I want to create a c program that creates .o files for the linker on my computer (ld). Gcc does this when I do the command gcc -c myfile.c. Are there any resources that show how to make an object file for a linker?
In order to create a file similar to a file produced by gcc -c, it is important to first understand the format of the file produced.
First, create a file with gcc, and then see if it's format can be reverse-engineered. I will use the following C program (hello.c) to perform this task:
#include <stdio.h>
int main(void)
{
printf("Hello world\n");
return(0);
}
Now, compile the file without linking it:
gcc -Wall -c -o hello.o hello.c
The above code will create hello.o from hello.c. This file is compiled, but not yet linked.
Many files today contain bytes in the header of the file that help identify its format. These identifying bytes are termed 'file magic'. Google 'file magic' and you can find details on how to identify may types of files.
To identify this file type, look for a magic number in the first few lines of a hexdump of the file:
> hexdump -Cn 64 hello.o
00000000 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 |.ELF............|
00000010 02 00 3e 00 01 00 00 00 50 04 40 00 00 00 00 00 |..>.....P.@.....|
00000020 40 00 00 00 00 00 00 00 18 1a 00 00 00 00 00 00 |@...............|
00000030 00 00 00 00 40 00 38 00 09 00 40 00 2a 00 27 00 |....@.8...@.*.'.|
00000040
The identity of the file's format is revealed, in this case, by the first few bytes of the file; 45 4c 46 = ELF. Hence, the output of gcc -c is a file in the ElF format.
Hence, in order to create similar output from a C program, an understanding of the ELF file format is required.
Many *nix systems include a program called readelf that will translate the contents of an ELF file. For example:
> readelf -a hello.o
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x400450
Start of program headers: 64 (bytes into file)
Start of section headers: 6680 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 56 (bytes)
Number of program headers: 9
Size of section headers: 64 (bytes)
Number of section headers: 42
Section header string table index: 39
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .interp PROGBITS 0000000000400238 00000238
000000000000001c 0000000000000000 A 0 0 1
[ 2] .note.ABI-tag NOTE 0000000000400254 00000254
0000000000000020 0000000000000000 A 0 0 4
[ 3] .note.SuSE NOTE 0000000000400274 00000274
0000000000000018 0000000000000000 A 0 0 4
[ 4] .note.gnu.build-i NOTE 000000000040028c 0000028c
0000000000000024 0000000000000000 A 0 0 4
[ 5] .hash HASH 00000000004002b0 000002b0
0000000000000024 0000000000000004 A 7 0 8
[ 6] .gnu.hash GNU_HASH 00000000004002d8 000002d8
000000000000001c 0000000000000000 A 7 0 8
[ 7] .dynsym DYNSYM 00000000004002f8 000002f8
0000000000000060 0000000000000018 A 8 1 8
[ 8] .dynstr STRTAB 0000000000400358 00000358
000000000000003d 0000000000000000 A 0 0 1
[ 9] .gnu.version VERSYM 0000000000400396 00000396
0000000000000008 0000000000000002 A 7 0 2
[10] .gnu.version_r VERNEED 00000000004003a0 000003a0
0000000000000020 0000000000000000 A 8 1 8
[11] .rela.dyn RELA 00000000004003c0 000003c0
0000000000000018 0000000000000018 A 7 0 8
[12] .rela.plt RELA 00000000004003d8 000003d8
0000000000000030 0000000000000018 A 7 14 8
[13] .init PROGBITS 0000000000400408 00000408
0000000000000018 0000000000000000 AX 0 0 4
[14] .plt PROGBITS 0000000000400420 00000420
0000000000000030 0000000000000010 AX 0 0 16
[15] .text PROGBITS 0000000000400450 00000450
00000000000001e8 0000000000000000 AX 0 0 16
[16] .fini PROGBITS 0000000000400638 00000638
0000000000000016 0000000000000000 AX 0 0 4
[17] .rodata PROGBITS 0000000000400650 00000650
0000000000000010 0000000000000000 A 0 0 4
[18] .eh_frame_hdr PROGBITS 0000000000400660 00000660
0000000000000034 0000000000000000 A 0 0 4
[19] .eh_frame PROGBITS 0000000000400698 00000698
00000000000000dc 0000000000000000 A 0 0 8
[20] .ctors PROGBITS 0000000000600e30 00000e30
0000000000000010 0000000000000000 WA 0 0 8
[21] .dtors PROGBITS 0000000000600e40 00000e40
0000000000000010 0000000000000000 WA 0 0 8
[22] .jcr PROGBITS 0000000000600e50 00000e50
0000000000000008 0000000000000000 WA 0 0 8
[23] .dynamic DYNAMIC 0000000000600e58 00000e58
00000000000001a0 0000000000000010 WA 8 0 8
[24] .got PROGBITS 0000000000600ff8 00000ff8
