C strcpy into an array of strings in shared memory

Question

I want (or need) to do something along the lines of

char **my_array = malloc(1000*64);
strcpy(arr[0], "test");

While I know that arr[0] isn't pointing to a separate piece of allocated memory, I thought one should be able to copy a string into it like this (yet it segs). This works

arr[0] = "test";

However that is not going to work, for my actual goal is to do this in shared memory.

shm_array = shmget(IPC_PRIVATE, 1000 * 64, IPC_CREAT | 0644);
my_array = (char**) shmat(shm_array, (void**)0, 0);

Actually my question could be rephrased to: "How do you create an array of strings in shared memory?". I tried creating 1000 separate "string" shared memory segments, but apart of that it did not work it also seems wrong. Moreover, I thought one should simply be able to write into a big shared memory segment using relative pointer offsets.

Kerrek SB · Accepted Answer · 2012-04-07 11:02:05Z

1

You could just create one single piece of memory and write to specific offsets:

char * const buf = malloc(HUGE);

strcpy(buf + offset1, "hello");
strcpy(buf + offset2, "world");

It'd probably be better to use strncpy and pass a size of HUGE - offset along to make sure you don't run over the end. Managing the offsets is your own responsibility. Or you can use strncat, if efficiency doesn't matter so much.

answered Apr 7, 2012 at 11:02

Kerrek SB

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1 Comment

MarkM Over a year ago

This was what I was actually trying to accomplish. Works with the shared memory segment as well.

Sergey Kalinichenko · Accepted Answer · 2012-04-07 11:08:11Z

1

It looks like you are looking for a 2D array 1000 by 64. If this is indeed the case, you can do this:

struct shared_1000_by_64 {
    char strings[1000][64];
};

struct shared_1000_by_64 *shared = malloc(sizeof(struct shared_1000_by_64));
for (int i = 0 ; i != 1000 ; i++) {
    strcpy(shared->strings[i], "test");
}

This uses the standard trick of preventing the array from decaying into a pointer.

answered Apr 7, 2012 at 11:08

Sergey Kalinichenko

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Comments

caf · Accepted Answer · 2012-04-07 11:47:32Z

0

If you want to dynamically allocate an array of 64-character arrays, then use a pointer to an array rather than a pointer to a pointer:

char (*my_array)[64] = malloc(1000 * sizeof my_array[0]);
strcpy(my_array[0], "test");

or for the shared-memory case,

shm_array = shmget(IPC_PRIVATE, 1000 * sizeof my_array[0], IPC_CREAT | 0644);
my_array = shmat(shm_array, NULL, 0);

answered Apr 7, 2012 at 11:47

caf

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