Many subprogram declarations of GNAT Standard Library uses Pragma Inline in their specification files and I want to know how it work because other modules use only this specification during compilation and not body?
Related
In GCC10, gcc default to fno-common. That means, all tentative defined symbols are not common. I think gcc conforms to the C specification but it seems there are no common symbols in the native C program. Are common symbols only for extension syntax?
Does native C have common symbol?
Read the C11 standard n1570. Its index don't even mention common symbols.
Read carefully also the documentation of GCC and this draft report.
Perhaps you refer to the ELF file format used on Linux for object files and executables. There you can find a mention of common symbols, which tend to be deprecated .... Read the Linux ABI specification, etc here.
My recommendation is to declare all your public symbols as extern in some header file (#include-d in most of your *.c files), and define them once (without extern) in a single translation unit. You could use simple preprocessor tricks (such as X-macros).
You might be interested in using C code generators such as lemon or SWIG, or develop your script (with GNU awk or Guile or Python or GPP etc... ....) for simple metaprogramming techniques (autoconf could be inspirational) generating some C code. Configure your build automation tool (GNU make, ninja...) suitably.
You might be interested in using static analyzer options and precompiled headers of recent GCC. Look also into Clang static analyzer and clang tidy and Frama-C.
You surely want to pass -Wall -Wextra -g -H to gcc and read How to debug small programs and Modern C.
No, it has nothing to do with "extension syntax", and it has nothing to do with "common symbols" as a language construct. It simply refers to the behavior of variable declarations at file scope.
C says that if you place a declaration like int i; in a file, and don't elaborate on it anywhere else, then it will have external linkage and it will be considered to be defined to have a value of 0. This is called a "tentative definition". Declarations with the same name in different files, if they have external linkage, all refer to the same variable. Generally the way to use external linkage is to define a variable in one file, and use an extern declaration in any other files that make use of it.
In GCC with -fcommon, tentative definitions for the same variable can appear in more than one file. GCC will resolve this at link time, and allocate storage (initialized to zero) for the variable once.
In GCC with -fno-common, tentative definitions are resolved to definitions ASAP when the file is compiled. If more than one file contains a tentative definition for a variable, then this will cause a multiple definition error at link time.
As far as I can tell, the C standard doesn't require or prohibit either behavior. In particular, C does not have C++'s "one definition rule". However, the -fno-common behavior is generally less surprising, catches a forgotten extern sooner, and allows the compiler to optimize better (because it knows exactly where the variable lives when compiling, instead of waiting to find out later). For these reasons the default was changed in GCC.
I'm in the process of switching from a def file to using _declspec for a library I maintain. I have read several of the questions here on SO and the MSDN documentation. I understand how the feature works. I have created macros and use _declspec or gcc __attribute__ depending on the build environment. The macros also properly select between _declspec(dllexport) and _declspec(dllimport).
Is there any harm in using _declspec on the function definitions? Should _declspec only be used on the function prototypes?
I would prefer to have the marco on both the function prototype and definition. I did test using _declspec on both the function prototype and definition. Using VS the library compiled without warnings and I was able to use the library without issue.
NO harm on function defination - sing __declspec(dllimport) is optional on function declarations, but the compiler produces more efficient code if you use this keyword. However, you must use __declspec(dllimport) for the importing executable to access the DLL's public data symbols and objects.
It is mostly used for importing symbols from / exporting symbols to a shared library (DLL). Both Visual C++ and GCC compilers support __declspec(dllimport) and __declspec(dllexport).
You can also use it for variables.
I'm studying C language nowadays. In this book, it is said that the "compiler provides these library functions: 'printf','scanf'…".
I can't understand. Those functions are defined in the header file <stdio.h> aren't they?
Why does this book explain those functions are provided by the compiler?
printf, scanf, and the other standard library functions are provided as part of the implementation.
A C implementation is made up of several components. The compiler is just one of them. The library is another; it consists of headers (commonly provided as source files like stdio.h) and some form of object code files containing the code that actually implements the library functions.
