Q:

listing conditionally an array of strings

I'm trying to list all files and folders in a given directory in C, the following code errors out and i cant figure out whats wrong 
#include <sys/types.h>
#include <dirent.h>
#include <regex.h>
#include <stdio.h>

#include <unistd.h>
#include <pwd.h>

enum {
    WALK_OK = 0,
    WALK_BADPATTERN,
    WALK_BADOPEN,
};

int walk_directories(const char *dir, const char *pattern, char* strings[])
{
    struct dirent *entry;
    regex_t reg;
    DIR *d; 
    int i = 0;
    //char array[256][256];

    if (regcomp(&reg, pattern, REG_EXTENDED | REG_NOSUB))
    return WALK_BADPATTERN;
    if (!(d = opendir(dir)))
    return WALK_BADOPEN;
    while (entry = readdir(d))
    if (!regexec(&reg, entry->d_name, 0, NULL, 0) )
            //puts(entry->d_name);
        strings[i] = (entry->d_name);
        i++;
    closedir(d);
    regfree(&reg);

    return WALK_OK;
}

void main()
{
    struct passwd *pw = getpwuid(getuid());
    char *homedir = pw->pw_dir;
    strcat(homedir, "/.themes");

    int n = 0;
    char *array[256][100];
    char *array2[256][100];

    walk_directories(homedir, "", array);
        for (n = 0; n < 256; n++)
        {
            //do stuff here later, but just print it for now
            printf ("%s\n", array[n]);
        }

    walk_directories("/usr/share/themes", "", array2);
        for (n = 0; n < 256; n++)
        {
            //do stuff here later, but just print it for now
            printf ("%s\n", array2[n]);
        }
}

The error at compile time is 
test2.c: In function ‘main’:
test2.c:42:2: warning: incompatible implicit declaration of built-in function ‘strcat’ [enabled by default]
test2.c:48:2: warning: passing argument 3 of ‘walk_directories’ from incompatible pointer type [enabled by default]
test2.c:15:5: note: expected ‘char **’ but argument is of type ‘char * (*)[100]’
test2.c:52:6: warning: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘char **’ [-Wformat]
test2.c:55:2: warning: passing argument 3 of ‘walk_directories’ from incompatible pointer type [enabled by default]
test2.c:15:5: note: expected ‘char **’ but argument is of type ‘char * (*)[100]’
test2.c:59:6: warning: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘char **’ [-Wformat]

If it helps, I've implemented what I want already in python, this is the desired result for C
import os
DATA_DIR = "/usr/share"

def walk_directories(dirs, filter_func):
    valid = []
    try:
    for thdir in dirs:
        if os.path.isdir(thdir):
            for t in os.listdir(thdir):
                if filter_func(os.path.join(thdir, t)):
                     valid.append(t)
    except:
    logging.critical("Error parsing directories", exc_info=True)
    return valid

def _get_valid_themes():
    """ Only shows themes that have variations for gtk+-3 and gtk+-2 """
    dirs = ( os.path.join(DATA_DIR, "themes"),
         os.path.join(os.path.expanduser("~"), ".themes"))
    valid = walk_directories(dirs, lambda d:
            os.path.exists(os.path.join(d, "gtk-2.0")) and \
            os.path.exists(os.path.join(d, "gtk-3.0")))
    return valid

print(_get_valid_themes())

thank you
[EDIT]
thanks for the help, only problem im having now is the printf's all spit out rubbish instead of what i expected, ive tried a few things and the while loop looks like this now
    while (entry = readdir(d))
    if (!regexec(&reg, entry->d_name, 0, NULL, 0) )
            //printf("%s\n",entry->d_name);
        strcpy(strings[i], (entry->d_name));
        //strings[i] = (entry->d_name);
        printf("%i\n",i);
        i++;
    closedir(d);

the i doesnt get printed properly either, this is all i get from the 3 printf statements
0
Adwaita2

\@

0
Radiance

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\@

�K��
� `���
����
�

��
�
.N=

�O��
�

�

should mention that if i enable
       printf("%s\n",entry->d_name);

then it prints the expected output though

A:

You should include string.h to get the declaration of strcat(3).
In your declaration:
int walk_directories(const char *dir, const char *pattern, char* strings[])

The char  *strings[] is just syntactic sugar meaning char **strings.  Since you're passing a 2D array, that won't work.  It looks to me like you're intending to make two arrays of strings, but that's not what these declarations do:
char *array[256][100];
char *array2[256][100];

You probably don't want the *s there.  If you take them off, you can change the signature of walk_directories to this:
int walk_directories(const char *dir, const char *pattern, char strings[][100])

And it should work, with the necessary changes inside your function to match.  As a bonus, this change will make your printf calls start working, too.
It looks like you're missing some braces around your while loop body.

