The final images produced by compliers contain both bin file and extended loader format ELf file ,what is the difference between the two , especially the utility of ELF file.
A Bin file is a pure binary file with no memory fix-ups or relocations, more than likely it has explicit instructions to be loaded at a specific memory address. Whereas....
ELF files are Executable Linkable Format which consists of a symbol look-ups and relocatable table, that is, it can be loaded at any memory address by the kernel and automatically, all symbols used, are adjusted to the offset from that memory address where it was loaded into. Usually ELF files have a number of sections, such as 'data', 'text', 'bss', to name but a few...it is within those sections where the run-time can calculate where to adjust the symbol's memory references dynamically at run-time.
A bin file is just the bits and bytes that go into the rom or a particular address from which you will run the program. You can take this data and load it directly as is, you need to know what the base address is though as that is normally not in there.
An elf file contains the bin information but it is surrounded by lots of other information, possible debug info, symbols, can distinguish code from data within the binary. Allows for more than one chunk of binary data (when you dump one of these to a bin you get one big bin file with fill data to pad it to the next block). Tells you how much binary you have and how much bss data is there that wants to be initialised to zeros (gnu tools have problems creating bin files correctly).
The elf file format is a standard, arm publishes its enhancements/variations on the standard. I recommend everyone writes an elf parsing program to understand what is in there, dont bother with a library, it is quite simple to just use the information and structures in the spec. Helps to overcome gnu problems in general creating .bin files as well as debugging linker scripts and other things that can help to mess up your bin or elf output.
some resources:
ELF for the ARM architecture
http://infocenter.arm.com/help/topic/com.arm.doc.ihi0044d/IHI0044D_aaelf.pdf
ELF from wiki
http://en.wikipedia.org/wiki/Executable_and_Linkable_Format
ELF format is generally the default output of compiling.
if you use GNU tool chains, you can translate it to binary format by using objcopy, such as:
arm-elf-objcopy -O binary [elf-input-file] [binary-output-file]
or using fromELF utility(built in most IDEs such as ADS though):
fromelf -bin -o [binary-output-file] [elf-input-file]
bin is the final way that the memory looks before the CPU starts executing it.
ELF is a cut-up/compressed version of that, which the CPU/MCU thus can't run directly.
The (dynamic) linker first has to sufficiently reverse that (and thus modify offsets back to the correct positions).
But there is no linker/OS on the MCU, hence you have to flash the bin instead.
Moreover, Ahmed Gamal is correct.
Compiling and linking are separate stages; the whole process is called "building", hence the GNU Compiler Collection has separate executables:
One for the compiler (which technically outputs assembly), another one for the assembler (which outputs object code in the ELF format),
then one for the linker (which combines several object files into a single ELF file), and finally, at runtime, there is the dynamic linker,
which effectively turns an elf into a bin, but purely in memory, for the CPU to run.
Note that it is common to refer to the whole process as "compiling" (as in GCC's name itself), but that then causes confusion when the specifics are discussed,
such as in this case, and Ahmed was clarifying.
It's a common problem due to the inexact nature of human language itself.
To avoid confusion, GCC outputs object code (after internally using the assembler) using the ELF format.
The linker simply takes several of them (with an .o extension), and produces a single combined result, probably even compressing them (into "a.out").
But all of them, even ".so" are ELF.
It is like several Word documents, each ending in ".chapter", all being combined into a final ".book",
where all files technically use the same standard/format and hence could have had ".docx" as the extension.
The bin is then kind of like converting the book into a ".txt" file while adding as many whitespace as necessary to be equivalent to the size of the final book (printed on a single spool),
with places for all the pictures to be overlaid.
I just want to correct a point here. ELF file is produced by the Linker, not the compiler.
The Compiler mission ends after producing the object files (*.o) out of the source code files. Linker links all .o files together and produces the ELF.
Related
Is there a way I can find from how many Relocatable files an Elf execution file Built from?
And how can I associate a segment with its original Relocatable file?
