The True Story of Hello World

(or at least a good part of it)

Most of our computer science students have been through the
famous "Hello World" program at least once. When compared to a
typical application program ---almost always featuring a web-aware
graphical user interface, "Hello World" turns into an very
uninteresting fragment of code. Nevertheless, many computer science
students still didn't get the real story behind it. The goal of this
exercise is to cast some light in the subject by snooping in the
"Hello World" life-cycle.

The source code

Let's begin with Hello World's source code:

1. 2. 3. 4. 5. 6. 7. #include <stdio.h> int main(void) { printf("Hello World!\n"); return 0; }

Line 1 instructs the compiler to include the declarations needed
to invoke the printf C library (libc) function.

Line 3 declares function main, which is believed to be our
program entry point (it is not, as we will see later). It is
declared as a function that takes no parameter (we disregard command
line arguments in this program) and returns an integer to the parent
process --- the shell, in our case. By the way, the shell
dictates a convention by which a child process must return an 8-bit
number representing it status: 0 for normal termination, 0
> n < 128 for process detected abnormal termination, and
n > 128 for signal induced termination.

Line 4 through 8 comprise the definition of function main,
which invokes the printf C library function to output the
"Hello World!\n" string and returns 0 to the parent process.

Simple, very simple!

Compilation

Now let's take a look at the compilation process for "Hello
World". For the upcoming discussion, we'll take the widely-used GNU
compiler (gcc) and its associated tools (binutils). We
can compile the program as follows:

# gcc -Os -c hello.c

This produces the object file hello.o. More
specifically,

# file hello.o hello.o: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped

tells us hello.o is a relocatable object file, compiled for the
IA-32 architecture (I used a standard PC for this study), stored in
the Executable
and Linking Format (ELF), that contains a symbol table (not
stripped).

By the way,

# objdump -hrt hello.o hello.o: file format elf32-i386 Sections: Idx Name Size VMA LMA File off Algn 0 .text 00000011 00000000 00000000 00000034 2**2 CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE 1 .data 00000000 00000000 00000000 00000048 2**2 CONTENTS, ALLOC, LOAD, DATA 2 .bss 00000000 00000000 00000000 00000048 2**2 ALLOC 3 .rodata.str1.1 0000000d 00000000 00000000 00000048 2**0 CONTENTS, ALLOC, LOAD, READONLY, DATA 4 .comment 00000033 00000000 00000000 00000055 2**0 CONTENTS, READONLY SYMBOL TABLE: 00000000 l df *ABS* 00000000 hello.c 00000000 l d .text 00000000 00000000 l d .data 00000000 00000000 l d .bss 00000000 00000000 l d .rodata.str1.1 00000000 00000000 l d .comment 00000000 00000000 g F .text 00000011 main 00000000 *UND* 00000000 puts RELOCATION RECORDS FOR [.text]: OFFSET TYPE VALUE 00000004 R_386_32 .rodata.str1.1 00000009 R_386_PC32 puts

tells us hello.o has 5 sections:

.text: that's "Hello World" compiled program,
i.e. IA-32 opcodes corresponding to the program. This will be used
by the program loader to initialize the process' code
segment.
.data: "Hello World" has neither initialized global
variables nor initialized static local variables, so this section
is empty. Otherwise, it would contain the variable initial values
to be loaded into the data segment.
.bss: "Hello World" also doesn't have any
non-initialized variable, either global or local, so this section
is also empty. Otherwise, it would indicate how many bytes must be
allocated and zeroed in the data segment in addition to
section .data.
.rodata: this segment contains the "Hello World!\n"
string, which is tagged read-only. Most operating systems do not
support a read-only data segment for processes (running
programs), so the contents of .rodata go either to the
process' code segment (because it's read-only), or to the
data segment (because it's data). Since the compiler
doesn't know the policy adopted by your OS, it creates this extra
ELF section.
.comment: this segment contains 33 bytes of comments
which cannot be tracked back to our program, since we didn't write
any comment. We'll soon see where it comes from.

