Where are Library definitions located? - c++

Could someone say me where I can found the source file where are the definitions of the standard libraries?
For example, where is the file string.c that contains the definitions of the function prototyped in string.h? And above all, it exists?

its all in compilled state, some of maybe optimized by asm. You need find sources of your compiler to see definitions

For GCC, which is open source, you can download the sources for the libstdc++ library from their mirror sites here. Included in the download is the source for the std library. Bear in mind that different vendors will have different implementations, so the link provided is merely how the developers of GCC decided to implement the standard library

You're probably not going to like this.
The C++ Standard does not say specifically where anything in the Standard Libraries are actually implemented. It says where things are declared, but only to the degree that it names the file(s) you must #include in order to bring the names in. For example, the Standard says that:
std::string
is a typedef for basic_string<...>, and in order to bring that typedef in to your program, you must #include <string>. It doesn't actually say that basic_string or string are defined in <string> however, and it doesn't say where, on your hard drive <string> is even located. In fact, it's often not in <string> in the real world. In my implementation, (MSVC10) string is defined in a different file, <xstring>, and it looks like this:
typedef basic_string<char, char_traits<char>, allocator<char> >
string;
Useful, huh?
There's another aspect. A lot of the stuff in the Standard Library is template stuff, like string, so because of the way templates work in C++ these facilities must be so-called "include libraries." But not everything in the Standard Library is made up of templates.
Consider sprintf. The Standard says that this declaration is provided by #include <cstdio> but that, like string isn't even where it's declared. And sprintf isn't a template thing. the implementation is in what's often called the CRT -- the C Runtime Library. This is a collection of DLLs and LIBs (in MSVC10, anyway) that your program links to to run code like sprintf.
Now the bad news is those components that are in the CRT are generally shipped without source code. You don't know where sprintf is implemented and you can't look at the source code. You're left with little alternative in these cases except get a job with MicroSoft so you can take a look at the source code. :)

Related

Stop bycatch of <stdio.h> definitions caused by C++ includes

In a comment to another answer, I was shown a code example that seemingly used printf and puts without including <stdio.h> but the online compiler didn't complain.[1] To understand what's going on, I copied it to my local IDE.
Reduced to relevant includes and output, it's basically this:
#include <string>
#include <map>
#include <optional>
int main() {
printf("Answer %d\n", 42);
puts("Question?");
}
Experimenting with gcc 8.1.0 (packaged with Clode::Blocks 20.03), I found out, that the includes can be further reduced to
<string> or <map> or <optional> in C++17 (ISO/GCC)
<string> or <map> in C++11/C++14 (ISO/GCC)
<string> in C++98 (ISO/GCC)
Also a sample test - C++14 (gcc 8.3) - on ideone.com compiles and runs fine:
#include <iostream>
int main() {
printf("printf without #include <stdio.h>\n");
return 0;
}
This is also true for other definitions from <stdio.h> like FILE.
I found no information at cppreference.com
std::printf, std::fprintf, std::sprintf, std::snprintf - cppreference.com
std::puts - cppreference.com
printf, fprintf, sprintf, snprintf, printf_s, fprintf_s, sprintf_s, snprintf_s - cppreference.com
puts - cppreference.com
I also tried several web and SO searches but wasn't successful so far.
While it may be handy for small examples to get some powerful functions for free, but a serious project may suffer from it: besides comparatively easy to fix compiler errors, I see the danger of serious runtime errors.
How can I effectively control/prevent this kind of inclusion?
[1] the referenced code now contains the include statement, but I'm pretty sure that it didn't at stage I copied it .. or maybe I copied just a portion of it? ... anyway the observed behavior is there as described above.
I am afraid you cannot.
The standard requires that the well known include files declare the relevant names, but does not prevent them to include other files/names, if the library implementation finds it useful.
Said differently, after including iostream you are sure that all the names belonging to it are correctly declared, but you cannot know (except by examining the file itself) if other names have been defined, or if other standard files have been included. Here, your implementation chooses to automatically include stdio.h, but a different (standard library) implementation could choose not to include it. You have reached the world of unspecifiedness...
Aside from any arguments about "purity", I don't see how including the declarations for these functions could in any way be harmful. As a matter of fact, not having them and letting the compiler implicitly assuming int printf() – albeit only C compilers do it, C++ won't – is calling for trouble.
printf is part of the C language standard. And although C++ isn't C, there's so much overlap between their ecosystems, that C++ considers printf as part of the implementation runtime environment as well, as it does with all the other functions from the C standard library.
And as such, all of these symbols are reserved. Outside of actually implementing the runtime library itself, you have no business defining symbols of those names yourself.
#include preprocessor directives are just inline text substitution with text pulled in from an additional file.
(extern) symbol declarations will not create symbols of those names, but they will make sure, that you're not going to redefine those symbols at your own.
So I really don't see where your worries about runtime errors do come from.