0000000000000008 0000000000000008 WA 0 0 8
[25] .got.plt PROGBITS 0000000000601000 00001000
0000000000000028 0000000000000008 WA 0 0 8
[26] .data PROGBITS 0000000000601028 00001028
0000000000000010 0000000000000000 WA 0 0 8
[27] .bss NOBITS 0000000000601038 00001038
0000000000000010 0000000000000000 WA 0 0 8
[28] .comment PROGBITS 0000000000000000 00001038
0000000000000039 0000000000000001 MS 0 0 1
[29] .comment.SUSE.OPT PROGBITS 0000000000000000 00001071
0000000000000006 0000000000000001 MS 0 0 1
[30] .debug_aranges PROGBITS 0000000000000000 00001080
0000000000000060 0000000000000000 0 0 16
[31] .debug_pubnames PROGBITS 0000000000000000 000010e0
000000000000005f 0000000000000000 0 0 1
[32] .debug_info PROGBITS 0000000000000000 0000113f
0000000000000232 0000000000000000 0 0 1
[33] .debug_abbrev PROGBITS 0000000000000000 00001371
0000000000000133 0000000000000000 0 0 1
[34] .debug_line PROGBITS 0000000000000000 000014a4
000000000000011e 0000000000000000 0 0 1
[35] .debug_frame PROGBITS 0000000000000000 000015c8
0000000000000058 0000000000000000 0 0 8
[36] .debug_str PROGBITS 0000000000000000 00001620
0000000000000115 0000000000000001 MS 0 0 1
[37] .debug_loc PROGBITS 0000000000000000 00001735
00000000000000fe 0000000000000000 0 0 1
[38] .debug_ranges PROGBITS 0000000000000000 00001833
0000000000000050 0000000000000000 0 0 1
[39] .shstrtab STRTAB 0000000000000000 00001883
0000000000000192 0000000000000000 0 0 1
[40] .symtab SYMTAB 0000000000000000 00002498
00000000000007c8 0000000000000018 41 65 8
[41] .strtab STRTAB 0000000000000000 00002c60
0000000000000244 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
There are no section groups in this file.
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
PHDR 0x0000000000000040 0x0000000000400040 0x0000000000400040
0x00000000000001f8 0x00000000000001f8 R E 8
INTERP 0x0000000000000238 0x0000000000400238 0x0000000000400238
0x000000000000001c 0x000000000000001c R 1
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000
0x0000000000000774 0x0000000000000774 R E 200000
LOAD 0x0000000000000e30 0x0000000000600e30 0x0000000000600e30
0x0000000000000208 0x0000000000000218 RW 200000
DYNAMIC 0x0000000000000e58 0x0000000000600e58 0x0000000000600e58
0x00000000000001a0 0x00000000000001a0 RW 8
NOTE 0x0000000000000254 0x0000000000400254 0x0000000000400254
0x000000000000005c 0x000000000000005c R 4
GNU_EH_FRAME 0x0000000000000660 0x0000000000400660 0x0000000000400660
0x0000000000000034 0x0000000000000034 R 4
GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000 RW 8
GNU_RELRO 0x0000000000000e30 0x0000000000600e30 0x0000000000600e30
0x00000000000001d0 0x00000000000001d0 R 1
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .note.ABI-tag .note.SuSE .note.gnu.build-id .hash .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame
03 .ctors .dtors .jcr .dynamic .got .got.plt .data .bss
04 .dynamic
05 .note.ABI-tag .note.SuSE .note.gnu.build-id
06 .eh_frame_hdr
07
08 .ctors .dtors .jcr .dynamic .got
Dynamic section at offset 0xe58 contains 21 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000c (INIT) 0x400408
0x000000000000000d (FINI) 0x400638
0x0000000000000004 (HASH) 0x4002b0
0x000000006ffffef5 (GNU_HASH) 0x4002d8
0x0000000000000005 (STRTAB) 0x400358
0x0000000000000006 (SYMTAB) 0x4002f8
0x000000000000000a (STRSZ) 61 (bytes)
0x000000000000000b (SYMENT) 24 (bytes)
0x0000000000000015 (DEBUG) 0x0
0x0000000000000003 (PLTGOT) 0x601000
0x0000000000000002 (PLTRELSZ) 48 (bytes)
0x0000000000000014 (PLTREL) RELA
0x0000000000000017 (JMPREL) 0x4003d8
0x0000000000000007 (RELA) 0x4003c0
0x0000000000000008 (RELASZ) 24 (bytes)
0x0000000000000009 (RELAENT) 24 (bytes)
0x000000006ffffffe (VERNEED) 0x4003a0
0x000000006fffffff (VERNEEDNUM) 1
0x000000006ffffff0 (VERSYM) 0x400396
0x0000000000000000 (NULL) 0x0
Relocation section '.rela.dyn' at offset 0x3c0 contains 1 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000600ff8 000300000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ + 0
Relocation section '.rela.plt' at offset 0x3d8 contains 2 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000601018 000100000007 R_X86_64_JUMP_SLO 0000000000000000 puts + 0
000000601020 000200000007 R_X86_64_JUMP_SLO 0000000000000000 __libc_start_main + 0
The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.