It also shows us a symbol table with symbol main
bound to address 00000000 and symbol puts
undefined. Moreover, the relocation table tells us how to
relocate the references to external sections made in section
.text. The first relocatable symbol corresponds to the "Hello
World!\n" string contained in section .rodata. The second
relocatable symbol, puts, designates a libc function
which was generated as a result of invoking printf. To better
understand the contents of hello.o, let's take a look at the
assembly code:

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. # gcc -Os -S hello.c -o -
.file "hello.c" .section .rodata.str1.1,"aMS",@progbits,1 .LC0: .string "Hello World!" .text .align 2 .globl main .type main,@function main: pushl %ebp movl %esp, %ebp pushl $.LC0 call puts xorl %eax, %eax leave ret .Lfe1: .size n,.Lfe1-n .ident "GCC: (GNU) 3.2 20020903 (Red Hat Linux 8.0 3.2-7)"

From the assembly code, it becomes clear where the ELF section
flags come from. For instance, section .text is to be 32-bit
aligned (line 7). It also reveals where the .comment section
comes from (line 20). Since printf was called to print a
single string, and we requested our nice compiler to optimize the
generated code (-Os), puts was generated
instead. Unfortunately, we'll see later that our libc
implementation will render the compiler effort useless.

And what about the assembly code produced? No surprises here: a
simple call to function puts with the string addressed by
.LC0 as argument.

Linking

Now let's take a look at the process of transforming
hello.o into an executable. One might think the following
command would do:

# ld -o hello hello.o -lc ld: warning: cannot find entry symbol _start; defaulting to 08048184

But what's that warning? Try running it!

Yes, it doesn't work. So let's go back to that warning: it tells
the linker couldn't find our program's entry point
_start. But wasn't it main our entry point? To be
short here, main is the start point of a C program from the
programmer's perspective. In fact, before calling main, a process
has already executed a bulk of code to "clean up the room for
execution". We usually get this surrounding code transparently from
the compiler/OS provider.

So let's try this:

# ld -static -o hello -L`gcc -print-file-name=` /usr/lib/crt1.o /usr/lib/crti.o hello.o /usr/lib/crtn.o -lc -lgcc

Now we should have a real executable. Static linking was used for
two reasons: first, I don't want to go into the discussion of how
dynamic libraries work here; second, I'd like to show you how much
unnecessary code comes into "Hello World" due to the way libraries
(libc and libgcc) are implemented. Try the following:

# find hello.c hello.o hello -printf "%f\t%s\n" hello.c 84 hello.o 788 hello 445506

You can also try "nm hello" or "objdump -d hello" to get an idea of what got linked into the
executable.

For information about dynamic linking, please refer to Program
Library HOWTO.

Loading and running

In a POSIX OS, loading a program for execution is accomplished by
having the father process to invoke the fork system
call to replicates itself and having the just-created child
process to invoke the execve system call to load and start
the desired program. This procedure is carried out, for instance, by
the shell whenever you type an external command. You can
confirm this with truss or strace:

# strace -i hello > /dev/null [????????] execve("./hello", ["hello"], [/* 46 vars */]) = 0 ... [08053d44] write(1, "Hello World!\n", 13) = 13 ... [0804e7ad] _exit(0) = ?

Besides the execve system call, the output shows the call
to write that results from puts, and the call to
exit with the argument returned by function main
(0).