How can I determine what header calls another header in c++?

I am reading from here : Can std::string be used without #include <string>?, <iostream> is calling <string>. But I do not see any includes of <string> in Standard library header <iostream> from c++ standard: https://en.cppreference.com/w/cpp/header/iostream. In <iostream> is only included <ios>, <streambuf>, <istream> and <ostream> according to that documentation. So this code works (file foo.cpp):
#include <iostream>
int main(){
std::cout << "enter srting\n";
std::string out; //std::string should not be included according to standard documentation
std::cin >> out;
std::cout << "test, " << out;
}
But I have to try to see dependecies generated from gcc:
cc -H foo.cpp |& vim -
and the output is (I have regex out files with string in it):
. /usr/include/c++/8/iostream
..... /usr/include/c++/8/bits/stringfwd.h
...... /usr/include/c++/8/string
....... /usr/include/c++/8/bits/basic_string.h
........ /usr/include/c++/8/ext/string_conversions.h
....... /usr/include/c++/8/bits/basic_string.tcc
So I can see that in the end, many "string" headers are indeed included
(so should I trust that documentation, when the did not mentioned <string> header in "Includes" list of <iostream>?).
Some of them are deeper on the #include stack (term from gcc manual), which brings me to question,
what calls what? And what is the "true" header, that define std::string of them? (is it /usr/include/c++/8/bits/basic_string.h?...)
from this question Why does omission of "#include <string>" only sometimes cause compilation failures?, they mentioned:
Some compilers on some platforms may on some time of the month compile even though you failed to include the header
But from the upper output of the "string headers", there is multiple of them , so how's possible for a compiler to compile only sometimes? Which of these headers are really important for successful compilation?
How to orient in cpp headers, which are meaningful for compiler, and could be tracked their "#include stack" (i.e. other meaningful headers)?
EDIT:
If it depends on my specific implementation of my stdlib++, then I want to know how can I determine from source whether that inclusion is made before I try to compile. Not by "If it compiles, then it works".
In general you cannot know what headers are transitively included by the headers you include, and any standard header is allowed to, but not required to, include any other header. And you shouldn't rely on transitive includes ever. You should include the headers you need for the things that you use and then you'll be good.
You should always check the documentation for any symbol you use and include any headers it is specified to require.
The C++ standard only specifies which symbols must be made available when a standard header is included. It does not place any limits on what other symbols are made available. In your example, the standard specifies the <iostream> must include <ios>, <streambuf>, <istream>, and <ostream>, but <iostream> may include any other headers its authors want. It may also forward-declare any symbols it may need.
(so should I trust that documentation, when the did not mentioned <string> header in "Includes" list of <iostream>?).
You should trust that the symbols specified as being available when you include <string> will be. That is all. You may not assume that those symbols will not be visible when including any other header.
what calls what? And what is the "true" header, that define std::string of them? (is it /usr/include/c++/8/bits/basic_string.h?...)
This is an implementation detail that can only be answered by inspecting the implementation's headers. Libstdc++ (the standard library implementation used by GCC) has the declaration of the std::string class in bit/stringfwd.h and its definition in bits/basic_string.h and bits/basic_string.tcc (for the current version, at least), but that is not required at all. If the libstdc++ maintainers decided they wanted to refactor and reorganize things, they would be free to do so. The only requirement that is guaranteed by the C++ language is that std::string must be available when <string> is included.
But from the upper output of the "string headers", there is multiple of them, so how's possible for a compiler to compile only sometimes?
Different standard library implementations or different versions of the same implementation could transitively include different headers. Different compiler flags (i.e. a debug flag or different standard compliance mode) could transitively include different headers.
Which of these headers are really important for successful compilation?
How to orient in cpp headers, which are meaningful for compiler, and could be tracked their "#include stack" (i.e. other meaningful headers)?
All of them are meaningful. The standard library's authors wouldn't include a header if they didn't need it for something. Just because you aren't using any symbols declared/defined in that header directly doesn't mean none are being used.
EDIT: If it depends on my specific implementation of my stdlib++, then I want to know how can I determine from source whether that inclusion is made before I try to compile. Not by "If it compiles, then it works".
The only way to know is to look at your standard library implementation's headers. There is nothing magical about them; they're just C++ code. If you want to know if <iostream> includes a declaration or definition of std::string, open your implementation's copy of <iostream> and look for a declaration or definition of std::string. Repeat this process for any headers that <iostream> includes.
<iostream> is calling <string>
Do you mean that it includes <string>? Calling is something that is done to functions.
But I do not see any includes of <string> in Standard library header <iostream> from c++ standard: https://en.cppreference.com/w/cpp/header/iostream.
What you're reading is documentation of <iostream>. Indeed, <iostream> is not documented to include <string>.
So this code works (file foo.cpp):
It may work with some standard library implementation. It may not work using other implementations.
(so should I trust that documentation, when the did not mentioned <string> header in "Includes" list of <iostream>?).
cppreference is fairly high quality. Unless it contradicts the standard, it is fairly safe to assume that it is correct. Indeed in this regard it is correct: <iostream> is not guaranteed to include <string>.
what calls what?
Some headers include some other headers. There is no need to know more accurately than that because you should not rely on transitive inclusions (except those that are documented).
And what is the "true" header, that define std::string of them?
As per documentation, std::string is defined in header named string (commonly stylised as <string> which matches the conventional inclusion syntax).
so how's possible for a compiler to compile only sometimes?
It could ask the current time from the operating system and use a branch to do one thing or another depending on that time. Compilers don't typically do that, but the point is that they could.
More realistically, you may at some point need to compile your program using another (version) of the compiler (or standard library implementation). Different implementations behave different from one another.
Which of these headers are really important for successful compilation?
The ones that are documented to define and declare the names whose definitions and declarations your program relies on. If you use std::string, then you must include the header that defines std::string which is <string>.

