Question

Okay look first thanks for this place 🙂
I would like to create a piece of shared memory to store 30 structures of the same type, but I am getting errors on compile..

struct nota
{
    int n;
    char text[30];
    char titulo[100];
    char login[20];
}

main(){
     int shmID;
     struct nota *evernota;
     struct nota um;

     shmID = shmget(1009, sizeof(struct nota)*30 , 0666 | IPC_CREAT);

     evernota = shmat(shmID, NULL, 0);

     evernota[0] = &um;  //Declaring one note here to the first position of shm..
     evernota[20] = &um;  //Declaring the same note on the 20 position of shm..

     printf("o meu int é %d\n",evernota[0]->n);  //Here I would get the int n of each structure, that should be 0, because its not initialized..
     printf("o meu int é %d\n",evernota[20]->n);  //and there the same n, because I use the same structure on position 0 and 20..  
}

But I have compile errors, someone see where is the problem??
Thanks alot in advance!!!

Answer 1

well for starters you need the includes:

#include <stdio.h>
#include <string.h>
#include <sys/shm.h>

then you need the semicolon after struct definition:

struct nota {
    ...
};

Add the flag IPC_EXCL, makes shmget fail if the memory key is already in use (use IPC_PRIVATE instead of 1009 to guarantee the memory segment is new). Then a check on shmID != -1 would be nice, and to copy the references use memcopy:

memcpy(&evernota[0], &um, sizeof(struct nota));  
memcpy(&evernota[20], &um, sizeof(struct nota));

about the printfs, use the . instead of ->. They will print garbage data, they’re not automatically 0-initialized on allocation, you have to do it by yourself.