CODEWITHSUNDEEP
c++stlcross-platform
Ronald joe
Ronald joe

Reputation: 349

'windows.h', what about other platforms?

I'm using visual studio on windows and I have this header file called windows.h which of course gives me access to the win32 API.

but when I try to use other platforms API like the linux API for example. I don't find any header file with that name, which mean that I can't use any other OS API than windows,

now the problem here is: what if I wan't to make a cross platform program for example ? when I don't have the appropriate access to its API, I tried to do some researches for a sometime but couldn't get any answer to my questions, which is really troubling my mind right now, so to be more specific these are my questions:

1- why I can only use the windows API out of all the other operating systems ? is it because I'm coding on windows, so if I was coding "somewhere else" it would be different ? or is it something related to the compiler itself ?

2- what if I want to use another API do I have to use an external library ? and if that's the case how is the Standard Library in c++ cross platform, I mean if it is cross platform then there should exist other platform specific headers provided by the library right ?

Upvotes: 1

Views: 3715

Answers (4)

datenwolf
datenwolf

Reputation: 162164

I'm using visual studio on windows and I have this header file called windows.h which of course gives me access to the win32 API.

Strictly speaking windows.h is sort of a meta-header. Some important macros, tokens and symbols are defined in windows.h, but it also pulls in a lot of other headers. If you look at the reference manual of each of the Win32 API functions it will tell you there, which header that function is declared in. Let's have a look at CreateFileA for example: At the bottom of the reference manual you'll find:

Requirements

Minimum supported client Windows XP [desktop apps only]

Minimum supported server Windows Server 2003 [desktop apps onliy]

Target Platform Windows

Header fileapi.h (include Windows.h)

Library Kernel32.lib

DLL Kernel32.dll

So this tells us, that CreateFileA actually is declared in fileapi.h and is exported from the kernel32.dll system library.

but when I try to use other platforms API like the linux API for example.

That is, because there is no linux.h API header (if you search a Linux development system for files named linux.h you'll find plenty, but those are not used for the system level API).

The reason for that is, that Linux doesn't have its very own, proprietary API, but folloes the POSIX industry standard for operating system APIs, and the Single Unix Specification maintained by the Open Group.

There are of course Linux specific APIs, but you can safely ignore them for "usual" application development; you need those if you're doing low level stuff, like writing a C runtime library, or custom memory allocators.

Of most interest for you, as a developer are the manpages in section 2 (calls into the operating system kernel = syscalls) and section 3 (library functions). https://linux.die.net/man/

The POSIX equivalent to CreateFileA would be open which you can find in section 2: https://linux.die.net/man/2/open (or the creat syscall, that exists for legacy reasons, but nobody uses that (or should use it)).

If you look at the manpage of open it tells you

open(2) - Linux man page

Name

open, creat - open and possibly create a file or device

Synopsis

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);
int creat(const char *pathname, mode_t mode);

That tells you, that in order to use open you need to include sys/types.h, sys/stat.h and fcntl.h. Unlike Windows there's no all overencompassing header that pulls in everything. The reason for this is, that for backwards and cross compatibility reasons the things that are exposed by a header can be configured by setting certain macros to specific values before including the header. Those are explained in feature_test_macros(7), and for functions where they apply these are also mentioned in the respective manual page.

I don't find any header file with that name, which mean that I can't use any other OS API than windows,

Just because you can't find it, doesn't mean it doesn't exist.

now the problem here is: what if I wan't to make a cross platform program for example?

Then you write all the platform specific things into a separate .c file, that encapsulates the OS stuff.

when I don't have the appropriate access to its API

Well, you do.

1- why I can only use the windows API out of all the other operating systems?

Because you can't. Windows.h is available only on Windows systems.

2- what if I want to use another API do I have to use an external library?

Then you just use it.

Keep in mind that in Linux the graphical environment is not part of the main operating system. It's in fact a regular program that is automatically launched on system start. Hence you won't find documentation for that in the aforementioned manpages.

The default graphics enviroment for Linux is Xorg (an open implementation of the X11 display system). There's also Wayland, but after over 10 years in development it's still poorly supported and leaves a lot to be desired.

If you want to program X11 on the same level as Win32, you'll have to deal with the Xlib or Xcb. However direct X11 programming is quite tedious. You probably want to use some nice application framework like Qt or GTK. Qt by itself is a cross platform framework, so if you limit yourself to use only, and only Qt functions, your program will be fully cross platform without extra effort. If you want to do 3D graphics on the GPU, use the cross platform APIs OpenGL or Vulkan.

Upvotes: 3

eerorika
eerorika

Reputation: 238351

but when I try to use other platforms API like the linux API for example. I don't find any header file with that name

There is no header called <linux> for the system API. The POSIX operating system specification (which Linux conforms to) lists several headers which contain the functionality that you might find from windows.h on that system. POSIX spec overlaps with the C standard library, and the C standard library implementation also provides the POSIX specific headers. See the Linux manual or POSIX spec for full list of headers.

what if I want to use another [system] API

You will need at least the header files of that API in order to compile programs that call them. Furthermore, you'll need the library archives in order to link your programs. And you'll need to tell the compiler which system you are targeting. By default, all compilers assume that you target the system that is running the compiler. You'll need to consult the documentation of your compiler about whether cross compilation is possible, and how to do it.

Or a simpler approach: Compile on the system where the API is provided. The headers may need to be installed separately.

... how is the Standard Library in c++ cross platform

Each system has their own standard library. Some of the implementations of standard library are cross platform, but not all. For example, the libstdc++ - which is the standard library that is part of the GNU project, and is the default standard library in Linux - is available on many platforms including Windows. By contrast, the Msvc standard library is only available on Windows.

Upvotes: 2

john
john

Reputation: 87959

The answer to part one is yes. You can only use the Windows API because you are programming on Windows. However you can run Linux on Windows. A Microsoft product called the Windows Subsystem for Linux allows you to do that. But even if you do that you won't find a header called linux.h, it's more complicated than that. There is also a product (called WINE I believe) that lets you use the Windows API on Linux, although I believe it has a few issues.

The standard library interface is cross platform, but it's implementation certainly isn't. It does this by abstracting away OS specific features. If you look at how the library is implemented on different platforms you will certainly see lots of differences.

Upvotes: 0

Rohan Bari
Rohan Bari

Reputation: 7726

  1. The Wikipedia about windows.h library says:

windows.h is a Windows-specific header file for the C and C++ programming languages which contains declarations for all of the functions in the Windows API, all the common macros used by Windows programmers, and all the data types used by the various functions and subsystems.

  1. Linux doesn’t provide a default API for window management as does Windows so if you are programming a graphical application then you need to choose a windowing library as well.

Upvotes: 0

Related Questions