Monthly Archives: March 2017


Just some notes for setting up a new OS to develop projects on GNU/Linux Fedora

If the project is in C++ and uses mysql then install

sudo dnf install mysql++-devel;

If the project is in C/C++ and you are missing talloc.h install

sudo dnf install libtalloc-devel;

Set your name and email for all git projects

git config --global --edit
Then fill-in the configuration file similar to below
# This is Git's per-user configuration file.
[user]
# Please adapt and uncomment the following lines:
#       name = Michael, George
#       email = [email protected]
[user]
        name = Michael, George
        email = [email protected]
[gui]
        editor = gedit

or use these individual commands to set the configuration

[[email protected] ~]$ git config --global user.name "Michael, George"
[[email protected] ~]$ git config --global user.email "[email protected]"

Increase amount of inotify watchers

If you are using CLion or IntelliJ IDEA by jetbrains increase the amount of inotify watchers.
CLion, IntelliJ (and other tools of jetbrains) use inotify on GNU/Linux to monitor directories for changes. It’s common to encounter the system limit on the number of files they monitor.

inotify requires a watch handle to be set for each directory in the project. Unfortunately, the default limit of watch handles will not be enough for sized projects, and reaching the limit will force the jetbrains platform to fall back to recursive scans of directory trees.

Create a file (as root) called /etc/sysctl.d/idea.conf and add the following content to it to increase the number of watchers to 512K

fs.inotify.max_user_watches = 524288

Then call sysctl to reload the settings and apply the new configuration

[[email protected] ~]$ sudo sysctl -p --system;
  •  -p[FILE] or --load[=FILE]: Load in sysctl settings from the file  specified  or /etc/sysctl.conf if none  given.
    Specifying - as filename means reading data from standard input. Using this option will mean arguments to sysctl are files, which are read in the order they are specified.
    The file argument may be specified as regular expression.
  •  --system: Load settings from all system configuration files.
     /run/sysctl.d/*.conf
     /etc/sysctl.d/*.conf
     /usr/local/lib/sysctl.d/*.conf
     /usr/lib/sysctl.d/*.conf
     /lib/sysctl.d/*.conf
     /etc/sysctl.conf
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How to “pause” (suspend) an active process

Recently, we were executing the following time-wasting application and we wanted to pause it somehow and release the CPU that was being used temporarily for other tasks.
Unfortunately, the process was not executing on an active console, so we could not press CTRL+Z and suspend it.
Conveniently, the kill command provides us with the suspend functionality as long as we know the PID of the process to be suspended.

Using ps x, we found the PID of the application even though it was not attached to an active console.

Then to suspend the application, we used

kill -TSTP "$PID";

which instructed the process to stop by sending it the SIGTSTP signal.

Fortunately, our application did not block the signal and it was suspended.

Note: In case an application ignores the SIGTSTP signal, you can still force it to suspend by sending it the SIGSTOP signal as follows

kill -STOP "$PID";

After we were done, we resumed the execution of the process by sending the SIGCONT signal to it

kill -CONT "$PID";

 


git: How to move locally committed (but not pushed) changes to a new branch

Recently, we’ve been working on a certain branch, we did some changes and performed a couple of commits that were not pushed on the remote system.

There was a complication and it was decided that the local changes should not be pushed to the branch that we were working on.
Rather, they changes should go to a new branch which eventually will be merged.

As mentioned above, we already had done some changes and we already had performed the commits.

git status would give us the following:

$ git status;
On branch scanner_pdu_parser_master
Your branch is ahead of 'origin/scanner_pdu_parser_master' by 2 commits.
  (use "git push" to publish your local commits)

So, we needed to change the branch for those local commits.

Solution – Move the local commits to a new branch

First we got the name of the current branch using the command:

git branch;

Then, we switched to a new local branch

git checkout -b banana_peeler;

And, we pushed the local branch to the remote system:

git push --set-upstream origin banana_peeler;

Afterwards, we switched back to the previous branch

git checkout apple_peeler;

And reset it back to its original form, removing our local commits from it:

git reset --hard origin/apple_peeler;

Please note that the last command will delete all changes that are not committed as well.
In other words, any file you modified and did not commit or push, they will be reverted back to the original code as well.