How to view standard library functions in C++?

For example, i want to see the code of function toupper() to understand how it works, is there any way? I have searched and opened string.h library, but didn't find anything.
From a strict language point of view, you cannot "see the code" of a standard function, because the C++ language standard only defines functions' prototypes and behaviours, not how they are implemented.
In fact, from a strict language point of view, a standard function like toupper does not even have to have source code, because a standard header, like <string.h> does not even have to be a file!
Of course, in practice, you will probably never encounter a C++ implementation in which standard headers are not files, because files are just a natural and simple implementation of headers. This means that in practice, for the header <string.h>, there is actually a C++ source file called "string.h" somewhere on your computer. Just find it and open it.
I have searched and opened string.h library, but didn't find anything.
Then you have not looked close enough. Hint: This file most likely includes one or more other header files.
Note that if you actually looked for toupper, that function is not in <string.h> anyway. Look in <ctype.h> instead. cppreference.com is a good online reference to tell you which headers contain which functions.
http://en.cppreference.com/w/c/string/byte/toupper
Again, this does not mean that the corresponding header file of your compiler contains that function directly, but it may directly or indirectly include some other file which contains it.
In any case, beware of what you will see inside of your compiler's header files. It will usually be a lot more complicated than you may think, and, more importantly, it will often use constructs you are not allowed to use in your own code; after all, the code in those files is internal to the compiler implementation, and the compiler has a lot of privileges you don't have, for example using otherwise forbidden identifiers like _STD_BEGIN. Also expect a lot of completely non-standard #pragmas and other non-portable stuff.
Another important thing to keep in mind is that you are not supposed to dig through a function's implementation to find out what it does. In badly written software, i.e. software with confusing interfaces and no documentation (which exists everywhere in the real world), you unfortunately have to do this, provided you have access to the source code.
But C++ standard functions are perfectly documented and have, with some arguable exceptions, well-designed interfaces. It may be interesting, and educating, and sometimes even necessary for debugging, to look into their implementation on your system, but don't let this possibility keep you from learning two important software-engineering skills:
Reading documentation.
Programming to interfaces, not to implementations.
Yes, of course, you could (not all realizations, maybe). For example, the glibc implementation defines toupper function as:
#define __ctype_toupper \
((int32_t *) _NL_CURRENT (LC_CTYPE, _NL_CTYPE_TOUPPER) + 128)
int
toupper (int c)
{
return c >= -128 && c < 256 ? __ctype_toupper[c] : c;
}

Custom headers higher than standard?

Is it reasonable to put custom headers higher in include section than standard headers?
For example include section in someclass.hpp:
#include "someclass.h"
#include "global.h"
#include <iostream>
#include <string>
Is it best practice? What is the profit if it is?
The reason is that if you forget to include a dependent header in someclass.h, then whatever implementation file includes it as the first header, will get a warning/error of undefined or undeclared type, and whatnot. If you include other headers first, then you could be masking that fact - supposing the included headers define the required types, functions, etc. Example:
my_type.h:
// Supressed include guards, etc
typedef float my_type;
someclass.h:
// Supressed include guards, etc
class SomeClass {
public:
my_type value;
};
someclass.cpp:
#include "my_type.h" // Contains definition for my_type.
