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Saturday, October 18, 2014
 
Blast from the Past : The Weekend Playlist #7

Previous playlists:

    #1 (50s, 60s and 70s) | #2 (80s) | #3 (80s) | #4 (80s) | #5 (80s) | #6 (90s)

Audio-Visual material courtesy: YouTube. Other information: Wikipedia.

1. Fatboy Slim / Norman Cook - Brimful of Asha (1998)

A remix. Original by UK band Cornershop.

2. Vanilla Ice - Ice Ice Baby (1990)

3. Beck - Loser (1993)

4. Primus - Mr. Krinkle (1993)

5. Tool - Stinkfist (1996)

if you don't mind watching dark videos, look for Stinkfist official video on youtube.

6. P.M. Dawn - Set Adrift On Memory Bliss (1991)

7. Primitive Radio Gods - Standing Outside A Broken Phone Booth (1996)

no traces of official video anywhere on web, for some reason.

8. Blues Traveler - Run-Around (1995)

Grammy winner.

9. KoRn - A.D.I.D.A.S. (1997)

Under Pressure mix. Another dark song that has nothing to do with sportswear brand, Adidas.

10. Chumbawamba - Tubthumping (1997)

one hit wonder.

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Tuesday, September 30, 2014
 
Programming in C: Few Tidbits #3

1) Not able to redirect the stdout output from a C program/application to a file

Possible cause:

Buffered nature of standard output (stdout) stream. Might be waiting for a newline character, for the buffer to be full, or for some other condition to be met based on implementation.

Few potential workarounds:


2) Printing ("escaping" maybe?) a percent sign (%) in a printf formatted string

Conversion/format specifiers start with a % sign, and using the slash sequence to escape the % sign in strings that are not format specifiers usually does not work. Check the following example out.

eg.,

Executing the following code:

        int pct = 35;
        printf("\n%d%", pct);

.. results in:

35, but not 35% as one would expect.

Format specifier "%%" simply prints the percent sign - so, the desired result can be achieved by replacing "%d%" with "%d%%" in printf statement.

        int pct = 35;
        printf("\n%d%%", pct);

.. shows:

35% as expected

(web search keywords: C printf conversion specification)


3) Duplicating a structure

If the structure has no pointers, assigning one struct to another struct duplicates the structure. The same effect can be achieved by using memcpy() too, but it is not really necessary. After the execution of struct assignment, there will be two copies of struct with no dependency - so, they can be operated independently without impacting the other. The following sample code illustrates this point.

eg., #1
 ...
 ...

 typedef struct human {
  int accno;
         int age;
 } person;

 ...
 ...

 person guy1, guy2;

 guy1.accno = 20202;
 guy1.age = 10;

 guy2 = guy1;

 printf("\nAddress of:\n\t-> guy1: %p. guy2: %p", guy1, guy2);

 printf("\n\nBefore update:\n");
 printf("\naccno of:\n\t-> guy1: %d. guy2: %d", guy1.accno, guy2.accno);
 printf("\nage of:\n\t-> guy1: %d. guy2: %d", guy1.age, guy2.age);

 guy1.age = 15;
 guy2.accno = 30303;

 printf("\n\nAfter update:\n");
 printf("\naccno of:\n\t-> guy1: %d. guy2: %d", guy1.accno, guy2.accno);
 printf("\nage of:\n\t-> guy1: %d. guy2: %d", guy1.age, guy2.age);

 ...
 ...
 

Execution outcome:

Address of:
        -> guy1: ffbffc38. guy2: ffbffc30

Before update:

accno of:
        -> guy1: 20202. guy2: 20202
age of:
        -> guy1: 10. guy2: 10

After update:

accno of:
        -> guy1: 20202. guy2: 30303
age of:
        -> guy1: 15. guy2: 10

On the other hand, if the structure has pointer variable(s), duplication of a structure using assignment operator leads to pointer variables in both original and copied structures pointing to the same block of memory - thus creating a dependency that could potentially impact both pointer variables with unintended consequences. The following sample code illustrates this.

eg., #2
 ...
 ...

 typedef struct human {
         int *accno;
         int age;
 } person;

 ...
 ...

