How to sent information between procecces in C using fork

Question

I'm trying to make a program that calculates the prime factors of a number (x for exemple) using fork()

In the child proces I want to iterate form 2 to x/2-1 and see witch number divides x. But i don't know how to let the parent proces to know that I modified a variable inside the child porces

And now my question is.. how do i sent information for child to parent?

int main(int argc, char *argv[]){
   int var = 0;
   pid = fork();
   if(pid > 0){
      if(var = 0){
         var++;
      else{do nothing}
else if(pid == 0){
  printf("%d\n", var);           here the program will print 0 but i want it to print 1. How?
}
}
}

}

Answer 1

the following proposed code:

1. cleanly compiles
2. performs the desired functionality
3. checks for (and handles) errors
4. documents why each header is included
5. demonstrates one interprocess communication (IPC) method (pipes)
6. demonstrates good use of horizontal and vertical spacing for readability and ease of understanding
7. uses int main( void ) so no warning messages from the compiler about unused parameters
8. used braces '{' and '}' in body of case 0: and default code blocks so can have local variables within those code blocks

and now, the proposed code:

#include <stdio.h>    // printf(), perror()
#include <sys/types.h>// pid_t
#include <unistd.h>   // fork(), pipe(), read()
#include <sys/wait.h> // wait()
#include <stdlib.h>   // exit(), EXIT_SUCCESS, EXIT_FAILURE

enum { READ_FROM, WRITE_TO };

int main( void )  
{
    int status; 

    int fd[2];
    if( pipe( fd ) )
    {
        perror( "pipe failed" );
        exit( EXIT_FAILURE );
    }

    pid_t pid = fork();

    switch( pid )
    {
        case -1:
            perror( "fork failed" );
            exit( EXIT_FAILURE );

        case 0: // child process
        {
            int var = 1;
            ssize_t bytesWritten;
            close( fd[ READ_FROM ] );

            if( ( bytesWritten = write( fd[ WRITE_TO ], &var, sizeof( var ) ) ) != sizeof( int ) )
            {
                perror( "write failed" );
                close( fd[ WRITE_TO ] );
                exit( EXIT_FAILURE );
            }

            close( fd[ WRITE_TO ] );
            exit( EXIT_SUCCESS );
        }

        default: // parent process
        {
            int buffer = 0;
            ssize_t bytesRead;
            close( fd[ WRITE_TO ] );

            if( ( bytesRead = read( fd[ READ_FROM ], &buffer, sizeof( buffer ) ) ) != sizeof( int ) )
            {
                perror( "read failed" );
                close( fd[ READ_FROM ] );
                wait( &status );  // waits for child to exit
                exit( EXIT_FAILURE );
            }

            printf( "%d\n", buffer );       
            close( fd[ READ_FROM ] );
            wait( &status );  // waits for child to exit
            break;
        }
    }
}

A typical/successful run of the program results in

1