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getting # includes right == |+|
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|−|There are two types of #include statements: <tt>#include <foo. h></tt> and <tt>#include "foo. h"</tt>. |+|
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|−|Say we have the file <tt>header1. h</tt> in <tt>/usr/include/mylib/</tt> that contains the following: |+|
|−|<code cpp n> |+|
|−|#include <header2. h> |+|
that the .
|−|#include "header3. h" |+|
|−|</code> | |
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|−|The preprocessor will search for the file < tt> header2.h</ tt> in all the paths given as < tt> -I</ tt> arguments and then replace the line with the contents of that file. |+|
<></> in the
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|−|For line 2 the preprocessor tries to use the file /usr/include/mylib/header3. h first and if it does not exist search for the file like for <tt>header2.h</tt>. The important part to note here is that the preprocessor does not look in the directory of the source file that includes <tt>header1. h</tt> but in the directory where <tt>header1.h</tt> resides. |+|
the to use the . the is not in the the . the .
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|−|Now, which include statement is the one to use? After all you can specify every directory you want using <tt>- I</tt> and thus could use <tt>#include <. ..></tt> everywhere. |+|
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|−|=== as an application developer === |+|
|−|* Include headers from external libraries using angle brackets. |+|
|−|<code cpp> |+|
switch to the
|−|#include <iostream> |+|
|−|#include <QtCore/QDate> |+|
|−|#include <zlib.h> |+|
a of the the
|−|* Include headers from your own project using double quotes. | |
|−|<code cpp> | |
|−|#include "myclass.h" | |
|−|</code>Using angle brackets works correctly if the first <tt>-I</tt> switch to the compiler is your own source directory. Ideally you would not need to specify <tt>-I./</tt> though, as that might break with library headers that have the same filename as a header of your project (i.e.: If a library has the header file <tt>foo. h</tt> and your project has a different file with the same filename the compiler will always pick the header from your project instead of the one from the library because the source directory of the project is specified first. ) | |
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|−|=== as a library developer === |+|
from and to .
|−|* Include headers from external libraries using angle brackets (see above for examples) | |
|−|* Include headers of your own library and libraries that belong to it using double quotes. | |
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|−|If you use subdirectories for the installed header files you need to have the exact same directory structure for the headers in the source directory. Example: |+|
the the the
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|−|<tt>/usr/include/libfoo/</tt> contains the directory <tt>bar</tt>. In <tt>libfoo</tt> resides the header <tt>header1.h</tt>, in <tt>libfoo/bar</tt> the file <tt>header2.h</tt>. The latter depends on the former so it includes it using |+|
|−|<code cpp> |+|
|−|#include "../ header1. h" |+|
/.is the (. )
|−|If the source directory structure of the library is not the same ( in this case: <tt>header2.h</tt> in a subdirectory of the directory where <tt>header1. h</tt> resides) this obviously will break. |+|
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|−|It is also possible to use angle brackets in library headers, but that requires the application to supply the exactly correct <tt>-I</tt> paths (e.g. libfoo above can be used either with <tt>-I/usr/include</tt> and <tt>#include <libfoo/header1.h></tt> or <tt>-I/usr/include/libfoo</tt> and <tt>#include <header1.h></tt>, if libfoo were to use angle brackets to include its own headers one of the include switches has to be present for the lib to find its own headers). Also angle brackets require the application to not put any include directory before the <tt>-I</tt> switch for the lib that contains header files of the same filename as for the library. An example how angle brackets can break the build: |+|
the to :
and to the
|−|<tt>/usr/include/libxyz/xyz.h</tt> includes <tt>foo.h</tt> using angle brackets and expects to have it replaced with the contents of the file <tt>/usr/include/libzyx/foo.h</tt>. Assuming there's another library that also ships a <tt>foo.h</tt> file in the directory <tt>/usr/include/anotherlib</tt>. If the application that uses both libraries compiles with |+|
|−|<tt>g++ -I/usr/include/libxyz -I/usr/include/ anotherlib ...</tt> libxyz will work as expected ( but anotherlib might break) . If the application compiles with <tt>g++ -I/usr/include/anotherlib -I/usr/include/libxyz ...</tt> the header <tt>xyz.h</tt> will include the file <tt>/usr/include/anotherlib/foo.h</tt> instead of the file that is shipped with libxyz. The same problem can appear if an application has a header file of the same name as a library and specifies <tt>-I./</tt> as the first include directory. | |
audio devices: use-cases
Carl: a power-user on the move
Carl uses his laptop for private use and work. He works at home, while
travelling on a train, or at the office. He uses the following hardware:
- built-in HDA soundcard with jacks for headphones and a microphone, the laptop contains builtin stereo speakers and a surround speaker option in the mixer.
