父子进程随机结束（linux，c，fork（））

Question

The task wast to sort in an ascending order the array from the input.dat, using fork and divide and conquer algorithm.

So i wrote a code using this, but the parent process end faster than the child. I have the following line in my code but the parents will not wait for their children.

while(wait(NULL)>0);

A little explanation for my code:

In the input.dat is an array with the following form:

(long of the array) (space) (first number) (space) (second number) ...

example:

6 2 3 1 4 5 6

So I read in the array, and I split it to two arrays created in two structs, for writhing into the pipe. After that I call the divitimp function , a little explanation for the form:

(in my code) divitimp(readwhilegoingdown(lefeleolvas),writewhilegoingdown(lefeleir),readwhilecomingup(felfeleolvas),writewhilecomingup(felfeleir))

After the divitimp function started I use fork, If it works(no errors) I look if it is the child or the parent process(the code is the same in the if and in the else), after that (in the if ) I read out one struct from the pipe if the array in the struct is not longer than two element I write the sorted struct to the writewhilecomingup pipe and not calling back the divitimp function, if the array is longer than 2, I split the array to two arrays created in two structs and i write them to writewhilegoingdown pipe and calling back the divitimp function.

After these I want to wait until the child process end and I use merge sort to sort the sorted arrays and then I write the results(eredmeny) in to the writewhilecomingup pipe.

After the divitimp function finished i go back to main and read the solution struct from the readwhilegoingup pipe, and write it to the output.dat.

The problem is the processes (parent and child) end in a random order.

Source code (very long sorry, but without the code I can't explain the problem):

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

typedef struct{
int n;
int a[100];
}TOMB;

void divitimp(int lefeleolvas, int lefeleir,int felfeleolvas,int felfeleir){
pid_t pid;
int i;
if ((pid = fork()) < 0){
    perror("fork hiba");
}
else{
    printf("pid= %i \n",pid);
    if(pid==0){
        TOMB valtozo;
        read(lefeleolvas,& valtozo, sizeof(valtozo));
        if(valtozo.n>2){
            TOMB sv,sv1;
            sv.n=valtozo.n/2;
            sv1.n=valtozo.n-valtozo.n/2;
            printf("Uj ismetles:%d ",sv.n);
            for(i=0;i<sv.n;i++){
                    sv.a[i]=valtozo.a[i];
                printf(" %d ",sv.a[i]);
            }
            int ai=sv1.n-1;
            printf(" *%d* ", sv1.n);
            for(i=0;i<sv1.n;i++){
                sv1.a[i]=valtozo.a[ai];
                ai++;
                printf(" %d ",sv1.a[i]);
            }
            printf("itt a vege\n");
            write(lefeleir,& sv,sizeof(sv));
            write(lefeleir,& sv1,sizeof(sv1));
            divitimp(lefeleolvas, lefeleir, felfeleolvas, felfeleir);
        }           
        else{
            if(valtozo.n==1){
                write(felfeleir,& valtozo,sizeof(valtozo));
            }
            else{
                if(valtozo.n==2);{
                    if(valtozo.a[0]<valtozo.a[1]){
                        write(felfeleir,& valtozo,sizeof(valtozo));
                    }
                    else{
                        int csere=valtozo.a[1];
                        valtozo.a[1]=valtozo.a[0];
                        valtozo.a[0]=csere;
                        write(felfeleir,& valtozo,sizeof(valtozo));

