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Top Forums Programming readdir and dynamic array memory corruption Post 302491516 by torbium on Thursday 27th of January 2011 04:34:26 PM
Old 01-27-2011
readdir and dynamic array memory corruption

Hi everyone

I am developing an utility.
At some part of it I read directory entries to a dynamic array: struct list
It stores pointers to items: list.entries,
which are structures: struct entry

If a number of files in a directory is greater then number of elements an array was initially allocated,
I reallocate memory for this array.
if (c > list_size)

And at this point something strange happens.
Pointers are correct.
While I can successfully allocate memory for new items
list.entries[c] = malloc(sizeof(struct entry));
list.entries[0]->pde->d_name is corrupted at some iteration, but always if list.entries was reallocated.

See the test code provided.

If I do not realloc list.entries everything goes fine.
I played with list_size values.
On Mac OS X (10.4.0 Darwin Kernel Version 10.4.0) memory is corrupted while list.entries[124] is processed.
On Ubuntu Linux 2.6.24-23-xen #1 SMP Wed Apr 1 23:47:10 UTC 2009 x86_64 GNU/Linux while list.entries[196] is processed.
If I don't use readdir and explicitly allocate
list.entries[c]->pde = malloc(sizeof(struct dirent));
memory is not corrupted.

What is wrong?
Where is the problem?
What is the best way to read dir entries to a dynamic array?

Code:
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dirent.h>
#include <math.h>
#include <pwd.h>
#include <dirent.h>
#include <unistd.h>

int
main (void)
{
    int i, c, ac;
    unsigned int list_size = 10;
    char * fname;
    
    DIR *pdir;
    struct dirent *pde;
    
    
    struct entry {
        struct stat st;
        struct dirent *pde;
    };
    
    struct list {
        int count;
        struct entry **entries;
    } list;
    
    struct entry **ppent;
    pdir = NULL;
    pde = NULL;
    
    pdir = opendir("/usr/bin");
    
    if ((list.entries = malloc(list_size * sizeof(struct entry *))) == NULL)
        return 1;
    
    c = 0;
    ac = 0; /* allocation counter */
    while ((pde = readdir(pdir)) != NULL) {
        if (c > list_size - 1) {
            list_size <<= 1;
            if ((list.entries = realloc(list.entries,
                                    list_size * sizeof(struct entry*))) == NULL) {
                perror("unable to realloc");
                return 1;
            }
            ac++;
        }

        list.entries[c] = malloc(sizeof(struct entry));
        list.entries[c]->pde = pde;

        if (strcmp(list.entries[0]->pde->d_name, ".") != 0) {
            printf("memory corruption. size of array %d items\n", c);
            printf("number of reallocations %d\n", ac);
            return 1;
        }
        
        printf("base address: %p pointer address: %p entry pointer value: %p name: %s\n",
               list.entries, &list.entries[c], list.entries[c], list.entries[c]->pde->d_name);
        c++;
    }
    
    return 0;
}

 

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DIRECTORY(3)						     Library Functions Manual						      DIRECTORY(3)

NAME
opendir, readdir, telldir, seekdir, rewinddir, closedir - directory operations SYNOPSIS
#include <sys/types.h> #include <sys/dir.h> DIR *opendir(filename) char *filename; struct direct *readdir(dirp) DIR *dirp; long telldir(dirp) DIR *dirp; seekdir(dirp, loc) DIR *dirp; long loc; rewinddir(dirp) DIR *dirp; closedir(dirp) DIR *dirp; DESCRIPTION
Opendir opens the directory named by filename and associates a directory stream with it. Opendir returns a pointer to be used to identify the directory stream in subsequent operations. The pointer NULL is returned if filename cannot be accessed, or if it cannot malloc(3) enough memory to hold the whole thing. Readdir returns a pointer to the next directory entry. It returns NULL upon reaching the end of the directory or detecting an invalid seekdir operation. Telldir returns the current location associated with the named directory stream. Seekdir sets the position of the next readdir operation on the directory stream. The new position reverts to the one associated with the directory stream when the telldir operation was performed. Values returned by telldir are good only for the lifetime of the DIR pointer from which they are derived. If the directory is closed and then reopened, the telldir value may be invalidated due to undetected direc- tory compaction. It is safe to use a previous telldir value immediately after a call to opendir and before any calls to readdir. Rewinddir resets the position of the named directory stream to the beginning of the directory. Closedir closes the named directory stream and frees the structure associated with the DIR pointer. Sample code which searchs a directory for entry ``name'' is: len = strlen(name); dirp = opendir("."); for (dp = readdir(dirp); dp != NULL; dp = readdir(dirp)) if (dp->d_namlen == len && !strcmp(dp->d_name, name)) { closedir(dirp); return FOUND; } closedir(dirp); return NOT_FOUND; SEE ALSO
open(2), close(2), read(2), lseek(2), dir(5) 4.2 Berkeley Distribution September 24, 1985 DIRECTORY(3)
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