C Programming - Segmentation fault when Manipulating Array of Strings

Question

I am struggling with a segmentation fault that occurs when the dns variable is changed from size 10 to size 32. I believe the declaration of the dns ariable in main is creating an array of 10 strings, and then when increased to 32, an array of 32 strings.

So when executed with dns variable of size 10, I get output of 4 directory names starting with the letter D. This is the expected output.

If I increase dns variable to size 32, I still get the 4 directory names and then immediately afterwards, a Segmentation fault.

It has been a very long time since I have coded and I am sure I am violating memory somewhere.

Any help is greatly appreciated.

Thanks,

Dan.

void ld_dirs(char *strings[])
{
   int count;
   DIR *dp;
   struct dirent *ep;

   dp = opendir ("/tmp/sysinfo/inputs");
   if (dp != NULL)
   {
      count = 0;

      while (ep = readdir (dp))
      {
         if (ep->d_name[0] == 'D')
         {
            strings[count] = malloc (80 * sizeof(char));
            strings[count] = ep->d_name;
            count ++;
         }
      }

      closedir (dp);
   }
   else
      perror ("Could not open the directory");
}

int main ()
{
   char cwd[120];
   int count, count2;
   char *dns[10];
   char *path;
   if (getcwd(cwd, sizeof(cwd)) != NULL)
      printf ("\nCurrent working directory : %s\n\n\n", cwd);
   else
      perror ("getcwd() error");

   ld_dirs (dns);

   count = 0;

   puts ("List of valid inputs :\n");

   while (dns[count] != NULL)
   {
      puts (dns[count]);
      count ++;
   }     

   printf ("There are %d valid inputs\n", count);
   return 0;
}

Posting this question has been a nightmare, I kept getting issues with incorrectly formatted code. The output of execution with char *dns[10]; is : \nCurrent working directory : /data/test \n List of valid inputs : \n DTest1.vga \n DTest234.video \n DTest123.vga \n DTest800.video .... when the declaration is changed to char *dns[32]; the output is the same however after the last line, there is 'Segmentation fault'. ..... sorry after many edits I realize this comment is not much better as far as format or readability. Dan. — Dan G
– Dan G, Commented Mar 17, 2015 at 22:21
Did you read the stackoverflow.com online help regarding how to format? — lurker
– lurker, Commented Mar 17, 2015 at 22:32

R Sahu · Accepted Answer · 2015-03-17 22:37:59Z

1

Problems that I see:

Size of array.

You have declared:
```
char *dns[10];
```
You don't have any code in ld_dirs to make sure that you don't use strings beyond what it can legally hold.
You have not initialized the items in dirs to NULL yet you are counting on the value of the items to be NULL in the following while loop.
```
while (dns[count] != NULL)
{
   puts (dns[count]);
   count ++;
}     
```

You can fix these problems by:

Initialize the items in dns to be NULL.
```
char *dns[10] = {0};
```
Pass the size of the array to ld_dir and make sure that you don't use any more than you are legally supposed to.

Change the function interface to:
```
void ld_dirs(char *strings[], int size)
```
Change the call to:
```
ld_dirs (dns, sizeof(dns)/sizeof(*dns));
```

answered Mar 17, 2015 at 22:37

R Sahu

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Comments

lurker · Accepted Answer · 2015-03-17 22:37:52Z

1

One problem I see right away is you are continuing to reuse the return value of readdir for different string table entries and overwriting the address you just allocated to hold the name. But I don't think that readdir is ensured to return a new, allocated directory entry structure for each call. I suspect not, and it's only a matter of time before accessing one of the prior ones results in a segmentation fault.

You'll need to copy out the data from each directory entry read with readdir:

  while (ep = readdir (dp))
  {
     if (ep->d_name[0] == 'D')
     {
        strings[count] = malloc(80 * sizeof(char));
        //ERROR: strings[count] = ep->d_name;
        strcpy(strings[count], ep->d_name);  //CORRECTED
        count ++;
     }
  }

I did not check for additional errors beyond this point.

answered Mar 17, 2015 at 22:37

lurker

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2 Comments

Dan G Over a year ago

Thank you for the response! I implemented the change, however am still seeing the segmentation fault. The strcpy is a nicer implementation, so definitely keeping it. Thanks again!

lurker Over a year ago

@DanG thanks for the feedback. I didn't check the code beyond the point I mentioned above since the other answer seemed to cover the other issues.

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