Can someone explain me what _IO_stdin_used is in the following line:
114a: 48 8d 3d b3 0e 00 00 lea rdi,[rip+0xeb3] # 2004 <_IO_stdin_used+0x4>
Sorry for the noob question.
Expanding on https://stackoverflow.com/users/224132/peter-cordes 's comment (What is _IO_stdin_used ), some tools can help make sense of this. Starting with burt.c:
#include <stdio.h>
void main (void) {
puts("hello!");
}
built like so: LDFLAGS=-Wl,-Map=burt.map make burt
We will look at the map file in a moment, first inspect the executable:
$ objdump -xsd -M intel burt
...
0000000000002000 g O .rodata 0000000000000004 _IO_stdin_used
...
Contents of section .rodata:
2000 01000200 68656c6c 6f2100 ....hello!.
...
1148: 48 8d 05 b5 0e 00 00 lea rax,[rip+0xeb5] # 2004 <_IO_stdin_used+0x4>
114f: 48 89 c7 mov rdi,rax
1152: e8 d9 fe ff ff call 1030 <puts#plt>
...
Note, the string's location (2004h) does not have an entry in the symbol table (where the 2000h entry _IO_stdin_used occurs). So, objdump's disassembly comments mention it relative to another symbol it knows about.
But why does it know about _IO_stdin_used, or, why does is that symbol included and why does it have a name? burt.map shows it came from Scrt1.o:
...
.rodata 0x0000000000002000 0xb
*(.rodata .rodata.* .gnu.linkonce.r.*)
.rodata.cst4 0x0000000000002000 0x4 /usr/lib/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../lib/Scrt1.o
.rodata 0x0000000000002004 0x7 /tmp/ccYG1XvK.o
...
On my computer, Scrt1.o belongs to glibc, and the symbol can be traced back here: https://sourceware.org/git/?p=glibc.git;a=blob;f=csu/init.c;h=c2f978f3da565590bcab355fefa3d81cf211cb36;hb=63fb8f9aa9d19f85599afe4b849b567aefd70a36.
To show that this is displayed in the objdump disassembly because there is no name for the address, we can give our string a name to add it to the symbol table, so that objdump will show it instead. To do this let's modify burt.c like so:
#include <stdio.h>
const char greeting[] = "hello!";
void main (void) {
puts(greeting);
}
Build, then inspect:
...
0000000000002004 g O .rodata 0000000000000007 greeting
...
0000000000002000 g O .rodata 0000000000000004 _IO_stdin_used
...
Contents of section .rodata:
2000 01000200 68656c6c 6f2100 ....hello!.
...
113d: 48 8d 05 c0 0e 00 00 lea rax,[rip+0xec0] # 2004 <greeting>
1144: 48 89 c7 mov rdi,rax
1147: e8 e4 fe ff ff call 1030 <puts#plt>
The offset relative to rip has changed a little bit, because the .text and .rodata sections are now a little bit further apart, although the .rodata contents remain the same, because the source modification was chosen to avoid modifying .rodata -- if greetings were not const, it would be located in .data instead. The map file below now shows burt.c's temporary object files's contribution to .rodata defining the greeting symbol, as compared to the previous map file where the same data had no name:
.rodata 0x0000000000002000 0xb
*(.rodata .rodata.* .gnu.linkonce.r.*)
.rodata.cst4 0x0000000000002000 0x4 /usr/lib/gcc/x86_64-pc-linux-gnu/12.2.1/../../../..
/lib/Scrt1.o
0x0000000000002000 _IO_stdin_used
.rodata 0x0000000000002004 0x7 /tmp/ccDnWXjG.o
0x0000000000002004 greeting
So I'm taking a class where I am given a single object file and need to reverse engineer it into c++ code. The command I'm told to use is "gdb assignment6_1.o" to open it in gdb, and "disass main" to see assembly code.
I'm also using "objdump -dr assignment6_1.o" myself since it outputs a little more information.
The problem I'm running into, is that using objdump, I can see that the program is trying to access what I believe is a variable or maybe a string, ".rodata+0x41". There are multiple .rodata's, that's just one example.
Is there a command or somewhere I can look to see what that's referencing? I also have access to the "Bless" program.
Below is a snippet of the disassembled code I have.
a3: 48 8d 35 00 00 00 00 lea 0x0(%rip),%rsi # aa <main+0x31>
a6: R_X86_64_PC32 .rodata+0x41
aa: 48 8d 3d 00 00 00 00 lea 0x0(%rip),%rdi # b1 <main+0x38>
ad: R_X86_64_PC32 _ZSt4cout-0x4
b1: e8 00 00 00 00 callq b6 <main+0x3d>
b2: R_X86_64_PLT32 _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc-0x4
b6: 48 8d 35 00 00 00 00 lea 0x0(%rip),%rsi # bd <main+0x44>
b9: R_X86_64_PC32 .rodata+0x53
bd: 48 8d 3d 00 00 00 00 lea 0x0(%rip),%rdi # c4 <main+0x4b>
c0: R_X86_64_PC32 _ZSt4cout-0x4
c4: e8 00 00 00 00 callq c9 <main+0x50>
c5: R_X86_64_PLT32 _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc-0x4
c9: 48 8d 35 00 00 00 00 lea 0x0(%rip),%rsi # d0 <main+0x57>
cc: R_X86_64_PC32 .rodata+0x5e
d0: 48 8d 3d 00 00 00 00 lea 0x0(%rip),%rdi # d7 <main+0x5e>
d3: R_X86_64_PC32 _ZSt4cout-0x4
d7: e8 00 00 00 00 callq dc <main+0x63>
d8: R_X86_64_PLT32 _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc-0x4
dc: 48 8d 35 00 00 00 00 lea 0x0(%rip),%rsi # e3 <main+0x6a>
df: R_X86_64_PC32 .rodata+0x6e
e3: 48 8d 3d 00 00 00 00 lea 0x0(%rip),%rdi # ea <main+0x71>
e6: R_X86_64_PC32 _ZSt4cout-0x4
ea: e8 00 00 00 00 callq ef <main+0x76>
eb: R_X86_64_PLT32 _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc-0x4```
Is there a way to see what's inside a ".rodata+(memory location)" in an object file?
Sure. Both objdump and readelf can dump contents of any section.
Example:
// x.c
#include <stdio.h>
int foo() { return printf("AA.\n") + printf("BBBB.\n"); }
gcc -c x.c
objdump -dr x.o
...
9: 48 8d 05 00 00 00 00 lea 0x0(%rip),%rax # 10 <foo+0x10>
c: R_X86_64_PC32 .rodata-0x4
...
