Putting A Struct Into Shared Memory

This is a discussion on Putting A Struct Into Shared Memory within the C Programming forums, part of the General Programming Boards category; I have an array of structs I'm trying to get into shared memory. This is what I have so far. ...

  1. #1
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    Putting A Struct Into Shared Memory

    I have an array of structs I'm trying to get into shared memory. This is what I have so far.

    Code:
    #include <sys/types.h>
    #include <sys/ipc.h>
    #include <sys/shm.h>
    #include <stdio.h>
    
    typedef struct
    {
      int num;
      int denom;
    } Fraction;
    
    Fraction FracArray[5];
    
    int main()
    {
      key_t key;
      int shmid;
    
      key = 5678;
      shmid = shmget(key, 1024, IPC_CREAT | 0666)
      FracArray[5] = shmat(shmid, NULL, 0);
    
      return 0;
    }
    When compiled I get
    incompatible types when assigning to type ‘Fraction’ from type ‘void *’

    So while trying to find a solution I saw someone mention to try and use pointers and stick that into shared memory so I came up with

    Code:
    #include <sys/types.h>
    #include <sys/ipc.h>
    #include <sys/shm.h>
    #include <stdio.h>
    
    typedef struct
    {
      int num;
      int denom;
    } Fraction;
    
    Fraction *FracArray[5];
    
    int main()
    {
      key_t key;
      int shmid;
    
      key = 5678;
      shmid = shmget(key, 1024, IPC_CREAT | 0666);
      FracArray[5] = shmat(shmid, NULL, 0);
    
      printf("test\n");
    
      FracArray[1]->num = 5;
    
      return 0;
    }
    But when I run that program test prints fine but then it seg faults. Can anyone explain which way I should be doing it and how to actually get the struct array into shared memory correctly?

  2. #2
    and the hat of wrongness Salem's Avatar
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    You should have something similar to how you would call malloc.
    Code:
      shmid = shmget(key, 5  * sizeof(Fraction), IPC_CREAT | 0666);
      Fraction *FracArray = shmat(shmid, NULL, 0);
    First you need a sizeof() expression of some sort to say exactly HOW much memory you want.
    Then you need a pointer assignment to be able to get at that memory.

    malloc does all this in one call, but shared memory takes two steps.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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