Mmap() Segmentation Fault Error While Reading The Stored Valued

November 10, 2021

I’m writing a program in C and using mmap() for processes to return the calculation to their main process.

Here is the code:

int *results = mmap ( NULL, count*sizeof(int),
 PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, 0, 0 );

for (i = 0; i < count; i++)
   {
      pid_t pid = fork();
      if (pid == 0)
      {
         /// some calculations in the child.
         }
         
         // I save the result to ith element of mmap
         results[i] = calculation;
      }
      else{
         wait(NULL);
      }
      
   }
    
   // The following part arises an error called segmentation fault.
    for (i = 0; i < count; i++)
   {
      printf(" results[%d] = %d.\n", i, results[i]);
   }

When I try to access the results, I get an error. How can I fix that?

Any help is appreciated.

>Solution :

To cite man mmap:

MAP_ANONYMOUS

The mapping is not backed by any file; its contents are initialized to zero. The fd argument is ignored; however, some implementations require fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is specified, and portable applications should ensure this.