1f: 48 8d 05 00 00 00 00 lea 0x0(%rip),%rax # 26 <foo+0x26>
22: R_X86_64_PC32 .rodata+0x1
...
Note that because the RIP used in these instructions is the address of the next instruction, the actual data we care about is at .rodata+0 and .rodata+5 (in your original disassembly, you care about .rodata+45, not .rodata+41).
So what's there?
objdump -sj.rodata x.o
x.o: file format elf64-x86-64
Contents of section .rodata:
0000 41412e0a 00424242 422e0a00 AA...BBBB...
or, using readelf:
readelf -x .rodata x.o
Hex dump of section '.rodata':
0x00000000 41412e0a 00424242 422e0a00 AA...BBBB...
Compiling this code with -O3:
#include <iostream>
int main(){std::cout<<"Hello World"<<std::endl;}
results in a file with a length of 25,890 bytes. (Compiled with GCC 4.8.1)
Can't the compiler just store two calls to write(STDOUT_FILENO, ???, strlen(???));, store write's contents, store the string, and boom write it to the disk? It should result in a EXE with a length under 1,024 bytes to my estimate.
Compiling a hello world program in assembly results in 17 bytes file: https://stackoverflow.com/questions/284797/hello-world-in-less-than-17-bytes, means actual code is 5-bytes long. (The string is Hello World\0)
What that EXE stores except the actual main and the functions it calls?
NOTE: This question applies to MSVC too.
Edit:
A lot of users pointed at iostream as being the culprit, so I tested this hypothesis and compiled this program with the same parameters:
int main( ) {
}
And got 23,815 bytes, the hypothesis has been disproved.
The compiler generates by default a complete PE-conformant executable. Assuming a release build, the simple code you posted might probably include:
all the PE headers and tables needed by the loader (e.g. IAT), this also means alignment requirements have to be met
CRT library initialization code
Debugging info (you need to manually drop these off even for a release build)
In case the compiler were MSVC there would have been additional inclusions:
Manifest xml and relocation data
Results of default compiler options that favor speed over size
The link you posted does contain a very small assembly "hello world" program, but in order to properly run in a Windows environment at least the complete and valid PE structure needs to be available to the loader (setting aside all the low-level issues that might cause that code not to run at all).
Assuming the loader had already and correctly 'set up' the process where to run that code into, only at that point you could map it into a PE section and do
jmp small_hello_world_entry_point
to actually execute the code.
References: The PE format
One last notice: UPX and similar compression tools are also used to reduce filesize for executables.
C++ isn't assembly, like C it comes with a lot of infrastructure. In addition to the overheads of C - required to be compatible with the C abi - C++ also has its own variants of many things, and it also has to have all the tear-up and -down code required to provide the many guarantees of the language.
Much of these are provided by libraries, but some of it has to be in the executable itself so that a failure to load shared libraries could be handled.
Under Linux/BSD we can reverse engineer an executable with objdump -dsl. I took the following code:
int main() {}
and compiled it with:
g++ -Wall -O3 -g0 test.cpp -o test.exe
The resulting executable?
6922 bytes
Then I compiled with less cruft:
g++ -Wall -O3 -g0 test.cpp -o test.exe -nostdlib
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000400150
Basically: main is a facade entry point for our C++ code, the program really starts at _start.
Executable size?
1454 bytes
Here's how objdump describes the two:
g++ -Wall -O3 -g0 test.cpp -o test.exe
objdump -test.exe
test.exe: file format elf64-x86-64
Contents of section .interp:
400200 2f6c6962 36342f6c 642d6c69 6e75782d /lib64/ld-linux-
400210 7838362d 36342e73 6f2e3200 x86-64.so.2.
Contents of section .note.ABI-tag:
40021c 04000000 10000000 01000000 474e5500 ............GNU.
40022c 00000000 02000000 06000000 12000000 ................
Contents of section .note.gnu.build-id:
40023c 04000000 14000000 03000000 474e5500 ............GNU.
40024c a0f55c7d 671f9eb2 93078fd3 0f52581a ..\}g........RX.
40025c 544829b2 TH).
Contents of section .hash:
400260 03000000 06000000 02000000 05000000 ................
400270 00000000 00000000 00000000 01000000 ................
400280 00000000 03000000 04000000 ............
Contents of section .dynsym:
400290 00000000 00000000 00000000 00000000 ................
4002a0 00000000 00000000 10000000 20000000 ............ ...
4002b0 00000000 00000000 00000000 00000000 ................
4002c0 1f000000 20000000 00000000 00000000 .... ...........
4002d0 00000000 00000000 8b000000 12000000 ................
4002e0 00000000 00000000 00000000 00000000 ................
4002f0 33000000 20000000 00000000 00000000 3... ...........
400300 00000000 00000000 4f000000 20000000 ........O... ...
400310 00000000 00000000 00000000 00000000 ................
Contents of section .dynstr:
400320 006c6962 73746463 2b2b2e73 6f2e3600 .libstdc++.so.6.
400330 5f5f676d 6f6e5f73 74617274 5f5f005f __gmon_start__._
400340 4a765f52 65676973 74657243 6c617373 Jv_RegisterClass
400350 6573005f 49544d5f 64657265 67697374 es._ITM_deregist
400360 6572544d 436c6f6e 65546162 6c65005f erTMCloneTable._
400370 49544d5f 72656769 73746572 544d436c ITM_registerTMCl
400380 6f6e6554 61626c65 006c6962 6d2e736f oneTable.libm.so
400390 2e36006c 69626763 635f732e 736f2e31 .6.libgcc_s.so.1
4003a0 006c6962 632e736f 2e36005f 5f6c6962 .libc.so.6.__lib
4003b0 635f7374 6172745f 6d61696e 00474c49 c_start_main.GLI
4003c0 42435f32 2e322e35 00 BC_2.2.5.
Contents of section .gnu.version:
4003ca 00000000 00000200 00000000 ............
Contents of section .gnu.version_r:
4003d8 01000100 81000000 10000000 00000000 ................
4003e8 751a6909 00000200 9d000000 00000000 u.i.............
Contents of section .rela.dyn:
4003f8 50096000 00000000 06000000 01000000 P.`.............
400408 00000000 00000000 ........
Contents of section .rela.plt:
400410 70096000 00000000 07000000 03000000 p.`.............
400420 00000000 00000000 ........
Contents of section .init:
400428 4883ec08 e85b0000 00e86a01 0000e845 H....[....j....E
400438 02000048 83c408c3 ...H....
