Thread: Seg fault when accessing double pointer

Hello There,

I wrote a function that reads out all the files in a certain directory and it's writing them into an array of character arrays that was passed. The function looks like this:

Code:

int getdir(char *path, char **filelist)
{
  int items=0;
  char **temp=NULL;
  char filename[1024]={0};
  struct dirent *dp;
  DIR *dir = opendir(path);
  if (dir==NULL){
    syslog(LOG_ALERT, "Error opening log_record directory %s", path);
    printf("Error opening log_record directory %s", path);
    return -1;
  }
  while((dp=readdir(dir))!=NULL){
    if(strstr(dp->d_name, "log_record")!=NULL){
      sprintf(filename,"%s%s",path,dp->d_name);      
      syslog(LOG_ALERT, "%s\n",filename);
      temp = realloc(filelist, (items+1) * sizeof(*temp));
      if (temp == NULL) {
        while (--items >= 0)
          free (filelist[items]);
        free (filelist);
        syslog(LOG_ALERT, "Error reallocating memory for filelist\n");
	printf("Error reallocating memory for filelist\n");
        return -1;
      }
      filelist = temp;
      filelist[items] = calloc(strlen(filename)+1,sizeof(char));
      if (filelist[items] == NULL) {
        syslog(LOG_ALERT, "Error reallocating memory for filename\n");
	printf("Error reallocating memory for filename\n");
        return -1;
      }
      strcpy (filelist[items], filename);
      ++items;
    }    
  }
  printf("filelist[%d]: %s\n",items-1,filelist[items-1]);
  return items;
}

now thaty printf on the bottom there works just fine. I do call the function like this:

Code:

      files=getdir("/usr/share/NovaxTSP/", list);
      printf("files %d\n",files);
      printf("list[0] %s\n ",list[0]);

where list is of type char**. Now my printf here (to print list[0]) seg faults out, why? I don't understand...
Any clues?
elp is appreciated!
Thank you!
Ron

Have you malloc'd storage for filelist before doing so for the array of char pointers it points to?

Originally Posted by itCbitC

Have you malloc'd storage for filelist before doing so for the array of char pointers it points to?

Well, I have declared char **list=NULL; in my caller function and i do a

Code:

temp = realloc(filelist, (items+1) * sizeof(*temp));
      if (temp == NULL) {
        while (--items >= 0)
          free (filelist[items]);
        free (filelist);
        syslog(LOG_ALERT, "Error reallocating memory for filelist\n");
	printf("Error reallocating memory for filelist\n");
        return -1;
      }
      filelist = temp;

in my getdir() function which should take care of that, wouldn't it? I'm passing the reference of list to getdir() where it's called filelist... Should work after me, no?

Originally Posted by bithub

Code:

temp = realloc(filelist, (items+1) * sizeof(*temp));

How many bytes of memory do you think this is allocating? I bet if you printed out the value of items and sizeof(*temp), you would be surprised.

I made a printf("DEBUG: realloc:%dBytes\n",(items+1) * sizeof(*temp)); right after the realloc and it prints: DEBUG: realloc:4Bytes which sounds pretty good to me, no?

Originally Posted by bithub

Nope. The memory you allocate in the function is lost since you don't return it to the caller.

Huh but I pass the function a reference from the caller and then allocate memory at this address - how does this get lost again and also how can i return it to the caller?
I didn't know of that, doesn't make sense to me....

Pass the address of list to getdir(), tho' now you have to deal with three levels of indirection.

Originally Posted by itCbitC

Pass the address of list to getdir(), tho' now you have to deal with three levels of indirection.

Yes, this actually worked out nicely, thank you!

Originally Posted by dwks

For what it's worth, calloc() zeros out the memory it allocates, but you go and overwrite the data with a strcpy() immediately afterwards -- so I'd be inclined to use malloc() instead of calloc() to avoid this.

What's the advantage of using malloc instead of calloc then?

Originally Posted by dwks

I guess I wasn't clear: if you use calloc(), you zero the memory and then overwrite it. If you use malloc(), you never spend time zeroing memory which is just going to be overwritten anyway.

Of course it doesn't make much difference at all really.

Nevertheless I changed it to

Code:

(*filelist[items]) = malloc(strlen(filename)+1);

Originally Posted by cerr

Nevertheless I changed it to

Code:

(*filelist[items]) = malloc(strlen(filename)+1);

Actually, I have another problem there, let's say that there's more than 1 file in this directory.
Code snippet:

Code:

(*filelist) = temp;
      printf("DEBUG: filename:%s, items:%d\n",filename,items);
      (*filelist[items]) = malloc(strlen(filename)+1);
      printf("DEBUG: After malloc %p\n",(*filelist[items]));
      if ((*filelist[items]) == NULL) {
        syslog(LOG_NOTICE, "nlog: Error reallocating memory for filename\n");
	printf("Error reallocating memory for filename\n");
        return -1;
      }

returns me following:
DEBUG: filename:/usr/share/NovaxTSP/log_record20091229, items:0
DEBUG: After malloc 0x8f0b3a0
DEBUG: filename:/usr/share/NovaxTSP/log_record20091230, items:1
Segmentation fault

Why can't i malloc space for the 2nd element in where items = 1. above I did reallocate filellist so that there should be enough room for it:

Code:

  while((dp=readdir(dir))!=NULL){
    if(strstr(dp->d_name, "log_record")!=NULL){
      sprintf(filename,"%s%s",path,dp->d_name);      
      syslog(LOG_NOTICE, "nlog: %s\n",filename);
      temp = realloc((*filelist), (items+1) * sizeof(*temp));
      if (temp == NULL) {
        while (--items >= 0)
          free ((*filelist[items]));
        free ((*filelist));
        syslog(LOG_NOTICE, "nlog: Error reallocating memory for filelist\n");
	printf("Error reallocating memory for filelist\n");
        return -1;
      }
      (*filelist) = temp;
      printf("DEBUG: filename:%s, items:%d\n",filename,items);
      (*filelist[items]) = malloc(strlen(filename)+1);
      printf("DEBUG: After malloc %p\n",(*filelist[items]));
      if ((*filelist[items]) == NULL) {
        syslog(LOG_NOTICE, "nlog: Error reallocating memory for filename\n");
	printf("Error reallocating memory for filename\n");
        return -1;
      }
      strcpy ((*filelist[items]), filename);
      ++items;
    }    
  }

Originally Posted by dwks

Code:

(*filelist[items]) = malloc(strlen(filename)+1);

Instead of that you probably want

Code:

(*filelist)[items] = malloc(strlen(filename)+1);

assuming that filelist is a char*** as bithub and itCbitC suggested.

The thing is that (*filelist[items]) says "get index items of filelist, then dereference", whereas you probably want "dereference filelist, then get index items".

Exaactly, I didn't take into account that [] is basically already dereferencing the actual value and hence i "only" need to dereference the list. Is that correct...Oh man, that's confusing
Thanks a lot for everyone's help!!!!