Multiple Child Creation Issues

Question

This code is part of my OS project, the project asks to do something where parallel processes are applied, I decided to create a client server poker project with two players, where I use the processes of sons and grandchildren to determine the value of the hand.

The scheme used in the code is as follows:

The problem with the code is that in the same game only the first hand is correctly evaluated, and in the second - incorrectly, and in the third game an error occurs, and the program ends, this happens for each new game

Here is the code:

void check_hand(int suits[5],int ranks[5],int *point){ pid_t son[2]; int i,j; for (i = 0; i < 2; i++){ son[i]=fork(); /*********************************************** straight flush son ************************************************/ if(son[i]==0 && i==0){ pid_t grandson[3]; int k; for(k=0;k<3;k++){ grandson[k]=fork(); if(grandson[k]==0 && k==0){ exit(F_highcard(ranks)); } else if(grandson[k]==0 && k==1){ exit(F_flush(suits)); } else if(grandson[k]==0 && k==2){ exit(F_straight(ranks)); } else if(grandson[k]<0){ puts("fork failed"); exit(-1); } } int exit_status_straight,exit_status_flush,exit_status_highcard; //waiting his sons waitpid(grandson[0],&exit_status_highcard,0); waitpid(grandson[1],&exit_status_flush,0); waitpid(grandson[2],&exit_status_straight,0); /**checkpoint A****/ //elaborate the exit statuses and exit with a value } /*********************************************** full house son ************************************************/ if(son[i]==0 && i==1){ pid_t grandson[2]; int k; for(k=0;k<2;k++){ grandson[k]=fork(); if(grandson[k]==0 && k==0){ exit(F_n_pairs(ranks)); } else if(grandson[k]==0 && k==1){ exit(F_tris_poker(ranks)); } else if(grandson[k]<0){ exit(-1); } } int exit_status_pair,exit_status_tris_or_poker; waitpid(grandson[0],&exit_status_pair,0); waitpid(grandson[1],&exit_status_tris_or_poker,0); /**checkpoint B****/ //elaborate the exit statuses and exit with a value } } if(son[i]<0){ puts("fork failed"); exit(-1); } } /*********************************************** analysis exit status of his 2 sons ************************************************/ pid_t pid; int status; int values[10]; //initialization for(j=0;j<10;j++)values[j]=0; for(j=0;j<2;j++ ){ pid = wait(&status); if(pid==son[0]){ values[WEXITSTATUS(status)]=1; } else if(pid==son[1]){ values[WEXITSTATUS(status)]=1; } else if(pid==-1){ puts("error"); exit(1); } } for(j=9;j>=0;j--){ if(values[j]==1)break; } *point=j; printf("point=%d\n",*point); } 

In the following code, I set some control point to find an error, the result at runtime for each new game is the same for player 1 and player 2:

1 hand (always correct)

  checkpoint A checkpoint B point=value 

2 hands

  checkpoint A point=value checkpoint B 

why did this happen? A father must wait for his son, in which case he does not wait

3 hands

 point=-1 and exit 

Thanks in advance.

Answer 1

I got such a strong feeling that your problems are related to your last wait cycle. You only expect twice for child processes. Regardless of whether the returned PID is one of the children you check.

My guess is that your grandchildren are waiting for a return or returning a value of <-1 for any reason

The proof would be to put an else clause in your wait loop that prints the PID and check that PID with the PID of the created processes.

What I would do in any case is to change the wait loop to increase j only if one of the two sons called the wait call back.