Contents of section .plt:
400440 ff351a05 2000ff25 1c052000 0f1f4000 .5.. ..%.. ...#.
400450 ff251a05 20006800 000000e9 e0ffffff .%.. .h.........
Contents of section .text:
400460 31ed4989 d15e4889 e24883e4 f0505449 1.I..^H..H...PTI
400470 c7c0e005 400048c7 c1f00540 0048c7c7 ....#.H....#.H..
400480 d0054000 e8c7ffff fff49090 4883ec08 ..#.........H...
400490 488b05b9 04200048 85c07402 ffd04883 H.... .H..t...H.
4004a0 c408c390 90909090 90909090 90909090 ................
4004b0 90909090 90909090 90909090 90909090 ................
4004c0 b88f0960 00482d88 09600048 83f80e76 ...`.H-..`.H...v
4004d0 17b80000 00004885 c0740dbf 88096000 ......H..t....`.
4004e0 ffe0660f 1f440000 f3c3660f 1f440000 ..f..D....f..D..
4004f0 be880960 004881ee 88096000 48c1fe03 ...`.H....`.H...
400500 4889f048 c1e83f48 01c648d1 fe7411b8 H..H..?H..H..t..
400510 00000000 4885c074 07bf8809 6000ffe0 ....H..t....`...
400520 f3c36666 6666662e 0f1f8400 00000000 ..fffff.........
400530 803d5104 20000075 5f5553bb 80076000 .=Q. ..u_US...`.
400540 4881eb78 07600048 83ec0848 8b053e04 H..x.`.H...H..>.
400550 200048c1 fb034883 eb01488d 6c241048 .H...H...H.l$.H
400560 39d87322 0f1f4000 4883c001 4889051d 9.s"..#.H...H...
400570 042000ff 14c57807 6000488b 050f0420 . ....x.`.H....
400580 004839d8 72e2e835 ffffffc6 05f60320 .H9.r..5.......
400590 00014883 c4085b5d f3c3660f 1f440000 ..H...[]..f..D..
4005a0 bf880760 0048833f 007505e9 40ffffff ...`.H.?.u..#...
4005b0 b8000000 004885c0 74f15548 89e5ffd0 .....H..t.UH....
4005c0 5de92aff ffff9090 90909090 90909090 ].*.............
4005d0 31c0c390 90909090 90909090 90909090 1...............
4005e0 f3c36666 6666662e 0f1f8400 00000000 ..fffff.........
4005f0 48896c24 d84c8964 24e0488d 2d630120 H.l$.L.d$.H.-c.
400600 004c8d25 5c012000 4c896c24 e84c8974 .L.%\. .L.l$.L.t
400610 24f04c89 7c24f848 895c24d0 4883ec38 $.L.|$.H.\$.H..8
400620 4c29e541 89fd4989 f648c1fd 034989d7 L).A..I..H...I..
400630 e8f3fdff ff4885ed 741c31db 0f1f4000 .....H..t.1...#.
400640 4c89fa4c 89f64489 ef41ff14 dc4883c3 L..L..D..A...H..
400650 014839eb 72ea488b 5c240848 8b6c2410 .H9.r.H.\$.H.l$.
400660 4c8b6424 184c8b6c 24204c8b 7424284c L.d$.L.l$ L.t$(L
400670 8b7c2430 4883c438 c3909090 90909090 .|$0H..8........
400680 554889e5 53bb6807 60004883 ec08488b UH..S.h.`.H...H.
400690 05d30020 004883f8 ff74140f 1f440000 ... .H...t...D..
4006a0 4883eb08 ffd0488b 034883f8 ff75f148 H.....H..H...u.H
4006b0 83c4085b 5dc39090 ...[]...
Contents of section .fini:
4006b8 4883ec08 e86ffeff ff4883c4 08c3 H....o...H....
Contents of section .rodata:
4006c8 01000200 ....
Contents of section .eh_frame_hdr:
4006cc 011b033b 20000000 03000000 04ffffff ...; ...........
4006dc 3c000000 14ffffff 54000000 24ffffff <.......T...$...
4006ec 6c000000 l...
Contents of section .eh_frame:
4006f0 14000000 00000000 017a5200 01781001 .........zR..x..
400700 1b0c0708 90010000 14000000 1c000000 ................
400710 c0feffff 03000000 00000000 00000000 ................
400720 14000000 34000000 b8feffff 02000000 ....4...........
400730 00000000 00000000 24000000 4c000000 ........$...L...
400740 b0feffff 89000000 00518c05 86065f0e .........Q...._.
400750 4083078f 028e038d 0402580e 08000000 #.........X.....
400760 00000000 ....
Contents of section .ctors:
600768 ffffffff ffffffff 00000000 00000000 ................
Contents of section .dtors:
600778 ffffffff ffffffff 00000000 00000000 ................
Contents of section .jcr:
600788 00000000 00000000 ........
Contents of section .dynamic:
600790 01000000 00000000 01000000 00000000 ................
6007a0 01000000 00000000 69000000 00000000 ........i.......
6007b0 01000000 00000000 73000000 00000000 ........s.......
6007c0 01000000 00000000 81000000 00000000 ................
6007d0 0c000000 00000000 28044000 00000000 ........(.#.....
6007e0 0d000000 00000000 b8064000 00000000 ..........#.....
6007f0 04000000 00000000 60024000 00000000 ........`.#.....
600800 05000000 00000000 20034000 00000000 ........ .#.....
600810 06000000 00000000 90024000 00000000 ..........#.....
600820 0a000000 00000000 a9000000 00000000 ................
600830 0b000000 00000000 18000000 00000000 ................
600840 15000000 00000000 00000000 00000000 ................
600850 03000000 00000000 58096000 00000000 ........X.`.....
600860 02000000 00000000 18000000 00000000 ................
600870 14000000 00000000 07000000 00000000 ................
600880 17000000 00000000 10044000 00000000 ..........#.....
600890 07000000 00000000 f8034000 00000000 ..........#.....
6008a0 08000000 00000000 18000000 00000000 ................
6008b0 09000000 00000000 18000000 00000000 ................
6008c0 feffff6f 00000000 d8034000 00000000 ...o......#.....
6008d0 ffffff6f 00000000 01000000 00000000 ...o............
6008e0 f0ffff6f 00000000 ca034000 00000000 ...o......#.....
6008f0 00000000 00000000 00000000 00000000 ................
600900 00000000 00000000 00000000 00000000 ................
600910 00000000 00000000 00000000 00000000 ................
600920 00000000 00000000 00000000 00000000 ................