- a USB headset: simple usb-audio device with stereo playback and one microphone. It also has two buttons to increase/decrease the volume, sending the volume media keys keyboard events
- a monitor with built-in USB soundcard (usb-audio device which also contains a mixer device): stereo speakers, built-in microphone. Playback to the monitor speakers is also possible through HDMI (actually DisplayPort, but Linux reports that as HDMI). In addition the monitor also has a built-in webcam, which is attached with the same USB plug.
- A HiFi setup using a Cinch-3.5mm cable to connect its line-in to the laptop's headphone jack. The speakers are arranged such that the balance must be adjusted slightly to the right for a centered stereo sound where the laptop user sits.
- standard 3.5mm jack headphones (for use in the train, and sometimes also for use at the desk at home or at work)
- Alesis io|2 USB pro-audio soundcard. Carl uses this devices at home (or when he's working as sound-engineer) to record stuff. His favorite tool for this job is Ardour.
- At home Carl sometimes also connects his laptop to his TV and can use HDMI to use the stereo speakers of the TV for audio output.
When Carl does home-office he uses Skype, a SIP application, and a H.323
application (e.g. Ekiga for both SIP and H.323) to provide VoIP connectivity to
his collegues and be reachable via a phone number that is not his private phone
number. To provide the best sound quality he wants to use his USB headset when
it's connected. If the headset is not connected he wants to be able to answer
calls with the built-in speakers and microphone. When he then connects the
headset, while in a call, he wants the sound to migrate automatically to the
When at work, he wants to use the microphone and speakers of the monitor for
VoIP applications. If he plugs a headphone in the 3.5mm jack of the laptop, he'd
like to use that as the output device. On unplugging the headphone jack the
output should migrate to the monitor again.
Event sounds (this includes ringing sounds of VoIP applications when being
called) should go to
- at work: to the monitor speakers, unless a headphone is plugged in, in which case it should go to both the headphones and the monitor speakers
- at home: to the internal sound card/headphone jack and, if plugged in, additionally to the USB headset
Sometimes Carl uses his laptop to play a short round of Wesnoth against his friend, while talking to him on Skype. Thus he uses the USB headset for communication and wants Wesnoth to use the USB headset for sound output as well. Since the Wesnoth action is not always very high ;) Carl also starts up Amarok in the background and wants it to play to the headset, too. This requires to have the Skype output louder than the Amarok and Wesnoth "noise", because talking to his friend is most important to him.
Ami: a desktop system in the living room
Ami has her desktop computer on a desk in the living room. The internal HDA
soundcard is connected to the monitor speakers via the 3.5mm front output jack
and to high quality active speakers via the 3.5mm back output jack. She also has
headphones with 3.5mm connectors which she can plug into either the headphone
jack of the desktop or the headphone jack of the monitor. A USB webcam with
built-in microphone is attached, providing the only microphone of this system,
for use with Skype. The nearby TV is attached to the graphics card via an HDMI
cable and can provide stereo audio output.
For most of her time at the computer, she only requires event sounds and
the audio of web videos on her monitor speakers.
If she wants to switch to higher quality playback, she turns on the active
speakers and migrates the music and video audio to the active speakers while
event sounds stay on the monitor speakers.
To watch a DVD or some of her videos she uses the TV and either wants to use the
TV speakers or the active speakers.
When using Skype, she wants to capture from the webcam and use the monitor speakers. But sometimes she'd rather move over to the couch and TV for a longer chat.
Implications for the GUI
Generally the system should be as smart as possible and provide the best defaults possible to minimize configuration tasks. What the computer can't recognize:
- what kind of device is used when a 3.5mm connector is plugged into the headphone (or any line out) jack
The following special events are possibly interesting for a smart audio device management to monitor and handle:
- move video window from one monitor to another
- reconfigure monitor setup (e.g. switch from monitor to TV output)
- when a Skype/SIP/H.323 call is active and who is on the other side (friend vs. business contact)
- incoming VoIP call
That leaves the following setup changes to manual intervention:
- switch between headphone pan centered and slightly adjusted to the right
- turn playback to different output jacks on/off (this might be configured as two or three setups which can then be recalled somehow)