c array of pointers

greg108 · 08-16-2005, 07:50 PM

I can assign a new character to my array of strings:

Code:

int main()
{
  char *days[] = {"monday", "tuesday", "wednesday", "thursday",
		  "friday", "saturday", "sunday"};

  days[0][0] = 'v'; /* here I  can aisgn */

  int size = sizeof(days) / sizeof(char *);
  int i;
  for (i = 0; i < size; i++)
    printf("%s\n", *(days + i));
  tableReverse(days, size);

but after I pass a pointer to tableReverse() function and I try to assign something I get a segmentation fault.

Code:

void tableReverse(char *table[], int size)
{
  table[0][0] = 'q'; /* here I get a segmentation fault */

Why is it like that?

sxzzsf · 08-16-2005, 09:43 PM

days[0][0] = 'v'; /* here I can aisgn */
<===
Can not assign too.
The address of days[0][0] is the same to which of table[0][0] in function tableReverse() and it is located in the read-only memory area. If write to this area, there will be Segmentation fault

greg108 · 08-16-2005, 11:22 PM

Quote:

Originally posted by sxzzsf
days[0][0] = 'v'; /* here I can aisgn */
<===
Can not assign too.
The address of days[0][0] is the same to which of table[0][0] in function tableReverse() and it is located in the read-only memory area. If write to this area, there will be Segmentation fault

I double checked and I'm sure that I can assign in the first case. No segmentation fault and characters get changed.

Dave Kelly · 08-16-2005, 11:32 PM

How big is char * days[]
How big is char * table[]

Check your allocations.

sxzzsf · 08-16-2005, 11:36 PM

void tableReverse(char *table[], int size)
{
printf("0x%8x\n",&table[0][0]);
}

int main()
{
int size,i;
char *days[] = {"monday","tuesday", "wednesday","thursday","friday", "saturday", "sunday"};

size = sizeof(days) / sizeof(char *);
for (i = 0; i < size; i++)
printf("%s\n", *(days + i));
tableReverse(days, size);
printf("0x%8x\n",&days[0][0]);
printf("0x%8x\n",&days[0]);
return 0;
}
=============
after compiling using gcc 3.3.1
monday
tuesday
wednesday
thursday
friday
saturday
sunday
0x 804852f <== days[0][0]
0x 804852f <== table[0][0]
0xbffff8f0 <== days

greg108 · 08-17-2005, 02:09 AM

OK, that proves that the pointer(address) is the same. But I still don't understand why I can assign to days[0][0] and not to table[0][0].

sxzzsf · 08-17-2005, 02:23 AM

But I can not

madhukirant · 08-17-2005, 04:36 AM

looks like you got it all wrong you cannot assign the value at either of the places...the segementation fault comes at the statement

days[0][0] = 'v';

when a array of pointers is initialised it becomes a constant pointer and you cannot change the value....

jtshaw · 08-17-2005, 07:37 AM

I also don't see how you are possibly not getting a segmentation fault on the first assignment. Just to be sure, I built it on my system and in fact do get a segmentation fault with just the following code:

Code:

int main()
{
    char *days[] = {"monday", "tuesday", "wednesday", "thursday",
        "friday", "saturday", "sunday"};

    days[0][0] = 'v'; /* here I  can aisgn */
    return 0;
}

The problem here is you are attempting to change the value of a const char array ("monday" for example, is a const char array). I'm guessing you'd have more luck with something like this:

Code:

int main()
{
    char *days[] = {"monday", "tuesday", "wednesday", "thursday",
        "friday", "saturday", "sunday"};


    int size = sizeof(days) / sizeof(char *);
    int i;
    for (i = 0; i < size; i++)
        printf("%s\n", *(days + i));
    days[0] = "blahday"; /* here I  can aisgn */
    for (i = 0; i < size; i++)
        printf("%s\n", *(days + i));
    return 0;
}

greg108 · 08-17-2005, 09:38 AM

Ok, so I can't assign it.
You, guys are correct.
I pasted the line 'days[0][0] = 'v'; /* here I can aisgn */' in a wrong spot in my post.
Actually I was assigning after calling reverseTable(), where memory is allocated for the reversed strings. I didn't know at first that the first declaration of 'days' makes it constant though.

So this is the program.

Code:

#include <stdlib.h>
#include <stdio.h>

void tableReverse(char *table[], int size);

int main()
{
  char *days[] = {"monday", "tuesday", "wednesday", "thursday",
		  "friday", "saturday", "sunday"};

  // days[0][0] = '!'; segmentation fault here

  int size = sizeof(days) / sizeof(char *);
  int i;
  for (i = 0; i < size; i++)
    printf("%s\n", *(days + i));

  tableReverse(days, size);

  days[0][0] = '&';  // can assign here
  days[1][0] = '*';
  days[2][1] = '$';

  for (i = 0; i < size; i++)
    printf("%s\n", *(days + i));

  return EXIT_SUCCESS;
}


void tableReverse(char *table[], int size)
{
  // table[0][0] = 'q'; segmentation fault here
  int j;
  for (j = 0; j < size; j++){
    int len = strlen(*(table + j));
    char *str = (char *)malloc(len);
    int i;
    for (i = 0; i < len; i++)
      str[i] = *(*(table + j) + (len - 1 - i));
    str[len] = '\0';
    *(table + j) = str;
  }
}

deiussum · 08-17-2005, 10:25 AM

Whenever you have a string literal (e.g. "monday", "tuesday", etc.) that string literal is stored in a READ-ONLY data section of your executable. You should NEVER, EVER attempt to write to this memory.

If instead, you declared your array of days as a multidimensional array instead of an array of pointers, then memory that can be written to will be declared for you on the stack...

e.g.

Code:

char days[7][10] = {"monday", "tuesday", "wednesday", "thursday",
    "friday", "saturday", "sunday"};