600930 00000000 00000000 00000000 00000000 ................
600940 00000000 00000000 00000000 00000000 ................
Contents of section .got:
600950 00000000 00000000 ........
Contents of section .got.plt:
600958 90076000 00000000 00000000 00000000 ..`.............
600968 00000000 00000000 56044000 00000000 ........V.#.....
Contents of section .data:
600978 00000000 00000000 00000000 00000000 ................
Contents of section .comment:
0000 4743433a 2028474e 55292034 2e342e37 GCC: (GNU) 4.4.7
0010 20323031 32303331 33202852 65642048 20120313 (Red H
0020 61742034 2e342e37 2d313129 00474343 at 4.4.7-11).GCC
0030 3a202847 4e552920 342e392e 782d676f : (GNU) 4.9.x-go
0040 6f676c65 20323031 35303132 33202870 ogle 20150123 (p
0050 72657265 6c656173 652900 rerelease).
Disassembly of section .init:
0000000000400428 <_init>:
_init():
400428: 48 83 ec 08 sub $0x8,%rsp
40042c: e8 5b 00 00 00 callq 40048c <call_gmon_start>
400431: e8 6a 01 00 00 callq 4005a0 <frame_dummy>
400436: e8 45 02 00 00 callq 400680 <__do_global_ctors_aux>
40043b: 48 83 c4 08 add $0x8,%rsp
40043f: c3 retq
Disassembly of section .plt:
0000000000400440 <__libc_start_main#plt-0x10>:
400440: ff 35 1a 05 20 00 pushq 0x20051a(%rip) # 600960 <_GLOBAL_OFFSET_TABLE_+0x8>
400446: ff 25 1c 05 20 00 jmpq *0x20051c(%rip) # 600968 <_GLOBAL_OFFSET_TABLE_+0x10>
40044c: 0f 1f 40 00 nopl 0x0(%rax)
0000000000400450 <__libc_start_main#plt>:
400450: ff 25 1a 05 20 00 jmpq *0x20051a(%rip) # 600970 <_GLOBAL_OFFSET_TABLE_+0x18>
400456: 68 00 00 00 00 pushq $0x0
40045b: e9 e0 ff ff ff jmpq 400440 <_init+0x18>
Disassembly of section .text:
0000000000400460 <_start>:
_start():
400460: 31 ed xor %ebp,%ebp
400462: 49 89 d1 mov %rdx,%r9
400465: 5e pop %rsi
400466: 48 89 e2 mov %rsp,%rdx
400469: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp
40046d: 50 push %rax
40046e: 54 push %rsp
40046f: 49 c7 c0 e0 05 40 00 mov $0x4005e0,%r8
400476: 48 c7 c1 f0 05 40 00 mov $0x4005f0,%rcx
40047d: 48 c7 c7 d0 05 40 00 mov $0x4005d0,%rdi
400484: e8 c7 ff ff ff callq 400450 <__libc_start_main#plt>
400489: f4 hlt
40048a: 90 nop
40048b: 90 nop
000000000040048c <call_gmon_start>:
call_gmon_start():
40048c: 48 83 ec 08 sub $0x8,%rsp
400490: 48 8b 05 b9 04 20 00 mov 0x2004b9(%rip),%rax # 600950 <_DYNAMIC+0x1c0>
400497: 48 85 c0 test %rax,%rax
40049a: 74 02 je 40049e <call_gmon_start+0x12>
40049c: ff d0 callq *%rax
40049e: 48 83 c4 08 add $0x8,%rsp
4004a2: c3 retq
4004a3: 90 nop
4004a4: 90 nop
4004a5: 90 nop
4004a6: 90 nop
4004a7: 90 nop
4004a8: 90 nop
4004a9: 90 nop
4004aa: 90 nop
4004ab: 90 nop
4004ac: 90 nop
4004ad: 90 nop
4004ae: 90 nop
4004af: 90 nop
4004b0: 90 nop
4004b1: 90 nop
4004b2: 90 nop
4004b3: 90 nop
4004b4: 90 nop
4004b5: 90 nop
4004b6: 90 nop
4004b7: 90 nop
4004b8: 90 nop
4004b9: 90 nop
4004ba: 90 nop
4004bb: 90 nop
4004bc: 90 nop
4004bd: 90 nop
4004be: 90 nop
4004bf: 90 nop
00000000004004c0 <deregister_tm_clones>:
deregister_tm_clones():
4004c0: b8 8f 09 60 00 mov $0x60098f,%eax
4004c5: 48 2d 88 09 60 00 sub $0x600988,%rax
4004cb: 48 83 f8 0e cmp $0xe,%rax
4004cf: 76 17 jbe 4004e8 <deregister_tm_clones+0x28>
4004d1: b8 00 00 00 00 mov $0x0,%eax
4004d6: 48 85 c0 test %rax,%rax
4004d9: 74 0d je 4004e8 <deregister_tm_clones+0x28>
4004db: bf 88 09 60 00 mov $0x600988,%edi
4004e0: ff e0 jmpq *%rax
4004e2: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
4004e8: f3 c3 repz retq
4004ea: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
00000000004004f0 <register_tm_clones>:
register_tm_clones():
4004f0: be 88 09 60 00 mov $0x600988,%esi
4004f5: 48 81 ee 88 09 60 00 sub $0x600988,%rsi
4004fc: 48 c1 fe 03 sar $0x3,%rsi
400500: 48 89 f0 mov %rsi,%rax
400503: 48 c1 e8 3f shr $0x3f,%rax
400507: 48 01 c6 add %rax,%rsi
40050a: 48 d1 fe sar %rsi
40050d: 74 11 je 400520 <register_tm_clones+0x30>
40050f: b8 00 00 00 00 mov $0x0,%eax
400514: 48 85 c0 test %rax,%rax
400517: 74 07 je 400520 <register_tm_clones+0x30>
400519: bf 88 09 60 00 mov $0x600988,%edi
40051e: ff e0 jmpq *%rax
400520: f3 c3 repz retq
400522: 66 66 66 66 66 2e 0f data32 data32 data32 data32 nopw %cs:0x0(%rax,%rax,1)
400529: 1f 84 00 00 00 00 00
0000000000400530 <__do_global_dtors_aux>:
__do_global_dtors_aux():
400530: 80 3d 51 04 20 00 00 cmpb $0x0,0x200451(%rip) # 600988 <__bss_start>
400537: 75 5f jne 400598 <__do_global_dtors_aux+0x68>
400539: 55 push %rbp
40053a: 53 push %rbx
40053b: bb 80 07 60 00 mov $0x600780,%ebx
400540: 48 81 eb 78 07 60 00 sub $0x600778,%rbx
400547: 48 83 ec 08 sub $0x8,%rsp
40054b: 48 8b 05 3e 04 20 00 mov 0x20043e(%rip),%rax # 600990 <dtor_idx.6648>
400552: 48 c1 fb 03 sar $0x3,%rbx
400556: 48 83 eb 01 sub $0x1,%rbx
