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C++ || Serial & Parallel Multi Process File Downloader Using Fork & Execlp

August 19, 2013 admin 3 Comments

The following is another homework assignment which was presented in an Operating Systems Concepts class. The following are two multi-process programs using commandline arguments, which demonstrates more practice using the fork() and execlp() system calls on Unix based systems.

==== 1. OVERVIEW ====

File downloaders are programs used for downloading ﬁles from the Internet. The following programs listed on this page implement two distinct type of multi-process downloaders:

1. a serial ﬁle downloader which downloads ﬁles one by one. 2. a parallel ﬁle downloader which dowloads multiple ﬁles in parallel.

In both programs, the parent process first reads a ﬁle via the commandline. This file which is read is the ﬁle that contains the list of URLs of the ﬁles to be downloaded. The incoming url file that is read has the following format:

[URL1] [URL2] . . . [URLN]

Where [URL] is an http internet link with a valid absolute file path extension.
(i.e: http://newsimg.ngfiles.com/270000/270173_0204618900-cc-asmbash.jpg)

After the url file is parsed, next the parent process forks a child process. Each created child process uses the execlp() system call to replace its executable image with that of the “wget” program. The use of the wget program performs the actual file downloading.

==== 2. SERIAL DOWNLOADER ====

The serial downloader downloads files one at a time. After the parent process has read and parsed the incoming url file from the commandline, the serial downloader proceeds as follows:

1. The parent forks off a child process. 2. The child uses execlp("/usr/bin/wget", "wget", [URL STRING1], NULL) system call in order to replace its program with wget program that will download the first file in urls.txt (i.e. the file at URL ). 3. The parent executes a wait() system call until the child exits. 4. The parent forks off another child which downloads the next file specified in url.txt. 5. Repeat the same process until all files are downloaded.

The following is implemented below:





		
		
			Serial File Downloader Using fork() & execlp('wget')
			
C++
			
			// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Aug 19, 2013
//    Taken From: http://programmingnotes.org/
//    File:  Serial.cpp
//    Description: File downloaders are programs used for downloading files
//       from the Internet. Using the fork() & execlp("wget") command, the
//       following is a multi-process serial file downloader which reads an
//       input file containing url file download links as a commandline
//       argument and downloads the files located on the internet one by one.
// ============================================================================
#include <iostream>
#include <cstring>
#include <fstream>
#include <cstdlib>
#include <unistd.h>
#include <sys/wait.h>
using namespace std;

// compile & run
// g++ Serial.cpp -o Serial
// ./Serial urls.txt

int main(int argc, char* argv[])
{
    // declare variables
    pid_t pid = -1;
    int urlNumber = 0;
    char urlName[256];
    ifstream infile;

    // check if theres enough command line args
    if(argc < 2)
    {
        cout <<"\nERROR -- NOT ENOUGH ARGS!"
            <<"\n\nUSAGE: "<<argv[0]<<" <file containing url downloads>\n\n";
        exit(1);
    }

    // try to open the file containing the download url links
    // exit if the url file is not found
    infile.open(argv[1]);
    if(infile.fail())
    {
        cout <<"\nERROR -- "<<argv[1]<<" NOT FOUND!\n\n";
        exit(1);
    }

    // get download url links from the file
    while(infile.getline(urlName, sizeof(urlName)))
    {
        ++urlNumber;

        // fork another process
        pid = fork();

        if(pid < 0)
        {   // ** error occurred
            perror("fork");
            exit(1);
        }
        else if(pid == 0)
        {   // ** child process
            cout <<endl<<"** URL #"<<urlNumber
                    <<" is currently downloading... **\n\n";
            execlp("/usr/bin/wget", "wget", urlName, NULL);
        }
        else
        {   // ** parent process
            // parent will wait for the child to complete
            wait(NULL);
            cout <<endl<<"-- URL #"<<urlNumber<<" is complete! --\n";
        }
    }
    infile.close();

    cout <<endl<<"The parent process is now exiting...\n";

    return 0;
}// http://programmingnotes.org/
			
				
					