#include "someclass.h" // Will compile because my_type is defined.
...
This will compile fine. But imagine you want to use use SomeClass in your program. If you don't include my_type.h before including someclass.h, you'll get a compiler error saying my_type is undefined. Example:
#include "someclass.h"
int main() {
SomeClass obj;
obj.value = 1.0;
}
It is fairly common practice to #include "widget.h" as the first thing in widget.cpp. What this does is ensure that widget.h is self-contained, i.e. does not inadvertently depend on other header files.
Beyond that, I think it's essentially a matter of personal preference.
There are two important observations to be made before delving in the specifics:
When you develop a new header/source pair, it is important to check that the header is self-contained. To do so, the easiest way is to include first in a file.
It is best not to include extraneous things before including a header you do not own, as this could create strange issues in case of conflict of macros or overload of functions.
Therefore, the answer depend if you have unit test or not.
A general rule of thumb is to include headers starting with the Standard Library, then 3rd party headers (including Open Source projects), then your own middleware, utilities, etc... and finally the headers local to this library. It more or less follows the order of dependencies to comply with observation 2.
The only exception I have seen was the one header corresponding to the current source file, which would be included first to make sure it is self-contained (observation 1)... but this only holds if you don't have unit tests, for if you do then the unit test source file is a very good place to check this.
While it is just personal choice, I would prefer to include standard headers first. Few reasons:
Any set of #ifdef..#define would be correctly mapped, rather than standard headers misinterpreting them. This goes for conditional compilation as well as values of some macros, while standard headers are being compiled.
Any change/new function in standard header may conflict with your function, and compiler would emit error in header file, which would be be complicated to solve.
All required standard headers should be placed in one header (preferbly some pre-compiled-header), include that header, and then include your custom header. This would reduce compilation time.
Start with the system headers.
If there are no dependencies between the headers both ways work, but since programming is essentially communication, not with the computer but with other humans, it is important to make it logical and easy to understand. And my opinion is that it is better to start with the system headers.
I base this one of my very first programming courses (in 1984, I think), where we programmed in Lisp and were taught to think like this: you start with the normal Lisp language, and then you create a new language that is more useful for your application by adding some functions and data types. If you for example add dates and the ability to manipulate dates, this new language could be called Lisp-with-dates. Then you could use Lisp-with-dates to create a new language with calendar functionality, which could be called Lisp-with-calendars. Like layers in an onion.
Similarly, you can view C as having a "core" language, without any headers, and then you can for example expand this language into a new, bigger language with I/O functionality by #including stdio.h. You add more and more stuff to the core language by #including more headers. (I am aware that the term "C language" in other contexts refers to the entire standard, with all the standard headers, but bear with me here.) Each new #included header creates a new, bigger language, and an additional layer of the onion.
Now, to me it seems that the standard headers obviously should be the inner part of this onion, and therefore before the custom headers. You can create the language C-with-monsters by adding stuff to C-with-I/O, but the people who created C-with-I/O did not start with C-with-monsters.
any place you include c++ compiler treats it as the same