To understand the details behind the loading procedure carried
out by execve, let's take a look at our ELF executable:

# readelf -l hello Elf file type is EXEC (Executable file) Entry point 0x80480e0 There are 3 program headers, starting at offset 52 Program Headers: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align LOAD 0x000000 0x08048000 0x08048000 0x55dac 0x55dac R E 0x1000 LOAD 0x055dc0 0x0809edc0 0x0809edc0 0x01df4 0x03240 RW 0x1000 NOTE 0x000094 0x08048094 0x08048094 0x00020 0x00020 R 0x4 Section to Segment mapping: Segment Sections... 00 .init .text .fini .rodata __libc_atexit __libc_subfreeres .note.ABI-tag 01 .data .eh_frame .got .bss 02 .note.ABI-tag

The output shows the overall structure of hello. The first
program header corresponds to the process' code
segment, which will be loaded from file at offset 0x000000 into
a memory region that will be mapped into the process' address
space at address 0x08048000. The code segment will be 0x55dac
bytes large and must be page-aligned (0x1000). This segment will
comprise the .text and .rodata ELF segments discussed
earlier, plus additional segments generated during the linking
procedure. As expected, it's flagged read-only (R) and executable
(X), but not writable (W).

The second program header corresponds to the process'
data segment. Loading this segment follows the same steps
mentioned above. However, note that the segment size is 0x01df4 on
file and 0x03240 in memory. This is due to the .bss section,
which is to be zeroed and therefore doesn't need to be present in
the file. The data segment will also be page-aligned (0x1000) and
will contain the .data and .bss ELF segments. It will
be flagged readable and writable (RW). The third program header
results from the linking procedure and is irrelevant for this
discussion.

If you have a proc file system, you can check this, as
long as you get "Hello World" to run long enough (hint: gdb),
with the following command:

# cat /proc/`ps -C hello -o pid=`/maps 08048000-0809e000 r-xp 00000000 03:06 479202 .../hello 0809e000-080a1000 rw-p 00055000 03:06 479202 .../hello 080a1000-080a3000 rwxp 00000000 00:00 0 bffff000-c0000000 rwxp 00000000 00:00 0

The first mapped region is the process' code segment, the
second and third build up the data segment (data + bss +
heap), and the fourth, which has no correspondent in the ELF
file, is the stack. Additional information about the running
hello process can be obtained with GNU time,
ps, and /proc/pid/stat.

Terminating

When "Hello World" executes the return statement in
main function, it passes a parameter to the surrounding
functions discussed in section linking. One of these functions
invokes the exit system call passing by the return
argument. The exit system call hands over that value to the
parent process, which is currently blocked on the wait
system call. Moreover, it conducts a clean process termination, with
resources being returned to the system. This procedure can be
partially traced with the following:

# strace -e trace=process -f sh -c "hello; echo $?" > /dev/null execve("/bin/sh", ["sh", "-c", "hello; echo 0"], [/* 46 vars */]) = 0 fork() = 8321 [pid 8320] wait4(-1, <unfinished ...> [pid 8321] execve("./hello", ["hello"], [/* 46 vars */]) = 0 [pid 8321] _exit(0) = ? <... wait4 resumed> [WIFEXITED(s) && WEXITSTATUS(s) == 0], 0, NULL) = 8321 --- SIGCHLD (Child exited) --- wait4(-1, 0xbffff06c, WNOHANG, NULL) = -1 ECHILD (No child processes) _exit(0)

Closing

The intention of this exercise is to call attention of new
computer science students to the fact that a Java applet doesn't get
run by magic: there's a lot of system software behind even the
simplest program. If consider it useful and have any suggestion to
improve it, please e-mail
me.

FAQ

This section is dedicated to student's frequently asked questions.

What is "libgcc"? Why is included in linkage?

Internal compiler libs, such as libgcc, are used to implement language constructs not directly implemented by the target architecture. For instance, the module operator in C ("%") might not be mappable to a single assembly instruction on the target architecture. Instead of having the compiler to generate in-line code, a function call might be preferable (specially for memory limited machines such as microcontrollers). Many other primitives, including division, multiplication, string manipulation (e.g. memory copy) are typically implemented on such libraries.

Reference:
http://www.lisha.ufsc.br/teaching/os/exercise/hello.html

Linux 下程序的記憶體映射

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Sunday, December 19, 2010