40055a: 48 8d 6c 24 10 lea 0x10(%rsp),%rbp
40055f: 48 39 d8 cmp %rbx,%rax
400562: 73 22 jae 400586 <__do_global_dtors_aux+0x56>
400564: 0f 1f 40 00 nopl 0x0(%rax)
400568: 48 83 c0 01 add $0x1,%rax
40056c: 48 89 05 1d 04 20 00 mov %rax,0x20041d(%rip) # 600990 <dtor_idx.6648>
400573: ff 14 c5 78 07 60 00 callq *0x600778(,%rax,8)
40057a: 48 8b 05 0f 04 20 00 mov 0x20040f(%rip),%rax # 600990 <dtor_idx.6648>
400581: 48 39 d8 cmp %rbx,%rax
400584: 72 e2 jb 400568 <__do_global_dtors_aux+0x38>
400586: e8 35 ff ff ff callq 4004c0 <deregister_tm_clones>
40058b: c6 05 f6 03 20 00 01 movb $0x1,0x2003f6(%rip) # 600988 <__bss_start>
400592: 48 83 c4 08 add $0x8,%rsp
400596: 5b pop %rbx
400597: 5d pop %rbp
400598: f3 c3 repz retq
40059a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
00000000004005a0 <frame_dummy>:
frame_dummy():
4005a0: bf 88 07 60 00 mov $0x600788,%edi
4005a5: 48 83 3f 00 cmpq $0x0,(%rdi)
4005a9: 75 05 jne 4005b0 <frame_dummy+0x10>
4005ab: e9 40 ff ff ff jmpq 4004f0 <register_tm_clones>
4005b0: b8 00 00 00 00 mov $0x0,%eax
4005b5: 48 85 c0 test %rax,%rax
4005b8: 74 f1 je 4005ab <frame_dummy+0xb>
4005ba: 55 push %rbp
4005bb: 48 89 e5 mov %rsp,%rbp
4005be: ff d0 callq *%rax
4005c0: 5d pop %rbp
4005c1: e9 2a ff ff ff jmpq 4004f0 <register_tm_clones>
4005c6: 90 nop
4005c7: 90 nop
4005c8: 90 nop
4005c9: 90 nop
4005ca: 90 nop
4005cb: 90 nop
4005cc: 90 nop
4005cd: 90 nop
4005ce: 90 nop
4005cf: 90 nop
00000000004005d0 <main>:
main():
4005d0: 31 c0 xor %eax,%eax
4005d2: c3 retq
4005d3: 90 nop
4005d4: 90 nop
4005d5: 90 nop
4005d6: 90 nop
4005d7: 90 nop
4005d8: 90 nop
4005d9: 90 nop
4005da: 90 nop
4005db: 90 nop
4005dc: 90 nop
4005dd: 90 nop
4005de: 90 nop
4005df: 90 nop
00000000004005e0 <__libc_csu_fini>:
__libc_csu_fini():
4005e0: f3 c3 repz retq
4005e2: 66 66 66 66 66 2e 0f data32 data32 data32 data32 nopw %cs:0x0(%rax,%rax,1)
4005e9: 1f 84 00 00 00 00 00
00000000004005f0 <__libc_csu_init>:
__libc_csu_init():
4005f0: 48 89 6c 24 d8 mov %rbp,-0x28(%rsp)
4005f5: 4c 89 64 24 e0 mov %r12,-0x20(%rsp)
4005fa: 48 8d 2d 63 01 20 00 lea 0x200163(%rip),%rbp # 600764 <__init_array_end>
400601: 4c 8d 25 5c 01 20 00 lea 0x20015c(%rip),%r12 # 600764 <__init_array_end>
400608: 4c 89 6c 24 e8 mov %r13,-0x18(%rsp)
40060d: 4c 89 74 24 f0 mov %r14,-0x10(%rsp)
400612: 4c 89 7c 24 f8 mov %r15,-0x8(%rsp)
400617: 48 89 5c 24 d0 mov %rbx,-0x30(%rsp)
40061c: 48 83 ec 38 sub $0x38,%rsp
400620: 4c 29 e5 sub %r12,%rbp
400623: 41 89 fd mov %edi,%r13d
400626: 49 89 f6 mov %rsi,%r14
400629: 48 c1 fd 03 sar $0x3,%rbp
40062d: 49 89 d7 mov %rdx,%r15
400630: e8 f3 fd ff ff callq 400428 <_init>
400635: 48 85 ed test %rbp,%rbp
400638: 74 1c je 400656 <__libc_csu_init+0x66>
40063a: 31 db xor %ebx,%ebx
40063c: 0f 1f 40 00 nopl 0x0(%rax)
400640: 4c 89 fa mov %r15,%rdx
400643: 4c 89 f6 mov %r14,%rsi
400646: 44 89 ef mov %r13d,%edi
400649: 41 ff 14 dc callq *(%r12,%rbx,8)
40064d: 48 83 c3 01 add $0x1,%rbx
400651: 48 39 eb cmp %rbp,%rbx
400654: 72 ea jb 400640 <__libc_csu_init+0x50>
400656: 48 8b 5c 24 08 mov 0x8(%rsp),%rbx
40065b: 48 8b 6c 24 10 mov 0x10(%rsp),%rbp
400660: 4c 8b 64 24 18 mov 0x18(%rsp),%r12
400665: 4c 8b 6c 24 20 mov 0x20(%rsp),%r13
40066a: 4c 8b 74 24 28 mov 0x28(%rsp),%r14
40066f: 4c 8b 7c 24 30 mov 0x30(%rsp),%r15
400674: 48 83 c4 38 add $0x38,%rsp
400678: c3 retq
400679: 90 nop
40067a: 90 nop
40067b: 90 nop
40067c: 90 nop
40067d: 90 nop
40067e: 90 nop
40067f: 90 nop
0000000000400680 <__do_global_ctors_aux>:
__do_global_ctors_aux():
400680: 55 push %rbp
400681: 48 89 e5 mov %rsp,%rbp
400684: 53 push %rbx
400685: bb 68 07 60 00 mov $0x600768,%ebx
40068a: 48 83 ec 08 sub $0x8,%rsp
40068e: 48 8b 05 d3 00 20 00 mov 0x2000d3(%rip),%rax # 600768 <__CTOR_LIST__>
400695: 48 83 f8 ff cmp $0xffffffffffffffff,%rax
400699: 74 14 je 4006af <__do_global_ctors_aux+0x2f>
40069b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
4006a0: 48 83 eb 08 sub $0x8,%rbx
4006a4: ff d0 callq *%rax
4006a6: 48 8b 03 mov (%rbx),%rax
4006a9: 48 83 f8 ff cmp $0xffffffffffffffff,%rax
4006ad: 75 f1 jne 4006a0 <__do_global_ctors_aux+0x20>
4006af: 48 83 c4 08 add $0x8,%rsp
4006b3: 5b pop %rbx
4006b4: 5d pop %rbp
4006b5: c3 retq
4006b6: 90 nop
4006b7: 90 nop
Disassembly of section .fini:
00000000004006b8 <_fini>:
_fini():
4006b8: 48 83 ec 08 sub $0x8,%rsp
4006bc: e8 6f fe ff ff callq 400530 <__do_global_dtors_aux>
4006c1: 48 83 c4 08 add $0x8,%rsp
4006c5: c3 retq
and the smaller file:
g++ -Wall -O3 -g0 test.cpp -o test.exe -nostdlib
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000400150
test.exe: file format elf64-x86-64
Contents of section .note.gnu.build-id:
400120 04000000 14000000 03000000 474e5500 ............GNU.
