Could someone check my stat struct code? (C)

smoothdogg00 · 03-15-2006, 10:07 PM

Variables and function call:

Code:

char * filelist[n];

sortByTime(filelist);

My function:

Code:

int sortByTime(char * arr[])
{
  int i, j;
  char * temp;
  for(i = (sizeof(arr)/sizeof(char *)); i >= 1; i--)
    {
      for(j = 1; j < (i - 1); j++)
	{
	  stat(&arr[j], &arr[j + 1]);
	  if(&arr[j].st_mtime > &arr[j + 1].st_mtime)
	    {
	      strcpy(temp, arr[j]);
	      strcpy(arr[j], arr[j + 1]);
	      strcpy(arr[j + 1], temp);
	    }
	}
    }
}

The problem is with my stat structure and accessing the st_mtime. (But if you see any other problems feel free to let me know :-))

Wim Sturkenboom · 03-15-2006, 11:02 PM

Code:

gcc -c -Wall sometest.c
sometest.c: In function `sortByTime':
sometest.c:23: warning: passing arg 1 of `stat' from incompatible pointer type
sometest.c:23: warning: passing arg 2 of `stat' from incompatible pointer type
sometest.c:24: error: request for member `st_mtim' in something not a structure
or union
sometest.c:24: error: request for member `st_mtim' in something not a structure
or union

line23:
The first argument of stat must be a pointer to a string, the second one must be a pointer to a struct stat.
line24:
arr is not of type struct stat, and for that reason does not contain a member st_mtime

It seems that you don't have any idea what your doing. Benefit of the doubt with regards to 'homework' question.

Code:

int sortByTime(char * arr[])
{
  struct stat sb1,sb2;
  int i, j;
  char * temp;
  for(i = (sizeof(arr)/sizeof(char *)); i >= 1; i--)
    {
      for(j = 1; j < (i - 1); j++)
        {
          stat(arr[j], &sb1);
          stat(arr[j+1], &sb2);
          if(sb1.st_mtime > sb2.st_mtime)
            {
              strcpy(temp, arr[j]);
              strcpy(arr[j], arr[j + 1]);
              strcpy(arr[j + 1], temp);
            }
        }
    }
    return 0;
}

With the correct include files, this compiles without errors. I did not check if it works.

smoothdogg00 · 03-15-2006, 11:37 PM

Ok, that worked great, thanks a lot.

Now, for the next question (probably even easier):

Code:

(sizeof(arr)/sizeof(char *))

This is giving me trouble in the above code.

I can access items within arr with the following:

Code:

printf("%s", arr[10])

So I know there is more than one element in the array. However, first code sample results in 1, meaning that the size of the array is 4 and the size of char* is 4 (normal). Why would it say the array size is 4, when I can access things up to [50]?

And "yes", it is true that I have almost no idea what I am doing in C. I am being forced to learn it and program with it, but it is not being taught to me.

Wim Sturkenboom · 03-16-2006, 03:58 AM

Code:

int sortByTime(char * arr[])

arr[] is an array (of pointers to char) so arr is a pointer
For that reason it has a size of 4 (on 32bit systems), In your situation, you have to pass the number of elements as a second argument.

smoothdogg00 · 03-16-2006, 11:04 AM

I'm actually changing to a qsort now, and I'm having problems.

My variables:

Code:

char * filelist[n];
int files;

My call to qsort:

Code:

qsort((void*)filelist, files, sizeof(char *), compare());

My comparator function:

Code:

int compare(const void * s1, const void * s2)
{
  char * string1 = (char *)s1;
  char * string2 = (char *)s2;

  struct stat struct1, struct2;
  int t1, t2;

  stat(string1, &struct1);
  stat(string2, &struct2);
  t1 = struct1.st_mtime;
  t2 = struct2.st_mtime;

  if(t1 > t2)
    return -1;
  else if(t1 < t2)
    return 1;
  else
    return 0;
}

My Error:

Code:

Segmentation fault (core dumped)

Wim Sturkenboom · 03-17-2006, 02:04 AM

When I compiled, I got a warning on the line where you call qsort. It disappeared when changed it to

Code:

qsort((void*)filelist, files, sizeof(char *), compare);

The code that you showed did not cause segfaults on my system. It however does not sort correctly due to pointer problems. This might cause the segfault or it might not (in which case you have to look in code that you did not post).

The sort-problem is in your compare routine. Use of a debugger or printf statements will help.

debugging using printf
----------------------
First add a simple printf as the first line after the variable declaration in the compare function.

Code:

printf("entering compare\n");
return 0;

Using the return, we force to skip the rest of the compare function. If you don't get a segfault, you're problem is (very likely) in the compare function.
Next add the following code before the new return

Code:

printf("%s | %s\n", (char*)s1,(char*)s2);

This might cause a segfault or print garbage which is an indication that there's a pointer problem.
Next you can start printing pointers. First change the %s in the above printf by %p

Code:

printf("%p | %p\n", (char*)s1,(char*)s2);

Also add a loop before you call qsort and print out the content of filelist. Mine code looked like

Code:

for(i=0;i<no_of_words;i++)
{
  printf("%2d: %p %s\n",i+1,words[i],words[i]);
}

You will now see that the pointers don't have any relation while you expect to see the values printed in the loop to show in the compare as well (s1 and s2). There also should not be a segfault (to my knowkedge)

explanation
-----------
Your compare routine takes two pointers to a void; however, you're passing an array of pointers as the first argument of qsort and for that reason your arguments in the compare-function are pointers to a pointer (and not pointers to void).
Some small changes solve it

Code:

char ** string1 = (char **)s1;
char ** string2 = (char **)s2;

and

Code:

stat(*string1, &struct1);
stat(*string2, &struct2);

PS I did not use filelist and files, but words and no_of_words

smoothdogg00 · 03-17-2006, 12:23 PM

For some reason when I use:

Code:

qsort((void*)filelist, files, sizeof(char *), compare);

I get a compilation error:

Code:

error: 'compare' undeclared (first use in this function)

Any idea what the problem might be?

It is declared as:

Code:

int compare(const void * s1, const void * s2)

Wim Sturkenboom · 03-20-2006, 12:47 AM

Sorry, can't explain why.

graemef · 03-20-2006, 07:09 AM

The compare function is a function that you will need to declare, that will determine how you compare two values. If the first value is to be considerd greater that then second then it should return a positive number. So a simple implementation for integers might just be

Code:

int compare(const void * s1, const void * s2)
{
   return (int)(*s1) - (int)(*s2);
}