how to initiate a dynamic array in struct with shared memory C -language

Question

I would to ask you, how initiate array in struct for size from keyboard(argv) with shared memory, I can't use malloc, because it initiate's privat or something like that.

For example piece of code

Struct with array which I want initiate

struct data
{
  char txt[size from keyboard];
};

How to resize this array, remembering that we will use shared memory IPCV?

Thanks form your help :)

edit:

so if I had one more variable in struct, Can I do it like this?

struct data
{
   int counter; 
   char *text;
}*shared data;

int shmid

int main(int argc, char* argv[])
{
  int m = atoi(argv[1]) /* number of slots*/ 
  int n = atoi(argv[2]) /*size of txt */

  shmid = shmget(12345, m * n * sizeof(struct my_data), IPC_CREAT|)600|IPc_EXCL)); 

shared_data = (struct data*)shmat(shmid, NULL, 0);

/*So now Can I write to txt??? */


}

Remy Lebeau · Accepted Answer · 2020-12-02 00:41:24Z

1

Change txt from char[] to char*, and then use shm_open() and mmap() to allocate shared memory once you know the desired size.

typedef struct data
{
  char *txt;
} data;

data d;
int shm_fd = shm_open("name", O_RDWR | O_CREAT);
d.txt = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd, 0);
...
munmap(d.txt, size);
shm_unlink("name");

edited Dec 2, 2020 at 0:41

answered Dec 2, 2020 at 0:34

Remy Lebeau

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4 Comments

Roundstic Over a year ago

Is it possible to it in IPCV? I ask, because shm_open and mmap are POSIX

Remy Lebeau Over a year ago

@Roundstic I don't know what IPCV is

Roundstic Over a year ago

linux System V IPC

Remy Lebeau Over a year ago

According to this, System V has another API for shared memory, so just replace shm_open()/mmap()/munmap()/shm_unlink() in my example with shmget()/shmat()/shmdt/shmctl() instead.

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