Why does gcc generate different executables for different sourcefilenames?
to test I have this c-programm called test.c and test2.c:
int main(){}
"gcc test.c -o test" and "gcc test2.c -o test2" generate different output files. Using a hex-editor I can see that there still is its source-filename hidden in it. Stripping the files still results in different results (the source-filename is gone). Why does gcc operate this way? I tested clang and tcc as well. Clang behaves the like gcc does, whereas tcc generates the same results for different filenames?
gcc version 4.9.1 (Debian 4.9.1-1)
clang 3.4.2-4
tcc version 0.9.25
Doing a diff on the hexdump of both binaries shows a small difference at around offset 0x0280. Looking through the sections (via objdump -x), the differences appear in the .note.gnu.build-id section. My guess is that this provides some sort of UUID for distinguishing different builds of otherwise similar code, as well as validate debug info (referenced here, about a third of the way down).
The -o option of gcc is to specify the output file. If you give him different -o targets, it will generate different files.
gcc test.c -o foo
And you have a foo executable.
Also, note that without a -o option, gcc will output a a.outexecutable.
Related
I have reinstalled mingw in my system and downloaded the gcc compiler.
I was shocked after compiling the first file which was "subject.c" but the name of the compiled file which gcc returned was "a.exe". It should be "subject.exe" but do not know why this happened.
Can anyone please explain the reason behind this ?
expected:
gcc subject.c
ls
subject.c subject.exe
tried:
gcc subject.c
ls
subject.c a.exe
-o can be used to give the name of the output file.
For example,
gcc -Wall -Wextra -pedantic subject.c -o subject.exe
(Do enable your compiler's warnings!)
gcc names its output files, in the absence of other instructions, a.out or a.exe depending on system environment because that is what it's supposed to do.
To override this default behavior, you can use the -o flag which tells gcc that the next argument is the desired name for the output file. For instance:
gcc -o subject.exe subject.c
There is no automatic functionality built into gcc to strip a source file of its file extension and add .exe to the end but this can be done manually with Makefiles or other similar scripts, for instance you can write a Makefile with the following contents:
%.exe: %.c
gcc -o $# $<
Then a command like make subject.exe would be translated to gcc -o subject.exe subject.c, which may be what you're looking for.
There is functionality built into gcc to strip source files of their extensions during different parts of the compilation process, which may have been what confused you. For instance a call like gcc -c subject.c can be expected to produce an object file called subject.o, likewise gcc -S subject.c can be expected to produce an assembly language file called subject.s, however this does not apply to executable files not only for historical reasons, but because programs can be compiled from multiple source files and there is not always a clear way to choose a name for the executable output.
I wonder if there is any benefit for using the -S GCC option in my Makefiles.
I've been compiling C files like the following for quite some time now:
gcc -c a.c -o a.o
gcc -c b.c -o b.o
---
gcc a.o b.o -o a.out
Now would it be better going:
gcc -S a.c -o a.s
gcc -S b.c -o b.s
---
gcc -c a.s -o a.o
gcc -c b.s -o b.o
---
gcc a.o b.o -o a.out
Also there is apparently the option of skipping the .o phase, assembling directly .s files into a binary. Which option you think is the best and why?
-S flags asks gcc to produce human readable assembly code - .o files are nice for a linker but rather cryptic for most human beings...
It is mainly used when you need low level optimization of a (short) piece of code that has been identified by profiling as being a bottleneck. You can compare how the compiler will translate various versions and choose the one that will give the most efficient machine code for that specific implementation.
It is not intended to be used in standard makefiles.
Also there is apparently the option of skipping the .o phase, assembling directly .s files into a binary.
Plain assembly is never transformed directly to executable binary code, there is always in intermediate object-file step.
gcc a.s b.s -o ab.exe
will always call the assembler (twice) which produces object code for either units, and then the objects are linked. Add -v to the command line to see which sub-commands are executed by gcc. gcc is not actually a compiler, it is just a driver program calling jobs depending on options and file extensions. The compiler proper is cc1 (for C code), cc1plus (for C++ code), etc.
Which option you think is the best and why?
-S has the advantage to producing assembly code, however the compiler will always generate assembly code as intermediate step. It's just the case that it's written to temporary files, with 2 notable exceptions:
-save-temps: This will not use some temporary-file names (for example in /tmp), but save the intermediate code in the same place as the objects (there are two flavors actually, -save-temps=obj and -save-temps=src).
-pipe: This will used pipes to transfer code from one sup-program to the next instead of files (except with -save-temps which nullifies -pipe).
Thus, if you want to see the generated assembly, -save-temps might be the way to go. However, that option also applies to the pre-processed code which is saved in .i for C, .ii for C++ and .s for assembly. This is often very appreciated when working with C macros.
In the case you intend to inspect the compiler-generated assembly, you might enjoy -fverbose-asm which injects asm comments that indicate the C/C++ source associated to the assembly. And it might be a good idea not to clutter assembly with debug-info in that case.
