Pointers inside shared memory segment

Question

I've been trying this for hours, and google all the things I kind think of, but I'm going crazy. I have a struct: ``` typedef struct { int rows; int collumns; int* mat; char* IDs_row; } mem; ``` I don't know the sizes of the int* (a Matrix) and char* untill later. When I do, I create the shared memory like this: ``` mem *ctrl; int size = (2 + ((i-1)*num_cons))*sizeof(int) + i*26*sizeof(char); //I have the real size now shmemid = shmget(KEY, size, IPC_CREAT | 0666); if (shmemid < 0) { perror("Ha fallado la creacion de la memoria compartida."); exit(1); } ctrl = (mem *)shmat(shmemid, 0, 0); if (ctrl <= (mem *)(0)) { perror("Ha fallado el acceso a memoria compartida"); exit(2); } ``` No problem here. Then I give a value to ctrl->rows and collumns, and assign 0 to all the matrix. But after that, I write something in the char* and bam, segmentation fault. Debugging the program I saw that both pointers, mat and IDs_row where null. How do I give them the correct values inside the shared memo…

Accepted Answer

```
ctrl = (mem *)shmat(shmemid, 0, 0); 
```

This only assigns valid memory to the `ctrl` pointer, not to `ctrl->mat` or `ctrl->IDs_row`.

You probably want:

```
mem *ctrl;
shmemid = shmget(KEY, sizeof(ctrl), IPC_CREAT | 0666);
//allocate memory for the structure
ctrl = (mem *)shmat(shmemid, 0, 0);

//allocate memory for the int*
shmemid = shmget(KEY,((i-1)*num_cons))*sizeof(int), IPC_CREAT | 0666);
ctrl->mat = (int*)shmat(shmemid, 0, 0);

//allocate memory for the char*
shmemid = shmget(KEY,i*26*sizeof(char), IPC_CREAT | 0666);
ctrl->IDs_row = (char*)shmat(shmemid,0,0);
```

Pointers inside shared memory segment

Solution

Problem