I was further discussing C ideas and this time we came across bit flags (also called bit fields). The idea behind them is conservation of memory, a boolean data type consumes 1 byte of memory but all that is really needed is a single bit. Therefore you can store several booleans in a single byte.
Here is a simple example, note that each has a specific value to ensure that each matches a single bit in the 8-bit variable:
enum options {
option_a = 0x01, /* 1 == 00000001 */
option_b = 0x02, /* 2 == 00000010 */
option_c = 0x04, /* 4 == 00000100 */
option_d = 0x08, /* 8 == 00001000 */
option_e = 0x10, /* 16 == 00010000 */
option_f = 0x20, /* 32 == 00100000 */
option_g = 0x40, /* 64 == 01000000 */
option_h = 0x80, /* 128 == 10000000 */
};So basically we have 8 boolean flags (0 off/false, 1 on/true). We can use these by utilising bitwise operations. There is really only 3 operations that you will use regularly on bit flags so lets see them.
Switching individual/multiple options “on” (or setting value true):
options |= option_a;
/* Translates to:
options = options | option_a;
options = 00000000 | 00000001;
options = 00000001;
*/
options = option_a | option_c | option_h;
/* Translates to:
options = 00000001 | 00000100 | 10000000;
options = 10000101;
*/
Equally switching options “off” (or setting false) can be done as follows (note we are using the options from above, thus a, c and h are on):
options &= ~option_a;
/* Translates to:
options = options & ~option_a;
options = 10000101 & ~00000001;
options = 10000101 & 11111110;
options = 10000100;
*/
Obviously you need a basic grasp of boolean mathematics to be able to handle this but learning the basics shouldn’t be too hard. Finally the last thing to do is check to see if a flag is “on” or “off” (again using the options from before, so c and h are “on”:
if( options & option_b ) printf("B is ON\n");
/* 10000100 & 00000010 = 00000000 .. i.e. false, so no output */
if( options & option_c ) printf("C is ON\n");
/* 10000100 & 00000100 = 00000100 .. i.e. true, so "C is ON" is outputting */As with most things in C, once you start using and abusing it should start to fall into place. I hope that this serves as a basic introduction to bit fields.
With C++ there is ways of handling optional function parameters or overloading functions to handle different amounts of parameters, obviously within C these abilities do not exist. However if we wanted to handle a variable list of parameters then C and C++ would work the same.
Let’s see how we’d do that:
#include <stdarg.h>
#include <stdio.h>
int Adding(int cnt, ...) { /* the three ... indicate a variable list */
int add, tmp, i;
va_list args; /* make a va_list */
va_start(args, cnt); /* variable list starts after 'cnt' */
for(i = 0; i < cnt; i++) {
tmp = va_arg(args, int); /* grab the next */
add = add + tmp; /* add to the sum */
}
va_end(args); /* finished using variable list */
return add;
}
int main( ) {
int a;
a = Adding(2, 1, 2);
printf("%u\n", a); /* Outputs 3 */
a = Adding(3, 1, 2, 3);
printf("%u\n", a); /* Outputs 6 */
return 0;
}So the first argument indicates how many variables we are going to pass and the rest are summed together. The function Adding() would be identical in C++.
Sometimes when people are using Linux they will find that they come across a simple security check that confirms the authenticity of the RPM/Deb files that they are installing. Every so often the key is missing but that is easy to remedy. Here I will use an example with the YUM install of Adobe Flash:
[root@marine]# ls
adobe-release-i386-1.0-1.noarch.rpm
[root@marine]# yum localinstall adobe-release-i386-1.0-1.noarch.rpm
Loaded plugins: fastestmirror, priorities
Setting up Local Package Process
Examining adobe-release-i386-1.0-1.noarch.rpm: adobe-release-i386-1.0-1.noarch
Marking adobe-release-i386-1.0-1.noarch.rpm to be installed
Loading mirror speeds from cached hostfile
* addons: anorien.csc.warwick.ac.uk
* base: anorien.csc.warwick.ac.uk
* centosplus: anorien.csc.warwick.ac.uk
* contrib: anorien.csc.warwick.ac.uk
* extras: anorien.csc.warwick.ac.uk
* rpmforge: fr2.rpmfind.net
* updates: anorien.csc.warwick.ac.uk
588 packages excluded due to repository priority protections
Resolving Dependencies
--> Running transaction check
---> Package adobe-release-i386.noarch 0:1.0-1 set to be updated
--> Finished Dependency Resolution
Dependencies Resolved
================================================================================
Package Arch Version Repository Size
================================================================================
Installing:
adobe-release-i386 noarch 1.0-1 /adobe-release-i386-1.0-1.noarch 1.9 k
Transaction Summary
================================================================================
Install 1 Package(s)
Update 0 Package(s)
Remove 0 Package(s)
Total size: 1.9 k
Is this ok [y/N]: y
Downloading Packages:
warning: rpmts_HdrFromFdno: Header V3 DSA signature: NOKEY, key ID f6777c67
Public key for adobe-release-i386-1.0-1.noarch.rpm is not installed
As you can see it attempted to install but because the key was missing it refused to install. So what do we do to find that key? First we must download the key from somewhere, there is various key servers around the world, some of the most popular are (note hkp is the HTTP keyserver protocol):
- hkp://subkeys.pgp.net
- hkp://pgp.mit.edu
- hkp://keys.gnupg.net
- hkp://wwwkeys.uk.pgp.net (where UK can be replaced by any country code)
So lets search for the missing key:
[root@marine]# gpg --keyserver hkp://wwwkeys.uk.pgp.net --recv-keys f6777c67
gpg: requesting key F6777C67 from hkp server wwwkeys.uk.pgp.net
gpg: /root/.gnupg/trustdb.gpg: trustdb created
gpg: key F6777C67: public key "Adobe Systems Incorporated (Linux RPM
Signing Key) " imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg: imported: 1
So now we have the key and see that it is indeed Adobe’s key. Now all we need to do is add into RPM:
[root@marine]# gpg --armor --export f6777c67 >tmp-gpg.key
[root@marine]# rpm --import tmp-gpg.key
Simple as that, the install will work now!
This post is a refreshed look at a previous post on the same matter.
So it has been a while since I posted last and I felt like updating and cleaning stuff up. The tags and categories have been improved and reduced to help with clarity. I have switched to a new theme, although it is still created by the previous designer.
I have also added a Google CSE to assist with searching for a particular post, although I don’t exactly have too many.
I am very impressed with how WordPress has improved over recent times. The automatic upgrade is impressive and will surely ensure that less WP installs are hacked. The theme/plug-in search is quick and very easy to use, thus insuring that people will not be stuck with one design, or missing a vital plug-in. These two can be upgraded on the fly. Overall a real improvement proving that WP is a mature and stable piece of software.
There is only one minor annoyance and that is the speed at which pages are served, I do feel it could be slightly quicker.
Hope you all enjoy the updates that I applied!
I was recently pointed to the English and Polish pages for the MS Business Productivity. The most striking thing about the difference between the pages is that one depicts a white individual in place of a black individual. What could be the purpose of altering the image, surely it isn’t racism, after all the Chinese individual is still in place.
Strange what goes through the brain of MS Business Productivity.
Polish Alternative
English Alternative