The header stdio.h only declares these functions; it doesn't define them. The declaration of printf is something like:
int printf(const char *format, ...);
The definition of printf is the code that actually does the job of parsing the format string, accessing the arguments, and sending the formatted output to stdout. That's typically (but not necessarily) written in C and provided as some kind of linkable object code.
For some C implementations, the compiler and the library are provided by the same organization. For others, they might be provided separately (for example MinGW combines the gcc compiler with Microsoft's library).
The functions are provided by the standard library, which is a collection of precompiled code that is typically written by the compiler authors (but it is indeed not a part of the compiler itself).
Note, though, that the functions are only declared in the header files. The definition resides in source files that have already been compiled.
By saying, "Compiler provides these library functions , 'printf','scanf'..", the author of the book is being sloppy.
A standard conforming C implementation provides declarations of those functions in header files and implementations of those functions in some short of library. A compiler is just one aspect of a C programming environment.
The compiler does not provide those functions. The goal of the compiler is to translate your high-level language code into another form, in particular an executable binary.
The standard C library contains the functions in stdio.h and stdlib.h.
The compiler links the standard library with your code so that your code can call those functions.
For almost all libraries, you have to tell the compiler what libraries you want to link. It so happens that for some compilers, the library (libc) for stdio.h and stlib.h is automatically linked without you needing to specify them.
Those functions provided by standard library and GCC includes built-in versions of many of the functions in the standard C library.
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
So I'm writing portable embedded ansi C code that is attempting to support multiple compilers and hardware targets. Each compiler/hardware vendor has different math.h functions it supports. Some support only C90, some support a subset of C99, others a full set of C99.
I'm trying to find a way to check if a given function exists during preprocessor so that I can use a custom macro if it doesn't exist. Some vendors have extern functions in the math.h, some use #define to remap to some internal call. Is there a piece of code that can tell if it is #defined or an extern function? I can use #ifdef for the define, but what about an actual function call?
The usual solution is instead to look at macros defined by the preprocessor itself, or passed into the build process as -D definitions, which identify the compiler and platform you're running on, and use those plus your knowledge of what special assists each environment needs to configure your code.
I suppose you could write a series of test .c files, try compiling them, look at the error codes coming back, and use those to set appropriate -D flags... but I'm not convinced that would be any cleaner.
My basic question is how the compilation process works to use standard library routines. When I #include <stdio.h> in C, does the preprocessor take the entire standard library and paste it into my source file?
If this is so, when I use a library routine, how is it that the linker is involved?
The preprocessor is, as its name implies, a program that runs before the compiler. All it does is simple text substitutions.
When a #include directive is found, it simply "pastes" the complete file into the place where the directive was. The same applies to macro expansions, when a macro "call" is detected, the body of the macro is "pasted" into its place.
The preprocessor has nothing to do with libraries. It's just that C (and C++) needs to have all its functions and variables declared before they are used, and so putting the declarations in a header file that is included by the preprocessor is a simple way to get these declarations from libraries.
There are basically two types of libraries: Header only libraries, and libraries you need to link with. The first type, header only libraries, are exactly what the name implies: They are fully contained in the header files you include. However, the vast majority of libraries are libraries you need to link with. This is done in a step after the compiler has done its work, by a special program. How this is used depends on the environment of course.
In general, compilation of a program can be divided into these steps:
Editing
Preprocessor
Compiler
Linker
The editing step is what you do to create your source.
The preprocessor and compilation steps are often put together into a single step, which is probably why there is some confusion among beginners as to what the preprocessor really does.
The final step, linking, is taking the input from the compiler, and uses that together with the libraries you specified to create the final executable.
When I pound include in C, does the preprocessor take the entire standard library and paste it into my source file?
Only the header files you #include.
If this is so, when I use a library routine, how is it that the linker is involved?
The standard library headers contain declarations only. The definition (implementation) of the functions is in a library file, most likely /usr/lib/libc.ext (ext being an OS-dependent extension).
When you #include something in your source code, the preprocessor pastes whatever you #include into your source file.
But specifically, if you include a header file from a library, you are just including function declarations like void a();, and the linker finds implementations of these functions in the library itself.