fork() help

Applications Programming
1

Hi everybody,
I wanna write a code to understand how fork works.

my target
--------------

-Parent creates a file(called temp) and writes into this file "1".Then it closes the file.

-Then parent creates a child and wait until execution of this child ends.

-Then child opens the same file and reads the last integer in it. Then, it will multiply this integer by 2 and add 1. Then child appends the result as a new line at the end of the file. After closing the file this child will fork another child process and wait until it finishes its execution.

-Then child which calculates the Nth integer will not fork anymore, and return after writing the result into the file.( N can be taken from user )

Thanks a lot men...

2

Ummm.
So, how can i fork in a child process for a new child process?

3

This looks like homework. Is it? Why don't you just write code that forks once and where the parent prints that it is the parent and the child prints that it is the child? This will get you the code that you need to figure out which is a parent process and which is a child. Then you can use this code to run fork in the child process.

4 
/* Includes */
#include <unistd.h>     /* Symbolic Constants */
#include <sys/types.h>  /* Primitive System Data Types */ 
#include <errno.h>      /* Errors */
#include <stdio.h>      /* Input/Output */
#include <sys/wait.h>   /* Wait for Process Termination */
#include <stdlib.h>     /* General Utilities */
 
int main()
{
    pid_t childpid; /* variable to store the child's pid */
    int retval;     /* child process: user-provided return code */
    int status;     /* parent process: child's exit status */

    /* only 1 int variable is needed because each process would have its
       own instance of the variable
       here, 2 int variables are used for clarity */
        
    /* now create new process */
    childpid = fork();
    
    if (childpid >= 0) /* fork succeeded */
    {
        if (childpid == 0) /* fork() returns 0 to the child process */
        {
            printf("CHILD: I am the child process!\n");
            printf("CHILD: Here's my PID: %d\n", getpid());
            printf("CHILD: My parent's PID is: %d\n", getppid());
            printf("CHILD: The value of my copy of childpid is: %d\n", childpid);
            printf("CHILD: Sleeping for 1 second...\n");
            sleep(1); /* sleep for 1 second */
            printf("CHILD: Enter an exit value (0 to 255): ");
            scanf(" %d", &retval);
            printf("CHILD: Goodbye!\n");    
            exit(retval); /* child exits with user-provided return code */
        }
        else /* fork() returns new pid to the parent process */
        {
            printf("PARENT: I am the parent process!\n");
            printf("PARENT: Here's my PID: %d\n", getpid());
            printf("PARENT: The value of my copy of childpid is %d\n", childpid);
            printf("PARENT: I will now wait for my child to exit.\n");
            wait(&status); /* wait for child to exit, and store its status */
            printf("PARENT: Child's exit code is: %d\n", WEXITSTATUS(status));
            printf("PARENT: Goodbye!\n");             
            exit(0);  /* parent exits */       
        }
    }
    else /* fork returns -1 on failure */
    {
        perror("fork"); /* display error message */
        exit(0); 
    }
}
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