN (__preinit_array_start = .);115 KEEP (*(.preinit_array))116 PROVIDE_HIDDEN (__preinit_array_end = .);117 }118 .init_array :119 {120 PROVIDE_HIDDEN (__init_array_start = .);121 KEEP (*(SORT(.init_array.*)))122 KEEP (*(.init_array))123 PROVIDE_HIDDEN (__init_array_end = .);124 }125 .fini_array :126 {127 PROVIDE_HIDDEN (__fini_array_start = .);128 KEEP (*(.fini_array))129 KEEP (*(SORT(.fini_array.*)))130 PROVIDE_HIDDEN (__fini_array_end = .);131 }132 .ctors :133 {134 /* gcc uses crtbegin.o to find the start of135 the constructors, so we make sure it is136 first. Because this is a wildcard, it137 doesn't matter if the user does not138 actually link against crtbegin.o; the139 linker won't look for a file to match a140 wildcard. The wildcard also means that it141 doesn't matter which directory crtbegin.o142 is in. */143 KEEP (*crtbegin.o(.ctors))144 KEEP (*crtbegin?.o(.ctors))145 /* We don't want to include the .ctor section from146 the crtend.o file until after the sorted ctors.147 The .ctor section from the crtend file contains the148 end of ctors marker and it must be last */149 KEEP (*(EXCLUDE_FILE (*crtend.o *crtend?.o ) .ctors))150 KEEP (*(SORT(.ctors.*)))151 KEEP (*(.ctors))152 }153 .dtors :154 {155 KEEP (*crtbegin.o(.dtors))156 KEEP (*crtbegin?.o(.dtors))157 KEEP (*(EXCLUDE_FILE (*crtend.o *crtend?.o ) .dtors))158 KEEP (*(SORT(.dtors.*)))159 KEEP (*(.dtors))160 }161 .jcr : { KEEP (*(.jcr)) }162 .data.rel.ro : { *(.data.rel.ro.local* .gnu.linkonce.d.rel.ro.local.*) *(.data.rel.ro* .gnu.linkonce.d.rel.ro.*) }163 .dynamic : { *(.dynamic) }164 . = DATA_SEGMENT_RELRO_END (0, .);165 .got : { *(.got.plt) *(.igot.plt) *(.got) *(.igot) }166 .data :167 {168 PROVIDE (__data_start = .);169 *(.data .data.* .gnu.linkonce.d.*)170 SORT(CONSTRUCTORS)171 }172 .data1 : { *(.data1) }173 _edata = .; PROVIDE (edata = .);174 __bss_start = .;175 __bss_start__ = .;176 .bss : //找到了177 {178 *(.dynbss)179 *(.bss .bss.* .gnu.linkonce.b.*)180 *(COMMON)181 /* Align here to ensure that the .bss section occupies space up to182 _end. Align after .bss to ensure correct alignment even if the183 .bss section disappears because there are no input sections.184 FIXME: Why do we need it? When there is no .bss section, we don't185 pad the .data section. */186 . = ALIGN(. != 0 ? 32 / 8 : 1);187 }188 _bss_end__ = . ; __bss_end__ = . ;189 . = ALIGN(32 / 8);190 . = ALIGN(32 / 8);191 __end__ = . ;192 _end = .; PROVIDE (end = .);193 . = DATA_SEGMENT_END (.);194 /* Stabs debugging sections. */195 .stab 0 : { *(.stab) }196 .stabstr 0 : { *(.stabstr) }197 .stab.excl 0 : { *(.stab.excl) }198 .stab.exclstr 0 : { *(.stab.exclstr) }199 .stab.index 0 : { *(.stab.index) }200 .stab.indexstr 0 : { *(.stab.indexstr) }201 .comment 0 : { *(.comment) }202 /* DWARF debug sections.203 Symbols in the DWARF debugging sections