Thanks in avance.
Is there a way I can find from how many Relocatable files an Elf execution file Built from?
No (at least not in general).
And how can I associate a segment with its original Relocatable file?
Any trace of the original ET_REL file will be gone by the time the linker is done linking.
In addition, with section reordering, a single .o can be "spread out" all over the final linked binary, and with identical code folding a single segment of the final binary can be associated with multiple .o files.
I think http://xyproblem.info is also relevant here.
The linker can output both the ELF and the MAP file. These files are especially relevant in the embedded systems world because the ELF file is usually used to read out the addresses of variables or functions. Additionally, the ELF file is used by different embedded measurement or analysis tools.
When I open a MAP file, then within it, I can see for each global variable and every external function the following information: allocated address, symbolic name, allocated bytes, memory unit, and memory section.
On the other hand, once I open the ELF file, it is a binary file and not human readable. However, some tools I use are able to read it out and interpret it. Those tools can interpret the ELF file, and obtain the information about the symbolic name of a variable/function and its address or even show a function prototype.
From my understanding the ELF and MAP files are basically containing the same information, it is just that the first one is binary and the latter one is the text file.
So what are the actual differences between these two files from the content perspective?
Thank you in advance!
The primary output of the linker (i.e. its main purpose) is to produce the fully linked executable code. That is the ELF (Executable Linkable Format) file. An ELF file may as you have observed contain symbols - these are used for debug. It may also contain meta-data that associates the machine code with the source code from which it was generated. But the bulk of its content (and the part that is not optional) is the executable machine code and data objects that are your application.
The MAP file is an optional information only human readable output that contains information about the location and size of code and data objects in your application. The MAP file includes a summary that shows the total size and memory usage of your code.
In an embedded cross-development environment, the symbol information in the ELF file is used when the code is loaded into a source-level symbolic debugger. The debugger takes the binary code/data segments in the ELF file and loads them onto the target (typically using a JTAG or other debug/programming hardware tool), it loads the symbols and source-level debug meta-data into the debugger, then while the real machine code is executing on the target, that execution is reflected in the debugger in the original source code where you can view, step and break-point the code at the source level.
In short, the ELF file is your program. The MAP file is, as its name suggests, a map of your executable - it tells you where things are in the executable.
As far as I know compiler convert source code to machine code. But this code do not have any OS-related sections and linker add them to file.
But is it's possible to make some executable without linker?
Answering your question very literally - yes, it is possible to make an executive file without a linker: you don't need a compiler or linker to generate machine code. Binaries are a series of opcodes and relevant information (offsets, addresses etc). If you open a binary editor then type out some opcodes and make a program. Save and run it.
Of course the binary will be processor specific, just as if you had compiled a binary (native) executive. Here's a reference to the Intel x86 opcodes.
http://ref.x86asm.net/coder32.html.
If you're however asking, "Can I compile a source file directly into an executive file without a linker?" then speaking purely: no - unless the compiler has aspects of a linker integrated within it. The compiler generates intermediate objects that are passed on to the linker to "link" them into a binary such as a library or executive. Without the link step the pipeline is not complete.
Let's first make a statement that is to be considered true, compilers do not generate machine code that can be immediately executed (JIT's do, but lets ignore that).
Instead they generate files (object, static, dynamic, executable) which describe what they contains as well as groups of symbols. Symbols can be global variables or functions.
But symbols just like the file itself contain metadata. This metadata is very important. See the machine code stored in a symbol is the raw instructions for the target architecture but it does not know where memory is stored.
While modern CPU's give each process its own address space, a symbol may not land and probably won't land in the same address twice. In very recent times this is a security measure, but in past its so that dynamic linking works correctly.
So when the OS loads up an executable or shared library it can place it wherever it wants and by doing so make it not repeatable. Otherwise we'd all have to start caring and saying "this file contains 100% of the code I intend to execute". Usually on load the raw binary in the symbol table get transformed by patching it with the symbol locations in RAM. Making everything just work.
