With reference to this question:
On an embedded project for a small micro I found my compiled code size was much larger than expected. It turned out it was because I had included code that used assert(). The use of assert was appropriate in the included code but caused my compiled code size to almost double.
The question is not around if/when assert should be used but how the compiler/linker decides to include all the necessary overhead for assert.
My original question from the other post:
It would be helpful if someone could explain to me how gcc decides to include library functions when assert is called? I see that assert.h declares an external function __assert_func. How does the linker know to reference it from a library rather than just say "undefined reference to __asert_func"?
When configuring a toolchain, the authors decide which libraries that should be linked to by default.
Often this is includes runtime startup/initializing code and a library named libc which includes an implementation of the C standard, and any other code the authors deem relevant (e.g. libc might also implement Posix, any custom board specific functions etc.) and for embedded targets it's not unusual to also link to a library implementing an RTOS for the target.
You can use the -nodefaultlibs flag to gcc to omit these default libraries at the linking stage.
In the case of assert(), it is a standard C macro/function , which is normally implemented in libc. assert() might print to stdout if it fails, so using assert() could pull in the entire stdio facility that implements FILE* handling/buffering, printf etc., all which is implemented in libc.
You can see the libraries that gcc links to by default if you run gcc -v for the linking stage.
The gcc (or g++) command is simply a driver. It runs other programs, including the compiler proper (cc1 for C code, cc1plus for C++ code) and the assembler and the linker.
What programs are run are determined by the spec file (and there is an implicit one, see the -dumpspecs developer option). BTW, running gcc with the -v option displays the actual programs involved.
The assert macro is defined (see file /usr/include/assert.h) to do some check in <assert.h> only if NDEBUG is not a defined preprocessor symbol. On my Linux/Glibc system it can call a __assert_failed internal function from the C standard library. Citing assert(3) documentation:
If the macro NDEBUG is defined at the moment <assert.h> was last
included, the macro assert() generates no code, and hence does
nothing at all.
Some projects are compiling with -DNDEBUG their code in production mode.
You should read the Invoking GCC chapter of the documentation.
Perhaps you want to compile with -ffreestanding to avoid any extra libraries, even the standard ones?
On your embedded system linking is static. Static linking works as follows.
A static library is an archive of object files. The linker considers each object separately.
A referenced function or variable that is found in a static library is included in the resulting executable, together with the entire object file that contains the referenced symbol. Object files that don't contain referenced symbols are not pulled in.
This isn't by any way specific to gcc. Linkers work this way since the dawn of time.
Related
How does some std-lib, external-libs or any other pre-compiled src code such as the well-known header file <iostream> with its corresponding object file or static/dll lib get linked into my own application automatically? Does the compiler do it implicitly/under-the-hood or something like a compiler pre-linked list operation?
If such a case exist how do we use its functionality in our accord, Is there a way to put my own obj, dll, static-lib or src file into that ideal list via writing some special syntax without changing the initial directories of each of it, neither the help of an IDE config and outside-software, the goal is to drop the linking phase explicitly at terminal, want to do this configuration inside of the src-code.
Does every std-lib had a direct/inline special src-code that doing this kind of operation? If there are, then how do we take advantage of it? Or if everything is done by a compiler/handler and if it is generic-type then you could modified it with less problem but the delema is, it is fixed with the compiler and hate to forcefully modified/forked it. If there is alreadly a way to do this without explicitly tinkering it, for such doing it only at your onw src-code/write-time as a said at the first line of this block statement: "Does every std-lib/external-lib had a direct/inline special src-code that doing this kind of operation?".
// a.cpp
#include<iostream>
// there's no linking on iostream obj, src, dll, static-lib file
// love to have this kind of special features to our own none-std-lib/etc.
>c/cpp-compiler -c a.cpp
>c/cpp-compiler -o a a.o
Note: some of my terminologies are base on my own experience so watch out and be open-minded. For as I grow in the coding-community using terminology/standard way of communicating are a mess specially exploring from low to another low and to high to another high level prog-lang.
It depends on what you call "the compiler".
Most modern toolchains - including gcc, clang, Visual C++ - are based on a "compile then link" model, with several components. One of those components is the preprocessor (which does text substitution on C or C++ source code, to produce some modified source code), a "compiler" that translates preprocessed source code into object files, utility programs that produce libraries from sets of source files, a linker that produces an executable file from a set of object files and libraries, and - last but by no means least - a driver program that coordinates execution of other components.
The specifics are different between toolchains - e.g. VC++ does things quite differently than gcc/g++ or clang. The concepts are similar.
In what follows, I'll give a very over-simplistic (imprecise, details omitted) discussion of what gcc and g++ (in the gnu compiler collection) do.