				
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						// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Aug 19, 2013
//    Taken From: http://programmingnotes.org/
//    File:  Serial.cpp
//    Description: File downloaders are programs used for downloading files
//       from the Internet. Using the fork() & execlp("wget") command, the
//       following is a multi-process serial file downloader which reads an
//       input file containing url file download links as a commandline
//       argument and downloads the files located on the internet one by one.
// ============================================================================
#include <iostream>
#include <cstring>
#include <fstream>
#include <cstdlib>
#include <unistd.h>
#include <sys/wait.h>
using namespace std;
 
// compile & run
// g++ Serial.cpp -o Serial
// ./Serial urls.txt
 
int main(int argc, char* argv[])
{
    // declare variables
    pid_t pid = -1;
    int urlNumber = 0;
    char urlName[256];
    ifstream infile;
 
    // check if theres enough command line args
    if(argc < 2)
    {
        cout <<"\nERROR -- NOT ENOUGH ARGS!"
            <<"\n\nUSAGE: "<<argv[0]<<" <file containing url downloads>\n\n";
        exit(1);
    }
 
    // try to open the file containing the download url links
    // exit if the url file is not found
    infile.open(argv[1]);
    if(infile.fail())
    {
        cout <<"\nERROR -- "<<argv[1]<<" NOT FOUND!\n\n";
        exit(1);
    }
 
    // get download url links from the file
    while(infile.getline(urlName, sizeof(urlName)))
    {
        ++urlNumber;
 
        // fork another process
        pid = fork();
 
        if(pid < 0)
        {   // ** error occurred
            perror("fork");
            exit(1);
        }
        else if(pid == 0)
        {   // ** child process
            cout <<endl<<"** URL #"<<urlNumber
                    <<" is currently downloading... **\n\n";
            execlp("/usr/bin/wget", "wget", urlName, NULL);
        }
        else
        {   // ** parent process
            // parent will wait for the child to complete
            wait(NULL);
            cout <<endl<<"-- URL #"<<urlNumber<<" is complete! --\n";
        }
    }
    infile.close();
 
    cout <<endl<<"The parent process is now exiting...\n";
 
    return 0;
}// http://programmingnotes.org/

Since the serial downloader downloads files one at a time, that can become very slow. That is where the parallel downloader comes in handy!

==== 3. PARALLEL DOWNLOADER ====

The parallel downloader downloads files all at once and is implemented much like the serial downloader. The parallel downloader proceeds as follows:

1. The parent forks off n children, where n is the number of URLs in url.txt. 2. Each child executes execlp("/usr/bin/wget", "wget", [URL STRING], NULL) system call where each is a distinct URL in url.txt. 3. The parent calls wait() (n times in a row) and waits for all children to terminate. 4. The parent exits.

The following is implemented below:





		
		
			Parallel File Downloader Using fork() & execlp('wget')
			
C++
			
			// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Aug 19, 2013
//    Taken From: http://programmingnotes.org/
//    File:  Parallel.cpp
//    Description: File downloaders are programs used for downloading files
//       from the Internet. Using the fork() & execlp("wget") command, the
//       following is a multi-process parallel file downloader which reads an
//       input file containing url file download links as a commandline
//       argument and downloads the files located on the internet all at once.
// ============================================================================
#include <iostream>
#include <cstring>
#include <fstream>
#include <cstdlib>
#include <unistd.h>
#include <sys/wait.h>
using namespace std;

// compile & run
// g++ Parallel.cpp -o Parallel
// ./Parallel urls.txt

int main(int argc, char* argv[])
{
    // declare variables
    pid_t pid = -1;
    int urlNumber = 0;
    char urlName[256];
    ifstream infile;

    // check if theres enough command line args
    if(argc < 2)
    {
        cout <<"\nERROR -- NOT ENOUGH ARGS!"
            <<"\n\nUSAGE: "<<argv[0]<<" <file containing url downloads>\n\n";
        exit(1);
    }

