Making array name runtime and accessing its elements

Question

static const unsigned char pkt1[89] = {
0x00, 0x00, 0x5e, 0x00, 0x01, 0x12, 0x00, 0x18, /* ..^..... */
0x82, 0x5e, 0x5a, 0xf6, 0x08, 0x00, 0x45, 0xb8, /* .^Z...E. */
0x00, 0x4b, 0x9d, 0x0d, 0x00, 0x00, 0x3f, 0x11, /* .K....?. */                                          
};

/* Frame (89 bytes) */
static const unsigned char pkt2[89] = {
0x00, 0x00, 0x5e, 0x00, 0x01, 0x12, 0x00, 0x18, /* ..^..... */
0x82, 0x5e, 0x5a, 0xf6, 0x08, 0x00, 0x45, 0xb8, /* .^Z...E. */
0x00, 0x4b, 0x9d, 0x25, 0x00, 0x00, 0x3f, 0x11, /* .K.%..?. */
}

....
...

till pkt[100]; 


int main()
{

char buff[10]="pkt";
for(i=0;i<100;i++)
{
itoa(1,buff+3,10);
printf("%s",buff);
}

}

From the above code I am able to see the concatenated name as pkt1 / pkt2... till pkt100. But I dont know how can I access the elements of pkt1 now. Any suggestions how to do it are welcome. Can I do this runtime ?

unwind · Accepted Answer · 2014-01-21 12:21:45Z

1

Not without implementing it yourself, no. C does not retain variable names at run-time, so if you want to use the name to map to an array you're going to have to implement it yourself:

const unsigned char * get_pkt(const char *name)
{
  if(strcmp(name, "pkt1") == 0)
    return pkt1;
  if(strcmp(name, "pkt2") == 0)
    return pkt2;
  return NULL;
}

UPDATE: If the number of things you need to match is large, code like the above becomes both tedious (and error-prone!) to write, and can affect performance. In those cases, the proper solution is to put the objects into some key-value data structure, such as a hash table. Note that C does not come with a built-in hash table data type, so you're on your own. There are many implementations available, though: glib's GHashTable is one.

If you want to stay within the C standard library, I would go with putting the data in an array, sort it (using qsort()) and then binary-search it using bsearch(). For ~1000 keys, that should do no more than ~10 string comparisons.

Even with these tricks, you won't get away from a need to initialize the data structure, of course. With the arrays, you might be able to do the array initialization in the same motion as you define your packets:

static struct {
  const char *name;
  const unsigned char packet[89];
} packets[] = {
  { "pkt1", { 0x00, 0x00, 0x5e, 0x00, 0x01,  /* ... more */ } },
};

This merely re-organizes the source text you already have, and adds some punctuation. If you take care to give these in the proper order, you don't need to sort at run-time, either!

edited Jan 21, 2014 at 12:21

answered Jan 21, 2014 at 11:50

unwind

402k64 gold badges492 silver badges620 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Manish Kumar Over a year ago

if i have 1000 packets.. then I have to write 1000 if else statements.. which I dont think is the right way for programming

unwind Over a year ago

@Learner True, I failed to notice that part of your question. I updated my answer.

Manish Kumar Over a year ago

Can u give me an example here ? @unwind: so in the hash table I need to write thousand lines to map it one on one. Is it ?

Collectives™ on Stack Overflow

Making array name runtime and accessing its elements

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related