400130 d4b1e35c 21d1f541 b81d3ac9 d62bac7a ...\!..A..:..+.z
400140 606b1ad4 `k..
Contents of section .text:
400150 31c0c3 1..
Contents of section .eh_frame_hdr:
400154 011b033b 10000000 01000000 fcffffff ...;............
400164 2c000000 ,...
Contents of section .eh_frame:
400168 14000000 00000000 017a5200 01781001 .........zR..x..
400178 1b0c0708 90010000 14000000 1c000000 ................
400188 c8ffffff 03000000 00000000 00000000 ................
Contents of section .comment:
0000 4743433a 2028474e 55292034 2e392e78 GCC: (GNU) 4.9.x
0010 2d676f6f 676c6520 32303135 30313233 -google 20150123
0020 20287072 6572656c 65617365 2900 (prerelease).
Disassembly of section .text:
0000000000400150 <main>:
main():
400150: 31 c0 xor %eax,%eax
400152: c3 retq
It's worth noting that this executable doesn't work, it segfaults: to make it work, we'd actually have to implement _start instead of main.
We can see here that the bulk of the larger executable is glue code that deals with loading the dynamic library and preparing the broader environment required by the standard library.
--- EDIT ---
Even our smaller code still has to include exception handling, ctor/dtor support for globals, and so forth. It could probably elide such things and if you dig deeply enough you can probably find ways to elide them, but in general you probably don't need to, and it is probably easier to always include such basic support than to have the majority of new programmers stumbling over "how do I force the compiler to emit basic language support" than have a handful of new embedded programmers asking "how can I prevent the compiler emitting basic language support?".
Note also that the compiler generates ELF format binaries, this is a small contribution (maybe ~60bytes), plus emitting it's own identity added some size. But the bulk of the smaller binary is language support (EH and CTOR/DTOR).
Compiling with #include <iostream> and -O3 -g0 produces a 7625 byte binary, if I compile that with -O0 -g3 it produces a 64Kb binary most of which is text describing symbols from the STL.
Your executable is including the C runtime, which knows how to do things like get the environment, setup the argv vector, and close all open files after calling exit() but before calling _exit().
There are many things which could affect the final file size during compilation, as other posters have pointed out.
Dissecting your specific example is more work than I'm willing to put in, but I know of a similar example from many years ago that should help you to understand the general problem, and guide you towards finding the specific answer you seek.
http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html
This is done in C (rather than C++) using GCC, looking at the size of the ELF executable (not a Windows EXE), but as I said many of the same problems apply. In this case, the author looks at just return 42;
After you've read that document, consider that printing to stdout is considerably more complex than just returning a number. Also, since you are using C++ and cout <<, there's a lot of code hiding in there that you didn't write, and you can't really know how it's implemented without looking at that source.
people keep ignoring/forgetting that executables created in high level languages need engine to run properly. for example C++ engine is responsible for things like:
heap/stack management
when you call new,delete you are not actually accessing OS functions
instead the engine use its own allocated heap memory
so engine has it own memory management that takes code/space
local variables memory management
each time you call any function all the local variables must be allocated
and released before exiting it
classes/templates
to handle these properly you need quite a lot of code
In addition to this you have to link all the stuff you use like:
RTL most executables nowdays MSVCPP and MSVB does not link them so we need to install huge amount of RTLs in system to make exe to even run. but still the linking to used DLL's must be present in executable (see DLL linking on runtime)
debug info
frameworks linkage (similar to RTL you need the code to bind to frameworks libs too)
for high level winows/forms IDE's you also have the window engine present
included libs and linked objs (iostream classes and operators even if you use just << you need much more of them to make it work...)
You can look at the C++ engine as a small operating system within operating system
in standalone MCU apps they are really the OS itself
Another space is occupied by the executable format (like PE), and also code aligns add some space
When you put all these together then the 26KB is not so insane anymore
Compilers are not omnipotent.
std::cout is a stream object, with a set of data members for managing a buffer (allocating it, copying data to it and, when the stream is destroyed, releasing it).
The operator<< is implemented as an overloaded function which interprets its arguments and - when supplied a string - copies data to the buffer, with some logic that potentially flushes the buffer when it is full.
std::endl is actually an function which - in cooperation with all versions of a stream's operator<<() - affects data owned by the stream. Specifically, it inserts a newline into the streams buffer, and then flushes the buffer.
Flushing the stream's buffer calls other functions that copy data from the buffer to the standard output device (say, the screen).
All of the above is what the statement std::cout<<"Hello World"<<std::endl does.
In addition, as a C++ program, there is a certain amount of code that must be executed before main() is even called. This includes checking if the program was run with command line arguments, creating streams like std::cout, std::cerr, std::cin (there are others) ensuring those streams are connected with relevant devices (like the terminal, or pipes, or whatever). When main() returns, it is then necessary to release all the streams created (and flush their buffers), and things like that.
All of the above involves invoking other functionality. Creating a buffer for the stream means that buffer must be allocated and - after main() returns - released.