Symbol table '.dynsym' contains 4 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FUNC GLOBAL DEFAULT UND puts@GLIBC_2.2.5 (2)
2: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.2.5 (2)
3: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
Symbol table '.symtab' contains 83 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000400238 0 SECTION LOCAL DEFAULT 1
2: 0000000000400254 0 SECTION LOCAL DEFAULT 2
3: 0000000000400274 0 SECTION LOCAL DEFAULT 3
4: 000000000040028c 0 SECTION LOCAL DEFAULT 4
5: 00000000004002b0 0 SECTION LOCAL DEFAULT 5
6: 00000000004002d8 0 SECTION LOCAL DEFAULT 6
7: 00000000004002f8 0 SECTION LOCAL DEFAULT 7
8: 0000000000400358 0 SECTION LOCAL DEFAULT 8
9: 0000000000400396 0 SECTION LOCAL DEFAULT 9
10: 00000000004003a0 0 SECTION LOCAL DEFAULT 10
11: 00000000004003c0 0 SECTION LOCAL DEFAULT 11
12: 00000000004003d8 0 SECTION LOCAL DEFAULT 12
13: 0000000000400408 0 SECTION LOCAL DEFAULT 13
14: 0000000000400420 0 SECTION LOCAL DEFAULT 14
15: 0000000000400450 0 SECTION LOCAL DEFAULT 15
16: 0000000000400638 0 SECTION LOCAL DEFAULT 16
17: 0000000000400650 0 SECTION LOCAL DEFAULT 17
18: 0000000000400660 0 SECTION LOCAL DEFAULT 18
19: 0000000000400698 0 SECTION LOCAL DEFAULT 19
20: 0000000000600e30 0 SECTION LOCAL DEFAULT 20
21: 0000000000600e40 0 SECTION LOCAL DEFAULT 21
22: 0000000000600e50 0 SECTION LOCAL DEFAULT 22
23: 0000000000600e58 0 SECTION LOCAL DEFAULT 23
24: 0000000000600ff8 0 SECTION LOCAL DEFAULT 24
25: 0000000000601000 0 SECTION LOCAL DEFAULT 25
26: 0000000000601028 0 SECTION LOCAL DEFAULT 26
27: 0000000000601038 0 SECTION LOCAL DEFAULT 27
28: 0000000000000000 0 SECTION LOCAL DEFAULT 28
29: 0000000000000000 0 SECTION LOCAL DEFAULT 29
30: 0000000000000000 0 SECTION LOCAL DEFAULT 30
31: 0000000000000000 0 SECTION LOCAL DEFAULT 31
32: 0000000000000000 0 SECTION LOCAL DEFAULT 32
33: 0000000000000000 0 SECTION LOCAL DEFAULT 33
34: 0000000000000000 0 SECTION LOCAL DEFAULT 34
35: 0000000000000000 0 SECTION LOCAL DEFAULT 35
36: 0000000000000000 0 SECTION LOCAL DEFAULT 36
37: 0000000000000000 0 SECTION LOCAL DEFAULT 37
38: 0000000000000000 0 SECTION LOCAL DEFAULT 38
39: 0000000000000000 0 FILE LOCAL DEFAULT ABS init.c
40: 0000000000000000 0 FILE LOCAL DEFAULT ABS
41: 0000000000000000 0 FILE LOCAL DEFAULT ABS initfini.c
42: 000000000040047c 0 FUNC LOCAL DEFAULT 15 call_gmon_start
43: 0000000000400648 0 NOTYPE LOCAL DEFAULT 16 _real_fini
44: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
45: 0000000000600e30 0 OBJECT LOCAL DEFAULT 20 __CTOR_LIST__
46: 0000000000600e40 0 OBJECT LOCAL DEFAULT 21 __DTOR_LIST__
47: 0000000000600e50 0 OBJECT LOCAL DEFAULT 22 __JCR_LIST__
48: 00000000004004a0 0 FUNC LOCAL DEFAULT 15 __do_global_dtors_aux
49: 0000000000601038 1 OBJECT LOCAL DEFAULT 27 completed.6159
50: 0000000000601040 8 OBJECT LOCAL DEFAULT 27 dtor_idx.6161
51: 0000000000400510 0 FUNC LOCAL DEFAULT 15 frame_dummy
52: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
53: 0000000000600e38 0 OBJECT LOCAL DEFAULT 20 __CTOR_END__
54: 0000000000400770 0 OBJECT LOCAL DEFAULT 19 __FRAME_END__
55: 0000000000600e50 0 OBJECT LOCAL DEFAULT 22 __JCR_END__
56: 0000000000400600 0 FUNC LOCAL DEFAULT 15 __do_global_ctors_aux
57: 0000000000000000 0 FILE LOCAL DEFAULT ABS initfini.c
58: 0000000000000000 0 FILE LOCAL DEFAULT ABS 24173361_generating-an-ob
