Initializing an array after declaration

Question

Hello,

gcc 4.4.3 c89

I have the following code as a sample of what I am trying to do. I don't know the actual size of the array, until I enter the function. However, I don't think I can set the array size after I have declared it. I need it global as some other functions will need to access the device names.

Many thanks for any suggestions,

/* global */
char *devices_names[];

void fill_devices(size_t num_devices)
{
    devices_names[num_devices];

    /* start filling */
}

Answer 1

You'll need to allocate the memory dynamically using malloc:

char **device_names;

void fill_devices(size_t num_devices)
{
    device_names = malloc(num_devices * sizeof(char*));
}

And then use free(device_names); to free the memory when you no longer need it.

Answer 2

Make it dynamic:

char **g_device_names;
int g_num_devices;

void fill_devices(size_t num_devices) {
    g_device_names = malloc(sizeof(char*) * num_devices);
    g_num_devices = num_devices;
    ...
}

Answer 3

You should use a pointer, so when you enter the method the array is still not declared. You can use malloc to set the correct size. Look at this article: arrays and malloc

Answer 4

If you are using a global array then you need to know its size (or it's maximum size) at the time you declare it. E.g.

char *devices_names[MAX_DEVICES];

If you can't do this then you have no option but to use a pointer and dynamically allocated memory.

E.g.

char **devices_names = 0;

void fill_devices(size_t num_devices)
{
    devices_names = malloc( num_devices * sizeof *devices_names );

    /* ... */
}

Of course this has implications such as how do you prevent people accessing the array before it has been allocated and when do you free it?