The specification of C++ streams also involves error checking. The allocation of std::cout's buffer might fail (e.g. if the host system doesn't have much free memory). The standard output device might be redirected to a file, which has limited capacity - so writing data to it might fail. All of those things must be checked for and handled gracefully.
All of this stuff will be in this 26K executable (unless that code is in runtime libraries).
In principle, the compiler can recognise that the program is not using its command line arguments (so not include code to manage command line arguments), is only writing to std::cout (so no need to create all the other streams before main() and release them after main() returns), is only using two overloaded versions of operator<<() and one stream manipulator (so the linker need not include code for all other member functions of the stream). It might also recognise that the statement writes data to the stream and immediately flushes the buffer - and thereby eliminate std::cout's buffer and all code that manages it. If the compiler can read the programmer's mind (few compilers can, in practice) it might work out that none of the buffers are actually needed, that the user will never run the program with standard output redirected, etc - and eliminate the code and data structures associated with all those things.
So, how would a compiler recognise that all those things aren't needed? Compilers are software, so they have to conduct some level of analysis on their inputs (e.g. source files). The analysis to eliminate all the code that a human might deem unnecessary is significant - so would take time. If the compiler doesn't do the analysis, potentially the linker might. Whether that analysis to eliminate unnecessary code is done by the compiler or linker is irrelevant - it takes time. Potentially significant time.
Programmers tend to be impatient. Very few programmers would tolerate a build process for a simple "hello world" program that took more than a few seconds (maybe they will tolerate a minute, but not much more).
That leaves compiler vendors with a decision. They can get their programmers to design and implement all sorts of analysis to eliminate unwanted code. That will add weeks - or, if they are working to a tight deadline, months - to implement, validate, verify, and ship a working compiler to customers (other developers). That compiler will be painfully slow at compiling code. Instead, vendors (and their developers) choose to implement less of that analysis in their compiler, so they can actually ship a working compiler to developers who will use it within a reasonable time. This compiler will produce an executable in a time that is somewhat tolerable for most programmers (say, under a minute for a "hello world" program). So what if the executable is larger? It will work. Hardware (e.g. drives) is relatively inexpensive and developer effort is relatively expensive.
It's very old question. It have clear answer. The most problem is that one have to write many small pieces of information and make many small test which demonstrates different aspects of PE structures. I try to skip details and to describe the main parts of the problem based on Microsoft Visual Studio, which I know and use since many years. All other compilers do mostly the same, and I suppose that one need use just a little other options of compiler and linker.
First of all I suggest you to set breakpoint on the first line of the main, start debugging and to examine the Call Stack windows of the debugger. You will see something like
So the first thing, which is very important to understand, the main is not the first function which will be called in your program. The entry point of the program is mainCRTStartup, which calls __tmainCRTStartup, which calls main.
The CRT Startup code make many small things. One thing is very easy to understand: it uses GetCommandLineW Windows API to get the command line and parse the parameters, then it calls main with the parameters.
To reduce the size of the code there are two common approach:
use CRT from DLL
remove CRT from the EXE if it's not really used in the code.
It's very helpful if you start cmd.exe using "VS2013 x64 Native Tools Command Prompt" (or some close command prompt). Some additional paths will be set inside of the command prompt and you can use for example dumpbin.exe utility.
If you would use Multi-threaded DLL (/MD) compiler option then you will get 7K large exe file. "dumpbin /imports HelloWorld.exe" will show you that your program uses "MSVCR120.dll" together with "KERNEL32.dll".
Removing of CRT depends on the version of c/cpp compiler (the version of Visual Studio) which you use and even from the extension of the file: .c or .cpp. I understand your question as the common question for understanding the problem. So I suggest to start with the most simple case, rename .cpp file .c and the beginning and to modify the code to the following
#include <Windows.h>
int mainCRTStartup()
{
return 0;
}
One can see now
C:\Oleg\StackOverflow\HelloWorld\Release>dir HelloWorld.exe
Volume in drive C has no label.
Volume Serial Number is 4CF9-FADF
Directory of C:\Oleg\StackOverflow\HelloWorld\Release
21.06.2015 12:56 3.584 HelloWorld.exe
1 File(s) 3.584 bytes
0 Dir(s) 16.171.196.416 bytes free
C:\Oleg\StackOverflow\HelloWorld\Release>dumpbin HelloWorld.exe
Microsoft (R) COFF/PE Dumper Version 12.00.31101.0
Copyright (C) Microsoft Corporation. All rights reserved.
Dump of file HelloWorld.exe
File Type: EXECUTABLE IMAGE
Summary
1000 .data
1000 .rdata
1000 .reloc
1000 .rsrc
1000 .text
One can add the linker option /MERGE:.rdata=.text to reduce the size and to remove one section
C:\Oleg\StackOverflow\HelloWorld\Release>dir HelloWorld.exe
Volume in drive C has no label.
Volume Serial Number is 4CF9-FADF
Directory of C:\Oleg\StackOverflow\HelloWorld\Release
21.06.2015 18:44 3.072 HelloWorld.exe
1 File(s) 3.072 bytes
0 Dir(s) 16.170.852.352 bytes free
C:\Oleg\StackOverflow\HelloWorld\Release>dumpbin HelloWorld.exe
Microsoft (R) COFF/PE Dumper Version 12.00.31101.0
Copyright (C) Microsoft Corporation. All rights reserved.
Dump of file HelloWorld.exe
File Type: EXECUTABLE IMAGE
Summary
1000 .data
1000 .reloc
1000 .rsrc
1000 .text
To have "Hello World" program I suggest to modify the code to
#include <Windows.h>
int mainCRTStartup()
{
LPCTSTR pszString = TEXT("Hello world");
DWORD cbWritten;
WriteConsole(GetStdHandle(STD_OUTPUT_HANDLE), pszString, lstrlen(pszString), &cbWritten, NULL);
return 0;
}
One can easy verify that the code work and it's still small.
To remove CRT from .cpp file I suggest to follow the following steps. First of all we would use the following HelloWorld.cpp code
#include <Windows.h>
int mainCRTStartup()
{
LPCTSTR pszString = TEXT("Hello world");
DWORD cbWritten;
WriteConsole(GetStdHandle(STD_OUTPUT_HANDLE), pszString, lstrlen(pszString), &cbWritten, NULL);
return 0;
}
It's important that one verify some compiler and linker options and set/remove someone. I included the settings on the pictures below:
The last screen shows that we remove binding to default libraries which we don't need. The compiler uses directive like #pragma comment(lib, "some.lib") to inject usage of some libraries. By usage the options /NODEFAULTLIB we remove such libs and the exe will be compiled like we need.