In summary the compiler emits files that allow for dynamic patching of assembly
prior to execution. If it didn't, we would be living in a very restrictive and problematic world.
Linkers even have scripts to change how they operate. They are a very complex and delicate piece of software required to make our programs work.
Have a read of the PE-COFF and ELF standards if you want to get an idea of just how complex those formats really are.
I want to compile my C file with clang and then decompile it with with ndisasm (for educational purposes). However, ndisasm says in it's manual that it only works with binary and not executable files:
ndisasm only disassembles binary files: it has
no understanding of the header information
present in object or executable files. If you
want to disassemble an object file, you should
probably be using objdump(1).
What's the difference, exactly? And what does clang output when I run it with a simple C file, an executable or a binary?
An object file contains machine language code, and all sorts of other information. It sounds like ndisasm wants just the machine code, not the other stuff. So the message is telling you to use the objdump utility to extract just the machine code segment(s) from the object file. Then you can presumably run ndisasm on that.
And what does clang output when I run it with a simple C file, an executable or a binary?
A C compiler is usually able to create a 'raw' binary, which is Just The Code, hold the tomato, because for some (rare!) purposes that can be useful. Think, for instance, of boot sectors (which cannot 'load' an executable the regular way because the OS to load them is not yet started) and of programmable RAM chips. An Operating system in itself usually does not like to execute 'raw binary code' - pretty much for the same reasons. An exception is MS Windows, which still can run old format .com binaries.
By default, clang will create an executable. The intermediate files, called object files, are usually deleted after the executable is linked (glued together with library functions and an appropriate executable header). To get just a .o object file, use the -c switch.
Note that Object files also contain a header. After all, the linker needs to know what the file contains before it can link it to other parts.
For educational purposes, you may want to examine the object file format. Armed with that knowledge it should be possible to write a program that can tell you at what offset in the file the actual code starts. Then you can feed that information into ndisasm.
In addition to the header, files may contain even more data after the instructions. Again, ndisasm does not know and nor does it care. If your test program contains a string Hello world! somewhere at the end, it will happily try to disassemble that as well. It's up to you to recognize this garbage as such, and ignore what ndisasm does to it.
I already searched the web, especially the avr-gcc website.
I want to know the STRUCTURE of the output file, of sourcecode, compiled with avr-gcc.
Example of a standard Microsoft .EXE file:
00h DW Signature word.
"N" is low-order byte.
"E" is high-order byte.
02h DB Version number of the linker.
03h DB Revision number of the linker.
Can someone please tell me the avr-gcc output file structure?
Thank you. -MW
edit:
As Rev1.0 said, it's the Intel-HEX format.
Since you talk about AVR, you probably mean the format of the HEX-file? That is encoded as Intel HEX format.
EDIT Regarding your question from the comment:
I see the Header has the field "DATA". What exactly is in that field? The pure assembly?
Each line of the HEX file is called a "record". There are several types of records. Depending on the record type, the data contents have a different meaning. A "data record" holds the actual firmware/program data. That is lowest level machine code, not assembly. It represents exactly the data that resides in the flash memory after the device has been programmed.
Can someone please tell me the avr-gcc output file structure?
avr-gcc is just a driver program that calls sub-processes on different files and file formats. For example, the compiler proper reads pre-processed input (text) and writes assembly (text).
The GNU assembler reads assembly (*.s, text) as generated by the compiler and writes object files (*.o, ELF32)
The GNU linker / locator reads this object files and resolves references to libraries like libgcc (*.a, ELF32) and produces a final executable (ELF32).
Depending on your loader, you can use ELF directly (for example with avrdude).
If you want something else like Intex HEX or plain binary as an AVR sees it, you'll convert ELF32 to the desired output by means of avr-objcopy from GNU Binutils.
In general, one wants to keep ELF as long as possible because formats like IHEX are "dumb". They don't have additional information like symbol info or debug info, these formats are only used to upload a program to an AVR, but most modern tools understand ELF as well.