When you use gcc or g++ at the command line you're actually using a driver program, that orchestrates execution of a bunch of other programs (the preprocessor, the compiler, the linker, etc). Depending on what options you provide, the result produced differs. For example, gcc -E only completes preprocessing of source files, g++ -c means the process stops after compiling source files to produce object files. If used to produce an executable, the driver program will use the linker to (well!) link object files and libraries together to produce an executable.
So, if you think of gcc or g++ (the program you execute directly) as the compiler then you could claim the compiler does implicit linking. When being used to create an executable, both execute the linker - and provide it information needed (e.g. names of libraries). gcc automatically links in libraries needed by C programs (e.g. the C standard library) while g++ automatically links in libraries needed by C++ programs (e.g. parts of the C++ standard library as well as the C standard library).
However, if you take a narrower view of the compiler - it is the program that only translates source files into object files - then there is no implicit linking of libraries by the compiler. It is the driver program that orchestrates compiling and linking, not the compiler that orchestrates linking.
If you read documentation for your favourite toolchain, it will describe various means (extensions of source files, settings, command line options, values of environment variables, etc) to control what it does. There is typically flexibility to do preprocessing only, compilation only, output assembler, linking only, or a complete "compile multiple source files then link them together to produce an executable" process.
The linker search libraries in some oreder in which the standard libs folder is searched first.
There are somed default libraries that gets loaded by default like glibc.
this way you dont need to specify to the linker to link with standard libs.
Gcc even have flags for not linking with some standard libs
https://docs.oracle.com/cd/E19205-01/819-5262/auto29/index.html
Note that while it is not standard Microsoft's Visual C++ has a #pragma based language extension that allows specifying files to link in the source:
#pragma comment(lib, "yourfile.lib") // or yourfile.obj
The comment pragma can also be used to specify a few other linker command line options, for example:
#pragma comment(linker,"\"/manifestdependency:type='win32' name='Microsoft.Windows.Common-Controls' version='6.0.0.0' processorArchitecture='' publicKeyToken='6595b64144ccf1df' language=''"")
Note that the list of linker options that can be specified this way is fairly limited and that while there are a few other legal 'comment' types only lib and linker really have meaning.
In C or C++, does the compiler do implicit linking?
As the "compiler" (understood as the whole group of tools in the chain that generate the final executable) has whole control over creating that final executable, it does everything related to every stage of compilation, including implicit linking.
How does some std-lib, external-libs or any other pre-compiled src code such as the well-known header file with its corresponding object file or static/dll lib get linked into my own application automatically?
The same as any other library is linked - linker searches the library for symbols and uses them.
Does the compiler do it implicitly/under-the-hood or something like a compiler pre-linked list operation?
Yes (for the compilers I worked with).
But it's very specific to the compiler. From the point of C++ language, there is no requirement on compiler command line options. If the compiler -needs-this-option-to-link-with-standard-library, it's fine and specific to that compiler. It's a quality of implementation issue. Surely users would want some things to be done implicitly with sane defaults for that compiler.
how do we use its functionality in our accord, Is there a way to put my own obj/dll/static-lib/src file into that ideal list via writing some special syntax without changing the initial directories of each of i
Because the compiler does it implicitly, you have to modify the compiler. That strongly depends on the compiler, and specific system and specific compiler own very specific configuration and build settings.
For example on Linux with gcc you can use the method in Enable AddressSanitizer by default in gcc . You can also use the method in Custom gcc preprocessor but overwrite collect2 stage.
I have a static library file called libunp.a, I do know I could use gcc -lunp xx to link to the library.
I could use #pragma comment(lib,"xxx.lib") to tell the Microsoft C/C++ compiler to include the library; how could I do it under Linux/GCC?
There doesn't seem to be any mention of any equivalent pragmas in the GCC manual's page on pragmas.
One reason I saw for GCC not supporting linking in source code was that sometimes, correct linking depends on link order; and this would require you to make sure that the linking order happens correctly no matter the order of compilation. If you're going to go to that much work, you may as well just pass the linker arguments on the command line (or otherwise), I suppose.
Libraries should be specified during the linking step. Such information simply
doesn't belong inside a translation unit. A translation unit can be preprocessed,
compiled and assembled even without a linking stage.
Simply because #pragma comment(lib,"xxx.lib") is in the source file does not mean the compiler consumes it. In fact, it goes in as a comment and is subsequently used by the linker. Not much different than *nix.
Use this GCC flag to generate an error for unknown pragmas. It will quickly tell you if the compiler understands it.
-Werror=unknown-pragmas
Are there any differences in the linking process between gcc and g++?