are relative to the beginning204 of the section so we begin them at 0. */205 /* DWARF 1 */206 .debug 0 : { *(.debug) }207 .line 0 : { *(.line) }208 /* GNU DWARF 1 extensions */209 .debug_srcinfo 0 : { *(.debug_srcinfo) }210 .debug_sfnames 0 : { *(.debug_sfnames) }211 /* DWARF 1.1 and DWARF 2 */212 .debug_aranges 0 : { *(.debug_aranges) }213 .debug_pubnames 0 : { *(.debug_pubnames) }214 /* DWARF 2 */215 .debug_info 0 : { *(.debug_info .gnu.linkonce.wi.*) }216 .debug_abbrev 0 : { *(.debug_abbrev) }217 .debug_line 0 : { *(.debug_line) }218 .debug_frame 0 : { *(.debug_frame) }219 .debug_str 0 : { *(.debug_str) }220 .debug_loc 0 : { *(.debug_loc) }221 .debug_macinfo 0 : { *(.debug_macinfo) }222 /* SGI/MIPS DWARF 2 extensions */223 .debug_weaknames 0 : { *(.debug_weaknames) }224 .debug_funcnames 0 : { *(.debug_funcnames) }225 .debug_typenames 0 : { *(.debug_typenames) }226 .debug_varnames 0 : { *(.debug_varnames) }227 /* DWARF 3 */228 .debug_pubtypes 0 : { *(.debug_pubtypes) }229 .debug_ranges 0 : { *(.debug_ranges) }230 .gnu.attributes 0 : { KEEP (*(.gnu.attributes)) }231 .note.gnu.arm.ident 0 : { KEEP (*(.note.gnu.arm.ident)) }232 /DISCARD/ : { *(.note.GNU-stack) *(.gnu_debuglink) *(.gnu.lto_*) }233 }234 235 236 ================================================== 现在不使用默认的BSS段，使用自己写的BSS段。
OUTPUT_FORMAT("elf32-littlearm", "elf32-bigarm", "elf32-littlearm")OUTPUT_ARCH(arm)ENTRY(_start)SECTIONS{ . = 0x50000000; .text : { start.o *(.text) } . = ALIGN(4); laomi_data_start = .; .laomi : { *(.laomi) } laomi_data_end = .; .data : { *(.data) } . = ALIGN(4); _bss_start = .; .bss : { *(.bss) } . = ALIGN(4); _bss_end = .;} 1 #include2 #include 3 #include 4 #include 5 #include 6 #include 7 #include 8 9 u32 *T = (void *)0x60000000;10 11 int var0;12 int var1 = 0;13 14 int data = 9527;15 16 int glb0 __attribute__ ((unused, section(".jason")))= 7788;17 int glb1 __attribute__ ((unused, section(".jason")))= 8877;18 int glb2 __attribute__ ((unused, section(".jason")))= 8866;19 int glb3 __attribute__ ((unused, section(".jason")))= 8899;20 21 extern int jason_data_start;22 extern int jason_data_end;23 24 void entry(void)25 {26 int *p = &jason_data_start;27 28 while (p < &jason_data_end) {29 printf("*p = %d\n", *p);30 p++;31 }32 }
1 .PHONE:clean 2 3 TARGET := arm 4 BIN := $(TARGET).bin 5 LDADD := 0x50000000 6 ASM := start.o 7 OBJS := entry.o lib.o mmu.o 8 CROSS_COMPILE := arm-linux- 9 CC := $(CROSS_COMPILE)gcc10 LD := $(CROSS_COMPILE)ld11 OBJCOPY := $(CROSS_COMPILE)objcopy12 NM := $(CROSS_COMPILE)nm13 OBJDUMP := $(CROSS_COMPILE)objdump14 15 all:$(TARGET)16 $(OBJCOPY) -O binary $< $(BIN)17 $(TARGET):$(ASM) $(OBJS)18 $(LD) $(ASM) $(OBJS) -Tld.lds -o $@19 20 %.o:%.S21 $(CC) -c $^22 %.o:%.c23 $(CC) -c $^ -Iinclude -Wall -fno-builtin24 25 clean:26 rm -f *.o $(BIN) $(TARGET)27 28 29
并且指定链接脚本，拿到结果