Proper layout of a C++ header file

What is the proper layout of a C++ .h file?
What I mean is header guard, includes, typedefs, enums, structs, function declarations, class definitions, classes, templates, etc, etc
I am porting an old code base that is over 10 years old and moving to a modern compiler from Codewarrior 8 is proving interesting as things seem all over the place. I get a lot of dont name a type errors, forbidding declaring without a type, etc, etc.
There is no silver bullet regarding how to organize your headers.
However one important rule is to keep it consistent across the project so that all persons involved in the project know what to expect.
Usually typedefs and defines are at the top of the file in my headers, but that can not be regarded as a rule, then come class/template definitions.
A rule that I follow for C++ is one header per class, which usually keeps the headers small enough to allow grasping the content and finding things without scrolling too much.
It depends on what you mean by proper. If you mean language-enforced, there really isn't one. In fact, you don't even have to name it ".h". I've seen ".c" files #include'd in working commercial code (name withheld to protect the guilty). #include is just a preprocessor hack to get some kind of rough modularity in the language by allowing files to textually include other files. Anything else you tend to see as standard practice is just useful idioms people have developed over time.
That doesn't help your current issue though.
I'd guess that what you are actually seeing is a lot of missing symbols due to platform differences. Nothing due to weirdly-formed .h files at all.
It is possible that the old code was written to work with an old K&R-style C compiler. They had oddities like implicit function declarations (any reference to an undeclared routine assumed it returned int and all its parameters were int). You could try seeing if your compiler has a K&R flag, but a lot of the flagged stuff may actually be latent errors in the old code.
It sounds like you're running into assumptions made based on the previous implementation (Codewarrior). For example:
#include <iostream>
int main() {
std::cout << "string literal\n";
return 0;
}
This relies on iostream including something it's not required to declare: the operator<<(ostream&, char const*) overload (it's a free function, not a method of ostream like the others). And to be completely unambiguous, #include <ostream> is also required above. In C++, library headers are allowed to include any other library header, in general, so this problem crops up whenever someone inadvertently depends on this.
(That the extra header is required in this particular circumstance is considered a flaw by many, including me, and almost all implementations do provide the declaration of this function in iostream. It is still the shortest, common example I know of to illustrate this.)
It's often more subtle and complicated than this simple example, but the core issue is the same. The solution is to check every header to make sure it includes any libraries it requires, starting with the ones giving you the errors. E.g. #include <vector> and make sure you use std::vector (to avoid relying on it being in the global namespace, which is done in some, mostly old and obsolete now, implementations) when you get "vector does not name a type".
You might also be running into dependent types, in which case you'd add typename.
I think best thing you can do is to check out layout of any library files.