 person guy1, guy2;

 guy1.accno = malloc(sizeof(int));
 *(guy1.accno) = 20202;

 guy1.age = 10;
 guy2 = guy1;
 
 ...
 ...

 guy1.age = 15;
 *(guy2.accno) = 30303;

 ...
 ...
Execution outcome:
Address of:
        -> guy1: ffbffb48. guy2: ffbffb40

Before update:

accno of:
        -> guy1: 20202. guy2: 20202
age of:
        -> guy1: 10. guy2: 10

After update:

accno of:
        -> guy1: 30303. guy2: 30303
age of:
        -> guy1: 15. guy2: 10

Few people seem to refer this kind of duplication as shallow copy though not everyone agrees with the terminology.

If the idea is to clone an existing struct variable that has one or more pointer variables, then to work independently on the clone without impacting the struct variable it was cloned from, one has to allocate memory manually for pointer variables and copy data from source structure to the destination. The following sample code illustrates this.

eg., #3
 ...
 ...

 typedef struct human {
         int *accno;
         int age;
 } person;

 ...
 ...

 person guy1, guy2;

 guy1.accno = malloc(sizeof(int));
 *(guy1.accno) = 20202;

 guy1.age = 10;

 guy2.age = guy1.age;
 guy2.accno = malloc(sizeof(int));
 *(guy2.accno) = *(guy1.accno);

 ...
 ...

 guy1.age = 15;
 *(guy2.accno) = 30303;

 ...
 ...

Execution outcome:

Address of:
        -> guy1: ffbffaa8. guy2: ffbffaa0

Before update:

accno of:
        -> guy1: 20202. guy2: 20202
age of:
        -> guy1: 10. guy2: 10

After update:

accno of:
        -> guy1: 20202. guy2: 30303
age of:
        -> guy1: 15. guy2: 10

This style of explicit duplication is referred as deep copy by few people though not everyone agrees with the terminology.




Thursday, July 31, 2014
 
Programming in C: Few Tidbits #2

(1) ceil() returns an incorrect value?

ceil() rounds the argument upward to the nearest integer value in floating-point format. For example, calling ceil() with an argument (2/3) should return 1.

printf("\nceil(2/3) = %f", ceil(2/3));

results in:

ceil(2/3) = 0.000000

.. which is not the expected result.

However:

printf("\nceil((float)2/3) = %f", ceil((float)2/3));

shows the expected result.

ceil((float)2/3) = 1.000000

The reason for the incorrect result in the first attempt can be attributed to the integer division. Since both operands in the division operation are integers, it resulted in an integer division which discarded the fractional part.

Desired result can be achieved by casting one of the operands to float or double as shown in the subsequent attempt.

One final example for the sake of completeness.

printf("\nceil(2/(float)3) = %f", ceil(2/(float)2));
..
ceil(2/(float)3) = 1.000000

(2) Main difference between abort() and exit() calls

On a very high level: abort() sends SIGABRT signal causing abnormal termination of the target process without calling functions registered with atexit() handler, and results in a core dump. Some cleanup activity may happen.

exit() causes normal process termination after executing functions registered with the atexit() handler, and after performing cleanup activity such as flushing and closing all open streams.

If it is desirable to bypass atexit() registered routine(s) during a process termination, one way is to call _exit() rather than exit().

Of course, this is all high level and the information provided here is incomplete. Please check relevant man pages for detailed information.


(3) Current timestamp

The following sample code shows the current timestamp in two different formats. Check relevant man pages for more information.

#include <time.h>
..
char timestamp[80];
time_t now;
struct tm *curtime;

now = time(NULL);
curtime = localtime(&now);

strftime(timestamp, sizeof(timestamp), "%m-%d-%Y %X", curtime);

printf("\ncurrent time: %s", timestamp);
printf("\ncurrent time in a different format: %s", asctime(curtime));
..

Executing this code shows output

current time: 07-31-2014 22:05:42
current time in a different format: Thu Jul 31 22:05:42 2014

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Monday, June 30, 2014
 
Programming in C: Few Tidbits

.. with little commentary aside. Target audience: new programmers. These tips are equally applicable in C and C++ programming environments.

