Memory 为什么共享内存的价值没有改变？

我有一个带有共享变量计数器的多处理程序。父级和子级都会更改计数器的值，因此计数器的最终值必须是20，但它总是键入零而不做任何更改。我曾尝试以多种方式定义计数器：作为一个没有共享内存的全局变量，我从父级和子级获得从1到10的结果，作为一个共享内存，我在许多地方定义了它，并且总是得到0。 有人能告诉我为什么会这样吗？为什么计数器的值不是从1到20？？我的代码有错误吗？如何将数据大小更改为1MB或2MB

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <errno.h>
#include <sys/time.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#define SIZE1 sizeof(int)
#define DATASIZE (10<<20)
int *counter;
int main()
 {
double A[200][200];
double B[200][200];
double C[200][200];
int i, j, k, m; 
int outfile, result, count; 
unsigned seed;

struct timeval before, after;

char *buff;
pid_t child_pid ,wpid;
int child_status,shmid1;
key_t key1=2222;

buff = (char *)malloc(sizeof(char)*(1<<20));

srandom(seed);

for (i=0; i<200; i++)
for (j=0; j<200; j++)
    {
    A[i][j] = random()/100.0;
    B[i][j] = random()/100.0;
}

 gettimeofday(&before, NULL);
 /*  counter = mmap(NULL, sizeof *counter, PROT_READ | PROT_WRITE,  MAP_SHARED       | MAP_ANONYMOUS, -1, 0);
   *counter=1;*/
 child_pid=fork();
 shmid1 = shmget(key1, SIZE1, 0666 | IPC_CREAT);
 counter = shmat(shmid1, NULL, 0);
 *counter=1;
 switch (child_pid)
     {
        case -1:
        {
               perror("\nfork failed\n");
               exit(1);
        }
        case 0:
            {
             /* shmid1 = shmget(key1, SIZE1, 0666 | IPC_CREAT);
               counter = shmat(shmid1, NULL, 0);*/

           printf("\nIam the child ,\n");

               for (m=0 ; m < 10; m++) 
                  {

                  outfile = open("testfile", O_RDWR|O_CREAT|O_APPEND, 0777);
                  if (outfile <= 0)
                  perror("\nError opening file\n");
                  else 
                         {
                       result = write(outfile, buff, DATASIZE);
                       if (result <= 0)
                       perror("\nError writing file\n");
                     }
                 close(outfile);
                     *counter++;
                     printf("\ncounter from child %d",*counter);

               }              // end of for
             printf("\nI'm the Son, and exiting with status %d \n",child_status ); 
             exit(0);
          }                   // end of case 0
       default:                 // parent process - returns child ID
           {
                 /*shmid1 = shmget(key1, SIZE1, 0666 | IPC_CREAT);
                 counter = shmat(shmid1, NULL, 0);*/
          printf("\nIam the parent, my child process %d is \n",child_pid);

          for (m=0 ; m < 10; m++) 
              {

                     for (i=0; i<200; i++)
                       for (j=0; j<200; j++) 
                     {
                       C[i][j] = 0;
                       for (k=0; k<200; k++)
                           C[i][j] += A[i][k]*B[k][j];
                         }
                         *counter++;
                         printf("\ncounter from parent  %d",*counter);
             }// end of for

                  printf("\nIam the parent,  My child process  ID = %d \n",child_pid);

              }//end of default                     
                  waitpid(child_pid,&child_status,0);
    }//end of switch 
  //shmdt(counter);
  free(buff);
  gettimeofday(&after, NULL);
  count = (after.tv_sec - before.tv_sec) * 1e6;
  count += (after.tv_usec - before.tv_usec);

  printf("Total time in usec: %d\n", count);
  printf("\n multiprocessing end successfully ... \n");
  return 0;
   };