Question

I dont understand why in the parent process my data is not set to what its set to in my child process. I create the shared_data struct variable before I fork my program so it should be shared memory, correct?

Here is the code:

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

#define MAX_SEQUENCE 20

typedef struct
{
  long fib_sequence[MAX_SEQUENCE];
  int sequence_size;
} shared_data;

void fibonacci(shared_data* sdata);

int main ( int argc, char *argv[])
{

   pid_t pid, pid1;
  shared_data sdata;

 /* check for parameter values to program */
  int i;
  for(i = 0; i < argc; i++)
  {
     if(strcmp(argv[i], "-ms") == 0)
     {
         int j = ++i;
         if(argv[j])
         {
            /* set passed in value to the sequence_size */
            int paramValue = atoi(argv[j]);
            if(paramValue <= MAX_SEQUENCE)
            {
                sdata.sequence_size = paramValue;
            }
            else
            {
                sdata.sequence_size = MAX_SEQUENCE;
            }
        }
        break;
    }
}

printf("sequence size: %i\n", sdata.sequence_size);

/* fork a child process */
pid = fork();

if (pid < 0)
{
    /* error occurred */
    fprintf(stderr, "Fork Failed");
    return 1;
}
else if (pid == 0)
{
    /* child process */
    pid1 = getpid();
    printf("current child process id = %d\n",pid);
    printf("child's parent process id = %d\n",pid1);
    fibonacci(&sdata);
    printf("child: [%d] Fibonacci Result: %i\n", getpid(), sdata.fib_sequence[sdata.sequence_size - 1]);
}
else
{
    /* parent process */
    pid1 = getpid();
    printf("current parent process id = %d\n",pid); /* C */
    printf("current parent process parent id = %d\n",pid1); /* D */
    wait(NULL);
    int i =0;
    for(i = 0; i < sdata.sequence_size; i++)
    {
        printf("parent: [%d] Fibonacci Result: %i\n", getpid(), sdata.fib_sequence[i]);
       }
     }
   }
void fibonacci(shared_data* sdata)
{
 int first = 0;
 int second = 1;
 int next;

int i;
for(i = 0; i < sdata->sequence_size; i++)
{
    if(i <= 1)
    {
        next = i;
    }
    else
    {
        next = first + second;
        sdata->fib_sequence[i] = next;

        first = second;
        second = next;
    }
     printf("child: [%d] %i\n", getpid(), next);
   }
 }

Answer 1

During fork, all your memory (even preallocated one) becomes unshared. On the implementation level it’s likely to become copy-on-write, but visible effect is the same.

You can use POSIX shm_open (and friends) to allocate shared memory.