Using aliases for SSH

An extremely helpful feature of ssh is the ability to define aliases using its configuration files:

  • ~/.ssh/config
  • /etc/ssh/ssh_config

~/.ssh/config contains configuration that is only available to your user and any user can create one for themselves.
/etc/ssh/ssh_config contains configuration that applies to all users of the system and only administrators can modify it.

Note: ~/.ssh/config should only have read and write access rights by its owner only!
Be sure to execute the following after your create it:

chmod 600 ~/.ssh/config;

Example 1 – Creating an alias for a host name:

Assuming we are too bored to type the full domain of a server, we can define a shorter name as follows:

Host bf
    HostName bytefreaks.net

by having this configuration lines in your ~/.ssh/config file, you can shorten the command ssh bytefreaks.net; to ssh bf;.

Example 2 – Creating an alias for a host name with specific username:

In the next example, we create a new alias that not only will automatically set the host name but also the username

Host bf
    HostName bytefreaks.net
    User george

by having this configuration lines in your ~/.ssh/config file, you shorten the command ssh [email protected]; to ssh bf;.

Example 3 – Creating an alias for a host name with specific username and port:

In the next example, we create a new alias for a specific host name, username and ssh port number

Host bf
    HostName bytefreaks.net
    User george
    Port 22300

The above will shorten ssh [email protected] -p 22300 to ssh bf;.

Example 4 – Creating an alias for a host name with specific username and identity file:

Host bf
    HostName bytefreaks.net
    User george
    IdentityFile /path/to/needed/private/key/id_rsa

The above will shorten ssh [email protected] -i /path/to/needed/private/key/id_rsa; to ssh bf;

For more information on the capabilities of the configuration files, please review the following documentation page as it has a whole lot more of useful information: http://man.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man5/ssh_config.5

Repeated note: ~/.ssh/config should only have read and write access rights by its owner only!
Be sure to execute the following after your create it:

chmod 600 ~/.ssh/config;

C: Code to time execution with accuracy greater than a second

The following application computes the time needed for a process to finish using the method clock().
The result of the application is the time in seconds as a floating number (where 1.0 = 1 second).
It provides greater accuracy than seconds as the estimation is done using processor time used by the program.

#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include <limits.h>

int main()
{

    /* clock_t clock(void)
     The clock() function returns an approximation of processor time used by the program.
     The value returned is the CPU time used so far as a clock_t,
     to get the number of seconds used, divide by CLOCKS_PER_SEC.
     On error it returns -1. */
    const clock_t start = clock();

    /* svoid srand(unsigned int __seed)
     The srand() function sets its argument as the seed for a new sequence of pseudo-random
     integers to be returned by rand(). These sequences are repeatable by calling srand() with the
     same seed value.
     If no seed value is provided, the rand() function is automatically seeded with a value of 1. */
    /* time_t time(time_t *__timer)
     time() returns the time since the Epoch (00:00:00 UTC, January 1, 1970), measured in seconds.
     If the __timer variable is not NULL, the return value is also stored there. */
    srand(time(NULL));
    unsigned long i;
    for (i = 0; i < 10000000; i++)
    {
        /* int rand(void)
         The rand() function returns a pseudo-random integer in the range 0 to RAND_MAX inclusive. */
        rand();
    }
    const clock_t end = clock();

    /* ISO/IEC 9899:1999 7.23.1: Components of time
    The macro `CLOCKS_PER_SEC' is an expression with type `clock_t' that is
    the number per second of the value returned by the `clock' function. */
    /* CAE XSH, Issue 4, Version 2: <time.h>
    The value of CLOCKS_PER_SEC is required to be 1 million on all
    XSI-conformant systems. */
    const float seconds = (float) (end - start) / CLOCKS_PER_SEC;

    printf("Seconds elapsed %f\n", seconds);
    return 0;
}