59: 0000000000000000 0 FILE LOCAL DEFAULT ABS elf-init.c
60: 0000000000000000 0 FILE LOCAL DEFAULT ABS
61: 0000000000600e2c 0 NOTYPE LOCAL DEFAULT 20 __init_array_end
62: 0000000000600e58 0 OBJECT LOCAL DEFAULT 23 _DYNAMIC
63: 0000000000600e2c 0 NOTYPE LOCAL DEFAULT 20 __init_array_start
64: 0000000000601000 0 OBJECT LOCAL DEFAULT 25 _GLOBAL_OFFSET_TABLE_
65: 0000000000400560 2 FUNC GLOBAL DEFAULT 15 __libc_csu_fini
66: 0000000000601028 0 NOTYPE WEAK DEFAULT 26 data_start
67: 0000000000000000 0 FUNC GLOBAL DEFAULT UND puts@@GLIBC_2.2.5
68: 0000000000601038 0 NOTYPE GLOBAL DEFAULT 26 _edata
69: 0000000000400638 16 FUNC GLOBAL DEFAULT 16 _fini
70: 0000000000600e48 0 OBJECT GLOBAL HIDDEN 21 __DTOR_END__
71: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@@GLIBC_
72: 0000000000601028 0 NOTYPE GLOBAL DEFAULT 26 __data_start
73: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
74: 0000000000601030 0 OBJECT GLOBAL HIDDEN 26 __dso_handle
75: 0000000000400650 4 OBJECT GLOBAL DEFAULT 17 _IO_stdin_used
76: 0000000000400570 137 FUNC GLOBAL DEFAULT 15 __libc_csu_init
77: 0000000000601048 0 NOTYPE GLOBAL DEFAULT 27 _end
78: 0000000000400450 0 FUNC GLOBAL DEFAULT 15 _start
79: 0000000000601038 0 NOTYPE GLOBAL DEFAULT 27 __bss_start
80: 000000000040053c 21 FUNC GLOBAL DEFAULT 15 main
81: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _Jv_RegisterClasses
82: 0000000000400408 0 FUNC GLOBAL DEFAULT 13 _init
Histogram for bucket list length (total of 3 buckets):
Length Number % of total Coverage
0 1 ( 33.3%)
1 1 ( 33.3%) 33.3%
2 1 ( 33.3%) 100.0%
Version symbols section '.gnu.version' contains 4 entries:
Addr: 0000000000400396 Offset: 0x000396 Link: 7 (.dynsym)
000: 0 (*local*) 2 (GLIBC_2.2.5) 2 (GLIBC_2.2.5) 0 (*local*)
Version needs section '.gnu.version_r' contains 1 entries:
Addr: 0x00000000004003a0 Offset: 0x0003a0 Link: 8 (.dynstr)
000000: Version: 1 File: libc.so.6 Cnt: 1
0x0010: Name: GLIBC_2.2.5 Flags: none Version: 2
Notes at offset 0x00000254 with length 0x00000020:
Owner Data size Description
GNU 0x00000010 NT_GNU_ABI_TAG (ABI version tag)
OS: Linux, ABI: 2.6.4
Notes at offset 0x00000274 with length 0x00000018:
Owner Data size Description
SuSE 0x00000004 Unknown note type: (0x45537553)
Notes at offset 0x0000028c with length 0x00000024:
Owner Data size Description
GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
Build ID: 710ef458701fed230f91233a227b7b10c024e2ed
While, that is a lot of detail for such a little file. Here is how to break it down problematically. First, the elf(64) header:
typedef struct elf64_hdr {
unsigned char e_ident[EI_NIDENT]; /* ELF "magic number" */
Elf64_Half e_type;
Elf64_Half e_machine;
Elf64_Word e_version;
Elf64_Addr e_entry; /* Entry point virtual address */
Elf64_Off e_phoff; /* Program header table file offset */
Elf64_Off e_shoff; /* Section header table file offset */
Elf64_Word e_flags;
Elf64_Half e_ehsize;
Elf64_Half e_phentsize;
Elf64_Half e_phnum;
Elf64_Half e_shentsize;
Elf64_Half e_shnum;
Elf64_Half e_shstrndx;
} Elf64_Ehdr;
The secret to creating an elf file is to understand the above structure. This is the first bit of the file your C program must write. Although too lengthy to detail here, you can find this structure in elf.h; which is located (on most Linux systems) here: /usr/include/elf.h.