I use gcc compiled the hello.c:
dele-MBP:temp ldl$ ls
a.out hello.c
now, when I cat a.out:
$ cat a.out
??????? H__PAGEZERO?__TEXT__text__TEXTP1P?__stubs__TEXT??__stub_helper__TEXT???__cstring__TEXT??__unwind_info__TEXT?H??__DATA__nl_symbol_ptr__DATA__la_symbol_ptr__DATH__LINKEDIT ?"? 0 0h ? 8
P?
/usr/lib/dyldס??;K????t22
?*(?P
8??/usr/lib/libSystem.B.dylib&`)h UH??H?? ?E??}?H?u?H?=5??1ɉE??H?? ]Ð?%?L?yAS?%i?h?????Hello
P44?4
there shows the messy code.
I want to know what type of the a.out? is it assembly language? if is why there have so many ??? or %%%?
There are several intermediate file formats, depending on the compiler system you use. Most systems use the following steps, here shown with GCC as example:
Preprocessed C source (gcc -E test.c -o test.i), but this is before compilation, strictly speaking
Assembly source (gcc -S test.c -o test.s)
Object file containing machine code, not executable because calls to external functions are not resolved (gcc -c test.c -o test.o)
Executable file containing machine code (gcc test.c -o test)
Only the first two steps generate text files that you could read by cat or in a text editor. This is BTW a valuable source for insight. However, you can use objdump to see most informations contained in the other formats. Please read its documentation.
Each step does also all steps before it. So (gcc test.c -o test) generates assembly source and object file in temporary files that are removed automatically. You can watch that process by giving GCC the option -v.
Use gcc --help to see some entry points for further investigations.
There is at lot more to say about this process but it would fill a book.
I want clang to compile my C/C++ code to LLVM bitcode rather than a binary executable. How can I achieve that?
And if I have the LLVM bitcode, how can I further compile it to a binary executable?
I want to add some of my own code to the LLVM bitcode before compiling to a binary executable.
Given some C/C++ file foo.c:
> clang -S -emit-llvm foo.c
Produces foo.ll which is an LLVM IR file.
The -emit-llvm option can also be passed to the compiler front-end directly, and not the driver by means of -cc1:
> clang -cc1 foo.c -emit-llvm
Produces foo.ll with the IR. -cc1 adds some cool options like -ast-print. Check out -cc1 --help for more details.
To compile LLVM IR further to assembly, use the llc tool:
> llc foo.ll
Produces foo.s with assembly (defaulting to the machine architecture you run it on). llc is one of the LLVM tools - here is its documentation.
Use
clang -emit-llvm -o foo.bc -c foo.c
clang -o foo foo.bc
If you have multiple source files, you probably actually want to use link-time-optimization to output one bitcode file for the entire program. The other answers given will cause you to end up with a bitcode file for every source file.
Instead, you want to compile with link-time-optimization
clang -flto -c program1.c -o program1.o
clang -flto -c program2.c -o program2.o
and for the final linking step, add the argument -Wl,-plugin-opt=also-emit-llvm
clang -flto -Wl,-plugin-opt=also-emit-llvm program1.o program2.o -o program
This gives you both a compiled program and the bitcode corresponding to it (program.bc). You can then modify program.bc in any way you like, and recompile the modified program at any time by doing
clang program.bc -o program
although be aware that you need to include any necessary linker flags (for external libraries, etc) at this step again.
Note that you need to be using the gold linker for this to work. If you want to force clang to use a specific linker, create a symlink to that linker named "ld" in a special directory called "fakebin" somewhere on your computer, and add the option
-B/home/jeremy/fakebin
to any linking steps above.
If you have multiple files and you don't want to have to type each file, I would recommend that you follow these simple steps (I am using clang-3.8 but you can use any other version):
generate all .ll files
clang-3.8 -S -emit-llvm *.c
link them into a single one
llvm-link-3.8 -S -v -o single.ll *.ll
(Optional) Optimise your code (maybe some alias analysis)
opt-3.8 -S -O3 -aa -basicaaa -tbaa -licm single.ll -o optimised.ll
Generate assembly (generates a optimised.s file)
llc-3.8 optimised.ll
Create executable (named a.out)
clang-3.8 optimised.s
Did you read clang documentation ? You're probably looking for -emit-llvm.
I want to compile an application with debug information using gcc and gdb. When I do the following, the debug (.dSYM) files are generated correctly:
gcc -ggdb src/test.c -o build/test
If I, however, split this into a compile step and a link step, like this:
gcc -ggdb -c src/test.c -o build/test.o
gcc -ggdb build/test.o -o dist/bin/test
… no .dSYM files are generated at all, and therefore gdb does not show me the source line of code where a crash occurs, which makes debugging a lot more difficult. Since I have quite a bit of source files, compiling and linking them all in a single gcc invocation is not possible.
How can I let gcc generate the .dSYM files when using separate compile and link steps?
Check the second comment on the first answer in this post.
Its a quirk. Maybe you can run the "dsymutil" program manually to generate dSYM files.
You can also specify -g3 in you compilation options so that gcc puts debug symbols right into the binary, not in a separate file. Not sure if that is what you need.