C - iterative multiple forks

问题:

I need to make 4 forks 1000 times. I wrote this but it runs forever:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include  <sys/types.h>

#define   N  512

void  chunk0(void);
void  chunk1(void);
void  chunk2(void);
void  chunk3(void);
double get_time(void);

void  main(void)
{
    int i,j,k,iterations=0;
    unsigned int *a=(unsigned int *)malloc(N*N*(sizeof(unsigned int)));
    unsigned int *b=(unsigned int *)malloc(N*N*(sizeof(unsigned int)));
    unsigned int *c=(unsigned int *)malloc(N*N*(sizeof(unsigned int)));
    pid_t  pid;

    for(iterations=0;iterations<1000;iterations++){
        srand ( time(NULL) );
        double start=get_time();

        pid = fork();
        if (pid == 0) {
             chunk0();
        }else {
            pid = fork();
            if (pid == 0){ 
                 chunk1();
            }else {
                pid = fork();
                if (pid == 0){ 
                     chunk2();
                }else {
                    chunk3();
                    wait(NULL);
                    double end=get_time();
                    double diff=end-start;
                    printf("\n Time for run this code is: %lf seconds \n",diff);
                }
            }          
        }
    }       
}

void  chunk0(void)
{
/*  int i,j,k,iterations=0;
    unsigned int *a=(unsigned int *)malloc(N*N*(sizeof(unsigned int)));
    unsigned int *b=(unsigned int *)malloc(N*N*(sizeof(unsigned int)));
    unsigned int *c=(unsigned int *)malloc(N*N*(sizeof(unsigned int)));

    for(iterations=0;iterations<1000;iterations++){
        //printf("iteration #%d: Generating Matrix - ",iterations+1);
        for(i=0;i<N;i++){
            for(j=0;j<N;j++){
                //give int number between 0...1000 to a[i][j] , b[i][j] and reset c[i][j]
                *(a+(i*N+j))=(rand()%1001);
                *(b+(i*N+j))=(rand()%1001);
                *(c+(i*N+j))=0;
            }
        }
        //printf("Multiplying ... \n");
        for(i=0;i<N;i++){
            for(j=0;j<N;j++){
                for(k=0;k<N;k++){
                    *(c+(i*N+j))= *(c+(i*N+j)) + ((*(a+(i*N+k)))*(*(b+(k*N+j))));
                }
            }
        }
    }

    free(a);
    free(b);
    free(c);
*/  
     printf("   *** Child process 0 is done ***\n");
}
void  chunk1(void)
{
     int   i;
     printf("   *** Child process 1 is done ***\n");
}
void  chunk2(void)
{
     int   i;
     printf("   *** Child process 2 is done ***\n");
}
void  chunk3(void)
{
     int   i;
     printf("   *** Child process 3 is done ***\n");
}

double get_time(void){
    struct timeval stime;
    gettimeofday (&stime, (struct timezone*)0);
    return (stime.tv_sec+((double)stime.tv_usec)/1000000);
}

I know why, but don't know how to fix it

回答1:

Because after each fork() the child process continues the code from where the parent process is. Thus both parent and child processes continue running the for loop and not only parent process but also the child processes continue forking.

回答2:

You need to put a break; after each call to chunkXXX() but the last one (father process), and call exit() from the children chunks.

回答3:

wait(NULL) causes your main thread to wait until all children have exited. To break the infinite loop have chunk0, chunk1, and chunk2 call exit to terminate.

Furthermore, as others have pointed out, a break statement is needed after each call to chunk0-chunk2.