How c program runs in os?

Today we are going do discuss when you are going to run any user application or any program in Linux operating system. Here we are considering c program.
in the last post we seen how we are executing c program in Linux.
Now we are going to discuss how c program handled by operating system.
before starting with it. we need to go through some basic concepts of OS and kernel.
Basically there are two spaces in system
1) User space:- Where we are going to run user application. Most of the user program runs in this space.
2) Kernel space:- In the kernel space is reserved for kernel programs and device drivers.
In kernel space all file management , memory management and scheduling programs are executed.

In the following figure you can able to see the clear difference between user space and kernel space.
when you are going to run any user program its runs in user space.

In the figure there is linkage present between user space and kernel space and that is system calls.
Now we will see what is use of system call in simple way.
user processes can directly communicated to kernel or access hardware due to security purpose.
so they communicated through the system calls.
you can say system call is systematic way for user process to request kernel service such as hard disk access or any other device access, creation of any new process or process scheduling.....
  In the Linux operating system the system calls are handled by POSIX libraries.

lets discuss briefly about POSIX.
POSIX is Portable Operating System Interface.
POSIX defines Application Programming Interface(API) and some command shell and utilities.
lets understands in easy way lets consider I want to develop application for Linux operating system so I have to follow posix standard so my application will able to communicate with kernel.

You can get detailed function set of POSIX here.
you will understand this concept after this example.

Let us consider you written down simple C program. Now in the c program you used c system function  printf("hello world") which is function of c.

So when you are going to execute c program you have to tell to the OS that you have to display hello world on terminal but c program is running in user space so c program dont have privilege to access the output window. only kernel can access that so C program requests to POSIX and the POSIX carry forward that request to kernel.
and then we are able to see hello world message on terminal.

printf()----> write()---->kernel code

C program Function----->POSIX fuction------->Kernel code for write()

to see how the system call works:

run the c program to create temporary file for the same refer previous post.

that is $gcc -save-temps -o hi.c

now open the file hi.s

that is assembly code.

the below code is for hi.s

.global _start

     

        .text
_start_start:        # write(1, message, 13)        mov     $1, %rax                # system call 1 is write        mov     $1, %rdi                # file handle 1 is stdout        mov     $message, %rsi          # address of string to output        mov     $13, %rdx               # number of bytes        syscall                         # invoke operating system to do the write
        # exit(0)        mov     $60, %rax               # system call 60 is exit        xor     %rdi, %rdi              # we want return code 0        syscall                         # invoke operating system to exitmessage:        .ascii  "hi h r u"

In this you can understand from the comments.

This way you can able to see how c program runs by OS.

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How Compiler works on C program??

Last post we seen that how to compile C program.Now we will see how the compiler working on the C programming.
As we know that compiler converting source code into machine code that we are going to execute on the hardware then we will going to get output.
But we do not know how the compiler working internally on the C program. The answer is here. First we will see the internal architecture of compiler.
There are 4 major stages when compiler compiles the C program.
These steps are

1. Pre-processing
2. Compilation
3. Assembly
4. Linking

We are going to discuss these steps are in details.

1. Pre-Processing

In this step compiler will going to do

1. Macro substitution
2. Comments are stripped off
3. Expansion of the included files

In this step header files we added after pre-processor tag that is #include<stdio.h>

get expanded.

Let us see the example of it.
To see the output of pre-processing stage, we need to  run following command in terminal:
gcc -E hi.c -o bye

The output of this command will create file of name bye.
open that file then you will able to see expanded code.

you can able to see your written code at the bottom of that file.

you can easily get all intermediate file by simple command :

gcc -save-temps hi.c -o bye

 This command will generate three intermediate files.

1. hi.i  

2. hi.s 

3. hi.o

Here hi.i file is output of pre-processing step.

2. Compilation

In the next step the pre-processed file will work as input to Compilation process.

now we will submit hi.i file to compilation process.

This step will convert the code into Assembly code.

so the output of this step will be hi.s

As you are able to see the hi.s is assembly code.

3.Assembly

Now in this step the hi.s file is taken as input and it get converted into object code that is hi.o

This file is object code file which contains machine code which is we can not understand.

4.Linking

This is the final stage at which all the linking of function calls with their definitions are done.

The linker also does some extra work; it combines some extra code to our program that is required when the program starts and when the program ends. For example, there is code which is standard for setting up the running environment like passing command line arguments, passing environment variables to every program. Similarly some standard code that is required to return the return value of the program to the system.

Now we are able to see what actually happening when we are running c program.

In the next post we will discuss regarding how  operating system runs c program.

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How to run C program in Linux??

Most of the students are having habit of using Turbo-C to write C  and C++ programs.They think it as easy to use and easy to run. But Turbo-C software is not giving actual sense of running c program.

 When you runs c program in turbo-c you have to write getch() to block or freeze output window.

Actually you are not freezing output window that is called as incomplete execution of program.

your program is waiting to get character from keyboard to come out from that command prompt.

When you are using Linux operating system to run your C program there is need of GCC compiler that is GNU Compiler Collection is used to compile c program. Most of the time it is by default present in Linux operating system. The explanation given below is related with Ubuntu.

You can check GCC version and whether it is installed or not by using simple command on terminal.

Command: gcc -v

If It is not showing the result then you have to install it through terminal.

Command for the same: sudo apt-get install gcc

After successful installation of GCC major question is how to run your c program.

Type your c program in editor such as gedit or any other editor which you like.

Save that program with .c extension.now from your terminal go to respective directory where you saved your c program.

After that you have to compile your C program with GCC.

Command for that is: gcc hi.c -o bye

I would like to explain the command 

gcc is command for compilation. Instead of gcc you can also use cc.

hi.c is file name

-o is attribute for output 

bye is output file name.

This Command will create executable file having name "bye".

After successful compilation compiler will create executable file bye.

now we have just type ./bye to run that executable.

You successfully executed c program.

In Next post we will see what actually happen behind the screen when you compile c program.

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