POSIX IPC Shared memory (need help)

Question

Hi am new to this forum, I'd like to ask about how to make a shared memory in the following code using shmat(), shmget(), to path mata , matb , matc from parent to child process, because i tried using global variables but when the child process terminates it seems no update to the global variables

pls show me an example or sample code ...thnx inadvance

thnx

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
using namespace std;

class MATRIX{
public:
    int row;
    int col;
    int** MAT;
    MATRIX(int row , int col);
    ~MATRIX();
};
MATRIX::MATRIX(int row_i , int col_i){
    row=row_i;
    col=col_i;
MAT=new int* [row];
int i;
for(i=0;i<=row-1;i++){
    MAT[i]=new int [col];
}
}
MATRIX::~MATRIX (){
    int i;
    for(i=0;i<=row-1;i++){
        delete MAT[i];
    }
    delete MAT ;
}

void init_mat(MATRIX& mat){
int temp=1;
for(int i =0 ; i<= mat.row -1 ;i++){
    for(int j=0;j<=mat.col-1;j++){
        mat.MAT[i][j]=temp++; //Any initialization
    }
}

}

void print_mat(MATRIX& mat){
for(int i =0 ; i<= mat.row -1 ;i++){
    for(int j=0;j<=mat.col-1;j++){
        cout<<mat.MAT[i][j]<<" ";
    }
    cout<<endl;
}
}
void mul_col(MATRIX& mata ,MATRIX& matb,MATRIX& matc, int col){
int sum;
    for (int j =0 ;j<= mata.row-1;j++){
        sum=0;
        for(int i= 0 ;i<=mata.col-1;i++){
            sum=sum+mata.MAT[j][i]*matb.MAT[i][col];
        }
        matc.MAT[j][col]=sum;
    }

}

    MATRIX mata(3,3);
    MATRIX matb(3,3);
    MATRIX matc(3,3);

int main()
{


    init_mat(mata);
    init_mat(matb);
     //print_mat(mata);
    //print_mat(matb);


  pid_t pid;

   for (int k=0; k <= (matb.col-1);k++){

    pid = fork();

    if(pid == 0){
            cout<<"am in mul place"<<endl;
            mul_col(mata,matb,matc,k);
            print_mat(matc);
            cout<<k<<endl;
            exit(0);
    }
    else if(pid > 0){
        wait(NULL);
        cout<<"i passed by the wait  "<<endl;

    }
    else{
        cout<<"Can't fork"<<endl;
    }

    if (pid == 0)
    {
    break;
    }

    }

    if(pid > 0){
    cout<<"am in the print area"<<endl;
    print_mat(matc);
    }


}

Just copying someone's code is not going to help you much. You have to learn this stuff. You can start here. — Duck
if there is some explanation with code it will be helpful .. but thanx for ur help .. I'll read that page — Eng_Boody
Read, absorb, and take a stab at it. People are willing to help you with specific problems along the way but SO isn't a tutorial service. — Duck
I have read the page and understood the majority of it, but there are some points i couldn't get even after searching google , 1) Do i still need to attach memory to the process creating shared memory using shmget() or shmget do both jobs , 2) I couldn't find a document that contain a list of constants for the flag and it's corresponding functionality, what's the interpretation (meaning of) the key , 4) what's the difference between key and memory id ...thnx — Eng_Boody

Duck Duck · Accepted Answer · 2014-05-22T00:18:41

Starting with (3). The key is a discovery mechanism. It's a means by which two separate processes can find and attach to the same shared memory. As long as each knows the path to the appropriate file they can call ftok which will produce a unique (most of the time) "key". That key is then used by shmget to look up (or create) the memory segments it is managing and return the ID. The ID is what you really care about since it is the handle to the segment and used in the remainder of associated shared memory calls.
With (1) I think you are asking about processes that are "related" to each other. In other words, if a process opens shared memory in the parent and then forks it does not need to go through the whole ftok/shmget boilerplate again. It will inherit the open shared memory segment as part of the fork and it will have the ID wherever the parent stored it because the child starts as copy of the parent.

If two process are started separately then you have to do the shmget, etc., in both.
The flags to shmget are really 2 separate collections ORed together. (a) IPC_CREAT and IPC_EXCL; (b) file permissions common to any open call. The constants are usually found in fcntl.h - e.g. S_IRUSR, S_IWUSR, S_IXUSR, etc. So something like:

shmid = shmget(shmkey, sizeofsegment, 0666 | IPC_CREAT | IPC_EXCL);

or

shmid = shmget(shmkey, SIZEOFSHMSEG, IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR); //etc

Note: there are 2 kind of shared memory in POSIX. This is the "sysv" variant. There is also the "posix" variant which uses different calls and methods to achieve a similar effect.

POSIX IPC Shared memory (need help)

1 Answers