Aditech » C in linux/unix

Saving the output of a C program in a text file(Linux/Unix)

adithya — Sun, 30 Mar 2008 11:46:17 +0000

Consider the following c program

#include
main()
{
int i,n;
printf("Enter limit\n");
scanf("%d",&n);
printf("I will print %d times\n",n);
for(i=0;i
the output will be


Enter limit

4

I will print 4 times

I luv C

I luv C

I luv C

I luv C


This is a simple program which will print "I luv C" n times.Now, wouldn't be nice if we saved the output in a text file

then in such case you have to use directed symbol ">>"



Usage:



./a.out >> filename.txt


let the filename for the above c program be adi.c.In order to direct the output of this program to a text file.Issue commands


bash-2.05b$ cc adi.c

bash-2.05b$ ./a.out >> adi.txt


where adi.txt is the text file in which you want to save the output
When you type ./a.out >> adi.txt and press enter ,nothing will be displayed.

Now type 5 and press enter.

Open adi.txt file by issuing command vi adi.txt


bash-2.05b$vi adi.txt


the following will be it's contents


Enter limit

I will print 5 times

I luv C

I luv C

I luv C

I luv C

I luv C


when you direct the output to a text file,2 things are important
 1.Only scanning takes place in the terminal.Means no output will be there.You have to remember the sequence in which you should enter values.

2.Your input is not saved in the text file as you can see ,the file contains "Enter limit" but doesn't contain what i inputted.
1.If the text file already exists,then the output will be appended(added) to the existing contents

2.The text file is saved under the same directory.If you want to store it some other place then give the path

ex:



bash-2.05b$ ./a.out >> \home\adithya\output\adi.txt



will save the output to a text file named adi.txt saved in \home\adithya\output\ directory[/B]
Tip:
if you want the contents of the textfile to be of natural output ,then after every scanf() statement issue appropriate printf() statement of the value you scanned.

Ex:if you want to make the directed output file of the above program match exatly with real output then make this change


printf("Enter limit\n");

scanf("%d",&n);

printf("%d\n",n);


now the contents of the text file will be like this


Enter limit

5

I will print 5 times

I luv C

I luv C

I luv C

I luv C

I luv C



     
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Important instructions for programs using math.h library function

adithya — Sun, 23 Mar 2008 15:14:25 +0000

if your program uses math.h header file then sometimes the compilation will lead to errors.In such cases you have to suffix -lm to the compilation file command

Usage
cc filename.c -lm

lm means link mathematical header file.Note that this step is not always required use it only if your program is correct and compilation is leading to errors.
and all other procedures are same.

ex:
consider the following program

#include
#include
 main()
{
float num,sq;
printf("Enter the number\n");
scanf("%f",&num);
sq=sqrt(num);
printf("Sqrt=%f",sq);
}

suppose the name of this c file is adi.c
then sometimes compilation of this file will lead to an error like this

bash-2.05b$ cc adi.c /tmp/cck2Ps76.o(.text+0x42): In function `main': : undefined reference to `sqrt' collect2: ld returned 1 exit status

In such case compile like this

bash-2.05b$ cc adi.c -lm

Advantages of running c in Linux/Unix-1

adithya — Sat, 22 Mar 2008 13:59:28 +0000

C is strongly linked with linux/unix .The kernel of Unix/Linux is written in C.Running C in linux/unit has several advantages .The most important among it is full word length utilization.
Consider the following C program

#include
int main()
{
int a;
float b;
printf("Size of int datatype =%d bits\n",(sizeof(a)*8));
printf("Size of float datatype =%d bits\n",(sizeof(b)*8));
return 1;
}

sizeof() returns the amount of bytes that the variable is awarded

We want the answer in bits,hence we have to multiplied it by 8 (1 byte=8bits)
now run this c program in turbo c in windows
the output will be

Size of int datatype=16 bits Size of float datatype =32 bits

Even though your computer is 32 or 64 bit computer, Turbo C is utilizing only 16 bits to int which should have been 32 bits(Int is allocated with a space equal to wordlength of the computer)

now run the above c program in linux
the output will be

Size of int datatype=32 bits Size of float datatype =64 bits

^^if your processor is 32 bit

Size of int datatype=64 bits Size of float datatype =128 bits
^^if your processor is 64 bit

Ok…running C program in linux/unix utilizes full word length.what’s it’s use for me??

The maximum number of singed int datatype(normal int) that can be accomodated if you run c in windows is ((2^15)-1)=32767
that means if ‘a’ is made to store number greater than 32767 it will cause data overflow and you will get garbage values.(Read How C Compiler decides the Garbage values in case of data overflow for more information)

Now ,lets take the scenario in linux/unix

The maximum number of singed int datatype(normal int) that can be accomodated if you run c in linux is ((2^31)-1)=2147483647 (assuming your comp is 32 bit)

so the int variable ‘a’ can store a maximum value of 2147483647.Compare with this mere 32767..

This is for int datatype consider other datatypes such as float,long double,double,long int etc..
Imagine how many big values it can store

So you can play with larger values if you run a C program in linux/unix.
Apart from this the speed of execution will be more than double since it utilizes full available word length.

C in linux/unix-First step

adithya — Sat, 22 Mar 2008 13:26:50 +0000

First let’s learn how to run a c program in linux/unix.Nearly every distro support’s running c programs.

First thing is to type the c program.you can use any editor you want but here i use vi editor which comes with allmost all distro

vi editor

To create a new c file or to open an existing c file,type vi followed by filename.c
ex:
[student@localhost student]$vi adi.c

will create a new file adi.c or will open an exisiting file adi.c if it’s present

now press insert key to edit or modify or add new content
after you complete writing the program,press Esc key and type :w (colen w) and press enter to save the file then press :q (colen q) and press enter to exit the vi editor.

Next stop:Compilation

Compilation

To compile a c file enter cc followed by the filename
ex:
student@localhost student]$cc adi.c

if the compilation is successful ,then no message will be displayed.Otherwise a message along with line number where the error is present is displayed.
Next stop:Running the compiled file

Running the compiled file

To run the compiled file,enter ./a.out
ex:
[student@localhost student]$./a.out

the c program will be running.Just work the same way as you work in windows.
note that irrespective of the file name ,you have to always enter “./a.out” to run the c program which you just compiled.

let’s see an example program to print hi linux
[student@localhost student]$vi first.c

press insert and type the following program

#include main() { printf("\nHi linux\n"); }
note that i have given just main() not void main()

now press Esc key and type :w to save the file and :q to exit the vi editor

[student@localhost student]$cc first.c [student@localhost student]$./a.out

if you do these steps correctly, then you should get the output like this

Hi linux [student@localhost student]$

Noteif you get a error like this

[student@localhost student]$ cc adi.c bash: cc: command not found [student@localhost student]$

means there is no gcc package in your distro and you are unable to run c programs in that