    // try to open the file containing the download url links
    // exit if the url file is not found
    infile.open(argv[1]);
    if(infile.fail())
    {
        cout <<"\nERROR -- "<<argv[1]<<" NOT FOUND!\n\n";
        exit(1);
    }

    // get download url links from the file
    while(infile.getline(urlName, sizeof(urlName)))
    {
        ++urlNumber;

        // fork another process
        pid = fork();

        if(pid < 0)
        {   // ** error occurred
            perror("fork");
            exit(1);
        }
        else if(pid == 0)
        {   // ** child process
            cout <<endl<<"** URL #"<<urlNumber
                    <<" is currently downloading... **\n\n";
            execlp("/usr/bin/wget", "wget", urlName, NULL);
        }
    }
    infile.close();

    while(urlNumber > 0)
    {   // ** parent process
        // parent will wait for the child to complete
        wait(NULL);
        cout <<endl<<"-- URL #"<<urlNumber<<" is complete! --\n";
        --urlNumber;
    }

    cout <<endl<<"The parent process is now exiting...\n";

    return 0;
}// http://programmingnotes.org/
			
				
					
				
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						// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Aug 19, 2013
//    Taken From: http://programmingnotes.org/
//    File:  Parallel.cpp
//    Description: File downloaders are programs used for downloading files
//       from the Internet. Using the fork() & execlp("wget") command, the
//       following is a multi-process parallel file downloader which reads an
//       input file containing url file download links as a commandline
//       argument and downloads the files located on the internet all at once.
// ============================================================================
#include <iostream>
#include <cstring>
#include <fstream>
#include <cstdlib>
#include <unistd.h>
#include <sys/wait.h>
using namespace std;
 
// compile & run
// g++ Parallel.cpp -o Parallel
// ./Parallel urls.txt
 
int main(int argc, char* argv[])
{
    // declare variables
    pid_t pid = -1;
    int urlNumber = 0;
    char urlName[256];
    ifstream infile;
 
    // check if theres enough command line args
    if(argc < 2)
    {
        cout <<"\nERROR -- NOT ENOUGH ARGS!"
            <<"\n\nUSAGE: "<<argv[0]<<" <file containing url downloads>\n\n";
        exit(1);
    }
 
    // try to open the file containing the download url links
    // exit if the url file is not found
    infile.open(argv[1]);
    if(infile.fail())
    {
        cout <<"\nERROR -- "<<argv[1]<<" NOT FOUND!\n\n";
        exit(1);
    }
 
    // get download url links from the file
    while(infile.getline(urlName, sizeof(urlName)))
    {
        ++urlNumber;
 
        // fork another process
        pid = fork();
 
        if(pid < 0)
        {   // ** error occurred
            perror("fork");
            exit(1);
        }
        else if(pid == 0)
        {   // ** child process
            cout <<endl<<"** URL #"<<urlNumber
                    <<" is currently downloading... **\n\n";
            execlp("/usr/bin/wget", "wget", urlName, NULL);
        }
    }
    infile.close();
 
    while(urlNumber > 0)
    {   // ** parent process
        // parent will wait for the child to complete
        wait(NULL);
        cout <<endl<<"-- URL #"<<urlNumber<<" is complete! --\n";
        --urlNumber;
    }
 
    cout <<endl<<"The parent process is now exiting...\n";
 
    return 0;
}// http://programmingnotes.org/

QUICK NOTES:
The highlighted lines are sections of interest to look out for.

The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.

Also note, while the parallel downloader executes, the outputs from different children may intermingle.

C++ || Snippet – How To Use Fork & Execlp For Interprocess Communication

August 19, 2013 admin No comments

The following is sample code which demonstrates the use of the “fork” and “execlp” function calls on Unix based systems.

The “fork” function call creates a new process by duplicating the calling process; or in more simpler terms, it creates a duplicate process (a child) of the calling (parent) process.

This new process, referred to as the child, is an exact duplicate of the calling process, referred to as the parent.

