Programming


Git: Perform a stash addition using a custom/meaningful message

Did you ever wonder “Is there more to git stash?”, we did!
We wanted to see if there is a way to manually set the stash message to something meaningful instead of the automated message that derives from the last commit.

Fortunately, there is the command git stash save "Meaningful message"; which allows you to add new changes in your stash and at the same time use a custom message.

By using the git stash save "custom message"; command you will be enhancing the results of the git stash list; command as it will contain more useful information for you.

Example

$ git status;
On branch master
Changes not staged for commit:
 (use "git add <file>..." to update what will be committed)
 (use "git checkout -- <file>..." to discard changes in working directory)

modified: me

no changes added to commit (use "git add" and/or "git commit -a")
$ git stash save "custom message";
Saved working directory and index state On master: your message here
$ git stash list 
[email protected]{0}: On master: custom message
$ git stash show
 me | 1 +
 1 file changed, 1 insertion(+)

CentOS 7: C++: static linking cannot find -lstdc++ -lm and -lc

Recently, we were trying to compile a C++ application with the following compilation command on a CentOS 7 64bit :

g++ -static -O2 -lm -Wall -Wno-unused-result -std=c++11 -DCS_ACADEMY -DONLINE_JUDGE 510152025.cpp -o 510152025;

unfortunately, we got the following errors:

 /usr/bin/ld: cannot find -lstdc++
 /usr/bin/ld: cannot find -lm
 /usr/bin/ld: cannot find -lc
 collect2: error: ld returned 1 exit status

To resolve the issues, we performed the following installations to install the static versions of the glibc and libstdc libraries:

sudo yum install glibc-static libstdc++-static -y;

 


Fedora 26: C++: static linking cannot find -lstdc++ -lm and -lc

Recently, we were trying to compile a C++ application with the following compilation command on a Fedora 26 64bit :

g++ -static -O2 -lm -Wall -Wno-unused-result -std=c++14 -DCS_ACADEMY -DONLINE_JUDGE 510152025.cpp -o 510152025;

unfortunately, we got the following errors:

 /usr/bin/ld: cannot find -lstdc++
 /usr/bin/ld: cannot find -lm
 /usr/bin/ld: cannot find -lc
 collect2: error: ld returned 1 exit status

To resolve the issues, we performed the following installations to install the static versions of the glibc and libstdc libraries:

sudo dnf install glibc-static libstdc++-static -y;

 


C/C++: Get a random number that is in a specific range

Assuming you need to generate a random number that is in a specified range, you can do the following:

//int rand(void) creates a pseudo-random number in the range of 0 to RAND_MAX
//RAND_MAX is defined in stdlib.h and is the largest number rand will return (same as INT_MAX).
const int new_number = (rand() % (maximum_number + 1 - minimum_number)) + minimum_number;

The above code first creates a pseudo-random number that is in the range of [0, RAND_MAX].
Then it will divide it with the width (+1) of the range we want to use (maximum_number + 1 - minimum_number) and get the remainder (modulo).
The modulo will be in the range of [0, maximum_number - minimum_number], so we add to it the value of minimum_number to shift the result to the proper range.
This solution, as demonstrated in the example below, works for negative ranges as well.

Full example of generating 100000 random numbers that are all in the range [-31, 32].

const int maximum_number = 31;
const int minimum_number = -32;
unsigned int i;
for (i = 0; i <= 100000; i++) {
	const int new_number = (rand() % (maximum_number + 1 - minimum_number)) + minimum_number;
	printf("%d\n", new_number);
}

C/C++: Comparing the performance of syslog vs printf

The following code tries to compare the performance of syslog() with the printf() command. printf_vs_syslog.c (compressed) (136 downloads)
On our machine, it appears that syslog() is faster than printf().

To be as fair as possible, when the application was executing, we were monitoring the system logs as well, so that they will be printed on screen.
On CentOS 7, you can see the syslog in the file /var/log/messages.
The command we used was: sudo tail -f /var/log/messages

Results:

printf: Seconds elapsed 0.480000
syslog: Seconds elapsed 0.180000

Full source code for test:

printf_vs_syslog.c (compressed) (136 downloads)
#include <stdio.h>
#include <syslog.h>
// #include <stdlib.h> is needed for the resolution of EXIT_SUCCESS
#include <stdlib.h>
// #include <time.h> is needed for the clock() function and the macro CLOCKS_PER_SEC
#include <time.h>
// #include <unistd.h> and #include <sys/types.h> are needed for the functions uid_t getuid(void); and uid_t geteuid(void);
//getuid() returns the real user ID of the calling process.
//geteuid() returns the effective user ID of the calling process.
//These functions are always successful.
#include <unistd.h>
#include <sys/types.h>