I have a big C project and I just switched part of the code to C++. The code isn't using std C++ library yet, so -llibstdc++ isn't needed for now.
The main difference is that (assuming the files are detected as C++) g++ sets up the flags needed for linking with the C++ standard library. It may also set up exception handling. I wouldn't rely on the fact that just because your application doesn't use the standard library that it isn't needed when compiled as C++ (for example the default exception handler).
EDIT: As pointed out in comments you'll have trouble with any constructors (that do work) for static objects as well as not getting virtual function tables (so if you're using those features of C++ you still need to link that library).
EDIT2: Unless you're using C99 specific code in your C project I would actually just switch to compiling the whole thing as C++ as the first step in your migration process.
gcc and g++ are both just driver programs that don't do anything other than calling other programs, so you can use the -v option to see exactly what they do -- what other programs they invoke with what args. So you can see exactly what the difference is between linking with gcc and g++ for the specific version and architecture of gcc that you happen to have installed. You can't rely on that staying the same if you want portability, however.
Depending on what you are doing, you might also be interested in the -### argument
I think that the g++ linker will look for the CPP mangled function names, and it is different from the C ones. I'm not sure gcc can cope with that. (Provided you can explicitly use the C version rather than the C++ one).
Edit:
It should work if you have
extern "C" {
<declarations of stuff that uses C linkage>
}
in your code and the object file has been compiled with g++ -c. But I won't bet on this.
Every time I want to use CURL in one of my C++ programs, I have to add the flag -lcurl as a flag onto g++. This can be especially annoying when working with Eclipse. If /usr/include/curl/curl.h exists, what do I need to do to have CURL always be within the include path for g++?
tl;dr: you have to add the flag.
The linker needs libcurl, not the compiler. The compiler needs the header; the linker needs the lib.
To simplify things quite a bit, the header file tells the compiler that the declarations will be defined later. libcurl is what actually defines them.
The linker does not guess-and-check what to link against (doing so would be a horrible idea). You must explicitly tell it what to link against (except for the default libs). In particular, the linker has to know to use libcurl to find the declarations that curl.h laid out. Without libcurl, the linker is missing functions and thus cannot produce a complete binary.
I'm not familiar with Eclipse, but I'm nearly positive that it has an option where you can specify additional libraries. Yes, you'll have to do that once per project, but that shouldn't be a major overhead.
Try adding curl path in
Properties -> C/C++ General -> Paths and Symbols:
This is just how linking works in C and C++.
When you compile the program, you include the header file /usr/include/curl/curl.h. The compiler does this part. The header file contains all of the definitions for the library interface.
When you link the program, you link in the library /usr/lib/libcurl.so, or whatever it happens to be named. The linker does this part. The library contains the implementation in either a loadable (for dynamic libraries) or linkable (for static libraries) format.
The C and C++ languages have no way of specifying which libraries should be linked in, so you have to pass -lcurl to the linker. This is just the way it is.
There are some extensions to C and C++ that allow you to encode library dependencies in your source code, e.g., #pragma comment with MSC, but they're not supported by your typical ELF toolchain, as far as I know.
Note: Actually, the -lcurl flag is not for g++, but it is for the linker, ld. When you pass -lcurl to g++, g++ passes it through to the linker.
Are there any differences in the linking process between gcc and g++?
I have a big C project and I just switched part of the code to C++. The code isn't using std C++ library yet, so -llibstdc++ isn't needed for now.
The main difference is that (assuming the files are detected as C++) g++ sets up the flags needed for linking with the C++ standard library. It may also set up exception handling. I wouldn't rely on the fact that just because your application doesn't use the standard library that it isn't needed when compiled as C++ (for example the default exception handler).
EDIT: As pointed out in comments you'll have trouble with any constructors (that do work) for static objects as well as not getting virtual function tables (so if you're using those features of C++ you still need to link that library).
EDIT2: Unless you're using C99 specific code in your C project I would actually just switch to compiling the whole thing as C++ as the first step in your migration process.
gcc and g++ are both just driver programs that don't do anything other than calling other programs, so you can use the -v option to see exactly what they do -- what other programs they invoke with what args. So you can see exactly what the difference is between linking with gcc and g++ for the specific version and architecture of gcc that you happen to have installed. You can't rely on that staying the same if you want portability, however.
Depending on what you are doing, you might also be interested in the -### argument
I think that the g++ linker will look for the CPP mangled function names, and it is different from the C ones. I'm not sure gcc can cope with that. (Provided you can explicitly use the C version rather than the C++ one).
Edit:
It should work if you have
extern "C" {
<declarations of stuff that uses C linkage>
}
in your code and the object file has been compiled with g++ -c. But I won't bet on this.