The “execlp” function call is part of a family of functions which replaces a current running process image with a new process image. That means by using the “execlp” function call, you are basically replacing the entire current running process with a new user defined program.

Though fork and execlp are not required to be used together, they are often used in conjunction with one another as a way of creating a new program running as a child of another process.





		
		
			How To Use Fork & Execlp
			
C++
			
			// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Aug 19, 2013
//    Taken From: http://programmingnotes.org/
//    File:  ForkExample.cpp
//    Description: Demonstrate the use of the fork() command for parent and
//       child process manipulation.
// ============================================================================
#include <iostream>
#include <cstdlib>
#include <sys/wait.h>
#include <unistd.h>
using namespace std;

int main()
{
    // declare variable
    pid_t pid = -1;

    cout <<"\nParent is forking a child." << endl;

    // create a duplicate process of this current program
    pid = fork();

    // exit if something went wrong with the fork
    if(pid < 0)
    {
        perror("fork");
        exit(1);
    }
    // this code only gets executed by the child process
    else if(pid == 0)
    {
        cout <<"\nStarting the child process..\n" << endl;
        // NOTE: the "execlp" function replaces the current
        // child process with the terminal "ls" command.
        // the child process finally dies after the "execlp"
        // function is complete
        execlp("/bin/ls", "ls", "-l", NULL); // "ls -l" command
        cout <<"\nNOTE: This never gets executed - Why?!";
    }
    // the parent process executes here
    else
    {
        cout <<"\nParent is now waiting for child id #"<<pid<<" to complete.." << endl;
        // use the "wait" function to wait for the child to complete
        wait(NULL);
        cout <<"\nThe child process is complete and has terminated!\n";
    }

    cout <<"\nParent is now exiting...\n";

    return 0;
}// http://programmingnotes.org/
			
				
					
				
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						// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Aug 19, 2013
//    Taken From: http://programmingnotes.org/
//    File:  ForkExample.cpp
//    Description: Demonstrate the use of the fork() command for parent and
//       child process manipulation.
// ============================================================================
#include <iostream>
#include <cstdlib>
#include <sys/wait.h>
#include <unistd.h>
using namespace std;
 
int main()
{
    // declare variable
    pid_t pid = -1;
 
    cout <<"\nParent is forking a child." << endl;
 
    // create a duplicate process of this current program
    pid = fork();
 
    // exit if something went wrong with the fork
    if(pid < 0)
    {
        perror("fork");
        exit(1);
    }
    // this code only gets executed by the child process
    else if(pid == 0)
    {
        cout <<"\nStarting the child process..\n" << endl;
        // NOTE: the "execlp" function replaces the current
        // child process with the terminal "ls" command.
        // the child process finally dies after the "execlp"
        // function is complete
        execlp("/bin/ls", "ls", "-l", NULL); // "ls -l" command
        cout <<"\nNOTE: This never gets executed - Why?!";
    }
    // the parent process executes here
    else
    {
        cout <<"\nParent is now waiting for child id #"<<pid<<" to complete.." << endl;
        // use the "wait" function to wait for the child to complete
        wait(NULL);
        cout <<"\nThe child process is complete and has terminated!\n";
    }
 
    cout <<"\nParent is now exiting...\n";
 
    return 0;
}// http://programmingnotes.org/

QUICK NOTES:
The highlighted lines are sections of interest to look out for.

The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.

The following is sample output:

Parent is forking a child. Parent is now waiting for child id #10872 to complete..
Starting the child process.. total 1466 -rw-r--r-- 1 admin admin 468 Apr 27 2012 nautilus-computer.desktop -rwxrwxr-x 1 admin admin 9190 Aug 19 15:17 ForkExample -rw-rw-r-- 1 admin admin 1640 Aug 19 15:17 ForkExample.cpp The child process is complete and has terminated!
Parent is now exiting...

Tag Archives: execlp

C++ || Serial & Parallel Multi Process File Downloader Using Fork & Execlp

C++ || Snippet – How To Use Fork & Execlp For Interprocess Communication

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