1. Duplicating a file pointer

Steps: find the integer file descriptor associated with the file stream using fileno() call, make a copy of the file descriptor using dup() call, and finally associate the file stream with the duplicated file descriptor by calling fdopen().

eg.,
FILE *fptr = fopen("file", "mode");

FILE *fptrcopy = fdopen( dup( fileno(fptr) ), "mode");

2. Capturing the exit code of a command that was invoked using popen()

Using pipes is one way of executing commands programmatically that are otherwise invoked from a shell. While pipes are useful in performing tasks other than executing shell commands, this tip is mainly about the exit code of a command (to figure out whether it succeeded or failed) that was executed using popen() API.

To capture the exit code, simply use the value returned by pclose(). This function call returns the termination status of the command that was executed as a child process. However the termination status of the child process is in the top 16 bits of the return value, so dividing the pclose() return value by 256 gives the actual exit code of the command that was executed.

eg.,
...
FILE *ptr;
int rc;

if ((ptr = popen("ls", "r")) != NULL) {
 rc = pclose(ptr)/256;
 printf("\nls: exit code = %d", rc);
}

if ((ptr = popen("ls -W", "r")) != NULL) {
 rc = pclose(ptr)/256;
 printf("\nls -W: exit code = %d", rc);
}
...

% ./<executable>

ls: exit code = 0
ls: illegal option -- W
ls -W: exit code = 2

3. Converting an integer to a string

Standard C library has implementation for converting a string to an integer (atoi()), but not for converting an integer to a string. One way to achieve the desired result is by using sprintf() function call, which writes formatted data to a string.

eg.,
int weight = 30;
char *wtstr = malloc(sizeof(char) * 3);

sprintf(wtstr, "%d", weight);
...

sprintf() can also be used to convert data in other data types such as float, double to string. Also see: man page for snprintf().


4. Finding the length of a statically allocated array

When size was not specified explicitly, simply divide the total size of the array by the size of the first array element.

eg.,
static const char *greeting[] = { "Hi", "Hello", "Hola", "Bonjour", "Namaste", "Ciao", "Ni Hao" };
int numgreetings = sizeof(greeting)/sizeof(greeting[0]);

After execution, numgreetings variable holds a value of 7. Note that sizeof(greeting[0]) is actually the size of a pointer to a character array.

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Saturday, May 17, 2014
 
Blast from the Past : The Weekend Playlist #6

Compared to the 80s, music in the 90s was equally good if not better. Artists let their imagination run wild -- that resulted in many memorable, bold and dark themed renditions. While the 80s were dominated by the cassette tapes, the rise of compact discs with better quality sound might have added fuel to music industry's growth engine in the 90s.

Enjoy the recollection.

Access previous playlists from the following locations:

    #1 (50s, 60s and 70s) | #2 (80s) | #3 (80s) | #4 (80s) | #5 (80s)

Audio-Visual material courtesy: YouTube. Other information: Wikipedia.

1. Bon Jovi - Blaze of Glory (1990)

Love the guitar bits. Featured in western, Young Guns II.

2. Radiohead - Creep (1992)

Some cool guitar trick in the middle earns it extra credit

3. Depeche Mode - Enjoy The Silence (1990)

Masters of Electronic music. Enjoy.

4. Jimmy Cliff - I Can See Clearly Now (1993)

Cover for Johnny Nash's 1972 original. Great song nevertheless.

5. Godsmack - Voodoo (1999)

Beware: dark video. My brother's old buddy Sudhir gets the credit for digging this one out.

6. Sting - Fields Of Gold (1993)

Enjoy. Tidbit: Sting (Gordon Summer) was associated with "The Police" too.

7. Coolio - Gangsta's Paradise (1995)

Cool song. Won a Grammy.

8. Blue Man Group - Synaesthetic (1999)

Pump up the volume, sit back and Enjoy

9. The Prodigy - Mindfields (1997)

Love the guitar riff though sampled from John Barry's "Hip's Trip". Featured in The Matrix.

10. The Smashing Pumpkins - 1979 (1996)

Music video won the "MTV Video Music Award for Best Alternative Video" in 1996

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