#define RANGE (100000)

int main()
{
    {
        const clock_t start = clock();

        unsigned int i;
        for (i = 0; i < RANGE; i++){
            printf ("Program started by Real User %u (Effective User %u)\n", getuid(), geteuid());
        }
        printf("\n");

        const clock_t end = clock();
        const float seconds = (float) (end - start) / CLOCKS_PER_SEC;
        printf("printf: Seconds elapsed %f\n", seconds);
    }
    {
        const clock_t start = clock();

        setlogmask (LOG_UPTO (LOG_NOTICE));
        openlog ("bytefreaks", LOG_CONS | LOG_PID | LOG_NDELAY, LOG_LOCAL1);
        unsigned int i;
        for (i = 0; i < RANGE; i++){
            syslog (LOG_NOTICE, "Program started by Real User %u (Effective User %u)", getuid(), geteuid());
        }
        closelog ();

        const clock_t end = clock();
        const float seconds = (float) (end - start) / CLOCKS_PER_SEC;
        printf("syslog: Seconds elapsed %f\n", seconds);

    }
    return EXIT_SUCCESS;
}


printf_vs_syslog.c (compressed) (136 downloads)

C/C++: Set and Get the name of a pthread

Naming a pthread using meaningful names, can be a very useful feature for debugging multi-threaded applications as it can make your logs very informative.
For this reason, we are presenting two examples demonstrating the use of names in pthreads.

Example 1: The pthread decides for its name

The following code, creates a pthread which later, it will give itself a meaningful name.

pthread_self_named.c (compressed) (7 downloads)
// #define _GNU_SOURCE is needed for the resolution of the following warnings
//warning: implicit declaration of function ‘pthread_setname_np’ [-Wimplicit-function-declaration]
//warning: implicit declaration of function ‘pthread_getname_np’ [-Wimplicit-function-declaration]
#define _GNU_SOURCE
#include <stdio.h>
#include <sys/types.h>
#include <pthread.h>
#include <asm/errno.h>
#include <errno.h>
// #include <stdlib.h> is needed for the resolution of EXIT_SUCCESS
#include <stdlib.h>

//The thread name is a meaningful C language string, whose length is restricted to 16 characters, including the terminating null byte.
#define MAX_LENGTH_PTHREAD_NAME (16)

struct thread_info_t
{
    // Used to identify a thread.
    pthread_t thread_id;
};

// This is the thread that will be called by pthread_create() and it will be executed by the new thread.
void *self_named_thread(void *data)
{
    // We know that the input data pointer is pointing to a thread_info_t so we are casting it to the right type.
    struct thread_info_t *thread_info = (struct thread_info_t *) data;

    const int setname_rv = pthread_setname_np(thread_info->thread_id, "Tom Hanks");
    if (setname_rv)
    {
        errno = setname_rv;
        perror("Could not set pthread name");
    }

    char thread_name[MAX_LENGTH_PTHREAD_NAME];
    const int getname_rv = pthread_getname_np(thread_info->thread_id, thread_name, MAX_LENGTH_PTHREAD_NAME);
    if (getname_rv)
    {
        errno = getname_rv;
        perror("Could not get pthread name");
    }
    //This function always succeeds, returning the calling thread's ID.
    const pthread_t tid = pthread_self();
    //Usually pthread_t is defined as follows:
    //typedef unsigned long int pthread_t;
    //so we print pthread_t as an unsigned long int
    fprintf(stdout, "I am thread with ID '%lu', my name is '%s' and I gave me my name by myself\n", tid, thread_name );

    return NULL;
}

int main()
{
    struct thread_info_t thread_info;

    const int create_rv = pthread_create(&(thread_info.thread_id), NULL, &self_named_thread, (void *) &thread_info);
    if (create_rv)
    {
        errno = create_rv;
        perror("Could not create thread");
        return EXIT_FAILURE;
    }
    // The pthread_join() function suspends execution of the calling thread until the target thread terminates, unless the target thread has already terminated.
    const int join_rv = pthread_join(thread_info.thread_id, NULL);
    if (join_rv)
    {
        errno = create_rv;
        perror("Could not join thread");
    }
    return EXIT_SUCCESS;
}

pthread_self_named.c (compressed) (7 downloads)

Example 2: The parent decides for the pthread name

The next code, creates a pthread and the parent gives the thread a meaningful name.

pthread_named_by_parent.c (compressed) (8 downloads)
// #define _GNU_SOURCE is needed for the resolution of the following warnings
//warning: implicit declaration of function ‘pthread_setname_np’ [-Wimplicit-function-declaration]
//warning: implicit declaration of function ‘pthread_getname_np’ [-Wimplicit-function-declaration]
#define _GNU_SOURCE
#include <stdio.h>
#include <sys/types.h>
#include <pthread.h>
#include <asm/errno.h>
#include <errno.h>
// #include <stdlib.h> is needed for the resolution of EXIT_SUCCESS
#include <stdlib.h>
// #include <unistd.h> is needed for the resolution of unsigned int sleep(unsigned int seconds);
#include <unistd.h>

//The thread name is a meaningful C language string, whose length is restricted to 16 characters, including the terminating null byte.
#define MAX_LENGTH_PTHREAD_NAME (16)

struct thread_info_t
{
    // Used to identify a thread.
    pthread_t thread_id;
};