                    }
                }
            }
        }
    }
    else{
                    TOMB valtozo;
                    read(lefeleolvas,& valtozo, sizeof(valtozo));
                    if(valtozo.n>2){
                            TOMB sv,sv1;
                            sv.n=valtozo.n/2;
                            sv1.n=valtozo.n-valtozo.n/2;
                            printf("Uj ismetles:%d ",sv.n);
                            for(i=0;i<sv.n;i++){
                                    sv.a[i]=valtozo.a[i];
                                    printf(" %d ",sv.a[i]);
                            }
                            int ai=sv1.n-1;
            printf(" *%d* ", sv1.n);
                            for(i=0;i<sv1.n;i++){
                                    sv1.a[i]=valtozo.a[ai];
                                    ai++;
                                    printf(" %d ",sv1.a[i]);
                            }
                            printf("itt a vege\n");
                            write(lefeleir,& sv,sizeof(sv));
                            write(lefeleir,& sv1,sizeof(sv1));
                            divitimp(lefeleolvas, lefeleir, felfeleolvas, felfeleir);
            exit(0);
                    }
                    else{
                            if(valtozo.n==1){
                                    write(felfeleir,& valtozo,sizeof(valtozo));
                                    exit(0);
                            }
                            else{
                                    if(valtozo.n==2);{
                                            if(valtozo.a[0]<valtozo.a[1]){
                                                    write(felfeleir,& valtozo,sizeof(valtozo));
                                            exit(0);
                                            }
                                            else{
                                                    int csere=valtozo.a[1];
                                                    valtozo.a[1]=valtozo.a[0];
                                                    valtozo.a[0]=csere;
                                                    write(felfeleir,& valtozo,sizeof(valtozo));
                                            exit(0);
                                            }
                                    }
                            }
                    }
    }
    while(wait(NULL)>0);
    if(pid==0){
        TOMB sv,sv1,eredmeny;
        read(felfeleolvas,& sv,sizeof(sv));
        read(felfeleolvas,& sv1,sizeof(sv));
        int svi=0,sv1i=0;
        eredmeny.n=sv.n+sv1.n;
        for(i=0;i<sv.n+sv1.n;i++){
            if(svi<sv.n&&sv1i<sv1.n){
                if(sv.a[svi]<sv1.a[sv1i]){
                    eredmeny.a[i]=sv.a[svi];
                    svi++;
                }
                else{
                    eredmeny.a[i]=sv1.a[sv1i];
                    sv1i++;
                }
            }
            else{
                if(svi<sv.n){
                    eredmeny.a[i]=sv.a[svi];
                svi++;  
                }
                else{
                    eredmeny.a[i]=sv1.a[sv1i];
                    sv1i++;
                }
            }
            printf("e=%d sv=%d sv1=%d\n",eredmeny.a[i],sv.a[svi-1],sv1.a[sv1i-1]);

        }
        write(felfeleir,& eredmeny, sizeof(eredmeny));

    }
   }
 }

int main(){
    FILE *f= fopen("input.dat","r");
if(!f){
perror("nincs megfelelo allomany");
exit(1);
}
    FILE *g= fopen("output.dat","w");
int n;
fscanf(f,"%d",&n);
int a[n];
a[0]=n;
int i=0;
printf("%d",n);
for(i;i<n;i++){
fscanf(f,"%d",&a[i]);
}
//printf("%d \n",n);
TOMB betesz1, betesz2;
    betesz1.n=n/2;
    for(i=0;i<betesz1.n;i++){
    betesz1.a[i]=a[i];
printf("%d\n ",a[i]);   
}
    betesz2.n=n-n/2;
    int ai=betesz2.n;
    for(i=0;i<betesz2.n;i++){
    betesz2.a[i]=a[ai];
    ai++;
printf("%d\n ",a[ai-1]);
}
    int pfd[2];
    if (pipe(pfd) < 0){
    perror("pipe hiba");
}
    int pfd1[2];
    if (pipe(pfd1) < 0){
    perror("pipe hiba");
}
    write(pfd[1],& betesz1, sizeof(betesz1));
    write(pfd[1],& betesz2, sizeof(betesz2));

divitimp(pfd[0],pfd[1],pfd1[0],pfd1[1]);
TOMB eredmeny;
read(pfd1[0],& eredmeny,sizeof(eredmeny));
printf("%d\n",eredmeny.n);
for(i=0;i<eredmeny.n;i++){
    fprintf(g,"%d ",eredmeny.a[i]);
}
fclose(f);
fclose(g);
return 0;
}

Answer 1

I think you confused the parent and child processes. fork() returns 0 in the child process, but you have exit(0)s on the else branch, which basically means you terminate the parents.

Try switching your if statements to pid != 0.