One will see that the resulting HelloWorld.exe have only 3K (3.072 bytes) and there are exist dependency to one KERNEL32.dll only:
C:\Oleg\StackOverflow\HelloWorld\Release>dumpbin /imports HelloWorld.exe
Microsoft (R) COFF/PE Dumper Version 12.00.31101.0
Copyright (C) Microsoft Corporation. All rights reserved.
Dump of file HelloWorld.exe
File Type: EXECUTABLE IMAGE
Section contains the following imports:
KERNEL32.dll
402000 Import Address Table
402038 Import Name Table
0 time date stamp
0 Index of first forwarder reference
60B lstrlenW
5E0 WriteConsoleW
2C0 GetStdHandle
Summary
1000 .idata
1000 .reloc
1000 .rsrc
1000 .text
One can download the corresponding Visual Studio 2013 demo project from here. One need switch from default "Debug" compiling to "Release" and rebuild solution. One will have working HelloWorld.exe which length is 3K.
This does show how hard it can be to write a program with identical semantics.
<<std::endl will flush a stream if that stream is good(). That means the whole error handling code of ostream must be present.
Also, std::cout could have its streambuf swapped out from under it. The compiler cannot know it's actually going to STDOUT_FILENO. It has to use the whole streambuf intermediate layer.
When I look at the symbols in my library, nm mylib.a, I see some duplicate entries that look like this:
000000000002d130 S __ZN7quadmat11SpAddLeavesC1EPNS_14BlockContainerEPy
00000000000628a8 S __ZN7quadmat11SpAddLeavesC1EPNS_14BlockContainerEPy.eh
When piped through c++filt:
000000000002d130 S quadmat::SpAddLeaves::SpAddLeaves(quadmat::BlockContainer*, unsigned long long*)
00000000000628a8 S quadmat::SpAddLeaves::SpAddLeaves(quadmat::BlockContainer*, unsigned long long*) (.eh)
What does that .eh mean, and what is this extra symbol used for?
I see it has something to do with exception handling. But why does that use an extra symbol?
(I'm noticing this with clang)
Here's some simple code:
bool extenrnal_variable;
int f(...)
{
if (extenrnal_variable)
throw 0;
return 42;
}
int g()
{
return f(1, 2, 3);
}
I added extenrnal_variable to prevent the compiler from optimizing all the branches away. f has ... to prevent inlining.
When compiled with:
$ clang++ -S -O3 -m32 -o - eh.cpp | c++filt
it emits the following code for g() (the rest is omitted):
g(): ## #_Z1gv
.cfi_startproc
## BB#0:
pushl %ebp
Ltmp9:
.cfi_def_cfa_offset 8
Ltmp10:
.cfi_offset %ebp, -8
movl %esp, %ebp
Ltmp11:
.cfi_def_cfa_register %ebp
subl $24, %esp
movl $3, 8(%esp)
movl $2, 4(%esp)
movl $1, (%esp)
calll f(...)
movl $42, %eax
addl $24, %esp
popl %ebp
ret
.cfi_endproc
All these .cfi_* directives are there for the stack unwinding in case of an exception being thrown. They all compiled into into an FDE (Frame Description Entry) block and saved under the g().eh (__Z1gv.eh mangled) name. These directives specify where on the stack the CPU registers are saved. When an exception is thrown and the stack is being unwound the code in the function should not be executed (except for the destructors of locals), but the registers that were saved earlier should be restored. These tables store exactly that information.
These tables could be dumped via the dwarfdump tool:
$ dwarfdump --eh-frame --english eh.o | c++filt
The output:
0x00000018: FDE
length: 0x00000018
CIE_pointer: 0x00000000
start_addr: 0x00000000 f(...)
range_size: 0x0000004d (end_addr = 0x0000004d)
Instructions: 0x00000000: CFA=esp+4 eip=[esp]
0x00000001: CFA=esp+8 ebp=[esp] eip=[esp+4]
0x00000003: CFA=ebp+8 ebp=[ebp] eip=[ebp+4]
0x00000007: CFA=ebp+8 ebp=[ebp] esi=[ebp-4] eip=[ebp+4]
0x00000034: FDE
length: 0x00000018
CIE_pointer: 0x00000000
start_addr: 0x00000050 g()
range_size: 0x0000002c (end_addr = 0x0000007c)
Instructions: 0x00000050: CFA=esp+4 eip=[esp]
0x00000051: CFA=esp+8 ebp=[esp] eip=[esp+4]
0x00000053: CFA=ebp+8 ebp=[ebp] eip=[ebp+4]
Here you could find out about the format of this block. Here a bit more and some alternative more compact way of representing the same information. Basically this block describes which registers and where from on the stack to pop during the stack unwinding.
To see the raw content of these symbols you can list all the symbols with their offsets:
$ nm -n eh.o
00000000 T __Z1fz
U __ZTIi
U ___cxa_allocate_exception
U ___cxa_throw
00000050 T __Z1gv
000000a8 s EH_frame0
000000c0 S __Z1fz.eh
000000dc S __Z1gv.eh
000000f8 S _extenrnal_variable
And then dump the (__TEXT,__eh_frame) section:
$ otool -s __TEXT __eh_frame eh.o
eh.o:
Contents of (__TEXT,__eh_frame) section
000000a8 14 00 00 00 00 00 00 00 01 7a 52 00 01 7c 08 01
000000b8 10 0c 05 04 88 01 00 00 18 00 00 00 1c 00 00 00
000000c8 38 ff ff ff 4d 00 00 00 00 41 0e 08 84 02 42 0d
000000d8 04 44 86 03 18 00 00 00 38 00 00 00 6c ff ff ff
000000e8 2c 00 00 00 00 41 0e 08 84 02 42 0d 04 00 00 00
By matching the offsets you could see how each symbol is encoded.
When there are local variables present, they would have to be destroyed during the stack unwinding. For that there's usually more code embedded in the functions themselves and some additional bigger tables are created. You could explore that yourself by adding a local variable with non-trivial destructor into g, compiling and looking at the assembly output.
Further reading
Explanations of some of the .cfi_* directives
Exception handling in LLVM
It stands for stands for exception handler and is usually associated with the info below:
If you are using an exports list and building either a shared library, or an executable that will be used with ld's -bundle_loader flag, you need to include the symbols for exception frame information in the exports list for your exported C++ symbols. Otherwise, they may be stripped. These symbols end with .eh; you can view them with the nm tool.
from XcodeUserGuide20