// This is the thread that will be called by pthread_create() and it will be executed by the new thread.
void *self_named_thread(void *data)
{
    // We know that the input data pointer is pointing to a thread_info_t so we are casting it to the right type.
    struct thread_info_t *thread_info = (struct thread_info_t *) data;

    //Added an artificial delay for the sake of the example.
    //Making sure the parent thread gave the pthread a name.
    sleep(1);

    char thread_name[MAX_LENGTH_PTHREAD_NAME];
    const int getname_rv = pthread_getname_np(thread_info->thread_id, thread_name, MAX_LENGTH_PTHREAD_NAME);
    if (getname_rv)
    {
        errno = getname_rv;
        perror("Could not get pthread name");
    }
    //This function always succeeds, returning the calling thread's ID.
    const pthread_t tid = pthread_self();
    //Usually pthread_t is defined as follows:
    //typedef unsigned long int pthread_t;
    //so we print pthread_t as an unsigned long int
    fprintf(stdout, "I am thread with ID '%lu', my name is '%s' and my parent gave me my name\n", tid, thread_name );

    return NULL;
}

int main()
{
    struct thread_info_t thread_info;

    const int create_rv = pthread_create(&(thread_info.thread_id), NULL, &self_named_thread, (void *) &thread_info);
    if (create_rv)
    {
        errno = create_rv;
        perror("Could not create thread");
        return EXIT_FAILURE;
    }

    const int setname_rv = pthread_setname_np(thread_info.thread_id, "Bob Marley");
    if (setname_rv)
    {
        errno = setname_rv;
        perror("Could not set pthread name");
    }

    // The pthread_join() function suspends execution of the calling thread until the target thread terminates, unless the target thread has already terminated.
    const int join_rv = pthread_join(thread_info.thread_id, NULL);
    if (join_rv)
    {
        errno = create_rv;
        perror("Could not join thread");
    }
    return EXIT_SUCCESS;
}

pthread_named_by_parent.c (compressed) (8 downloads)

C/C++: Change position of bytes 1 and 2 with bytes 3 and 4 in a 32bit unsigned integer

The following function will produce a new 32bit value where bytes 1 and 2 were moved in place of bytes 3 and 4 and vice versa.

reorder-bytes.c (compressed) (101 downloads)

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

const unsigned int move_bytes_1_2_after_4 (const unsigned int input) {
  //We get the two leftmost bytes and move them to the positions of the two rightmost bytes.
  const unsigned int first_two_bytes = (input >> 16) & 0x0000FFFF;
  //We get the two rightmost bytes and move them to the positions of the two leftmost bytes.
  const unsigned int last_two_bytes = (input << 16) & 0xFFFF0000;
  //We combine the two temporary values together to produce the new 32bit value where bytes 1 and 2 were moved in place of bytes 3 and 4 and vice versa.
  return (first_two_bytes | last_two_bytes);
}

int main(void) {
  const unsigned int value = 0xABCD0123;
  printf ("Original: 0x%08x\n", value);
  const unsigned int modified = move_bytes_1_2_after_4(value);
  printf ("Modified: 0x%08x\n", modified);
  return EXIT_SUCCESS;
}

Executing the above code will produce the following output:

Original: 0xabcd0123
Modified: 0x0123abcd

reorder-bytes.c (compressed) (101 downloads)


How to undo a Git commit that was not pushed 1

To undo a Git commit that was not pushed, you are given a few major options:

  1. Undo the commit but keep all changes staged
  2. Undo the commit and unstage the changes
  3. Undo the commit and lose all changes

Method 1: Undo commit and keep all files staged

In case you just want to undo the commit and change nothing more, you can use

git reset --soft HEAD~;

This is most often used to make a few changes to your latest commit and/or fix your commit message. Leaves working tree as it was before reset.
soft does not touch the index file or the working tree at all (but resets the head to the previous commit). This leaves all your changed files Changes to be committed, as git status would put it.

Method 2: Undo commit and unstage all files

In case you want to undo the last commit and unstage all the files you can use the following

git reset HEAD~;

or

git reset --mixed HEAD~;

mixed will reset the index but not the working tree (i.e., the changed files are preserved but not marked for commit) and reports what has not been updated. This is the default action.

Method 3: Undo the commit and completely remove all changes

The following method will undo the commit and revert all changes so that your state is exactly as it was before you started making changes.

git reset --hard HEAD~;

hard resets the index and working tree. Any changes to tracked files in the working tree since the previous commit are discarded.

 

Note: In case you just want to rewrite the commit message, you could use git –amend instead.


C: Implicit declaration of function ‘read’ and ‘write’

While working on an socket-based application, we received the following warnings from the compiler:

implicit declaration of function 'read'
implicit declaration of function 'write'

read and write functions are declared in unistd.h which we forgot to include in our code.

Adding the directive

#include <unistd.h>

to the source file that used read and/or write removed the warnings.