Question

I need to fill a buffer space with file descriptors of files from a defined source directory. So I have the startup code:

int main(int argc, char* argv[])
{
    DIR *src=opendir(argv[1]);

    struct dirent *DirEntry;
    char* buffer[200];
    do {
        DirEntry = readdir(src);
        if(DirEntry != NULL) {
            //put file into buffer
        }
    }while(DirEntry!=NULL);
}

How do I complete this loop to place all file descriptors of a given directory into the array called ‘buffer’? Should I use an object of DirEntry like DirEntry->d_name to return a file descriptor that I then put into the array?

Answer 1

If you need to move files from a source directory to a destination directory, you are going to need file names much more than you need file descriptors. With the names, you can open and close descriptors whenever needed; without the names, you can’t create the files in the target directory sensibly. However, we can handle file descriptors too.

So, assuming you have strdup(), you might use:

typedef struct File
{
    char *name;
    int   fd;
} File;

And in your loop:

if (DirEntry != NULL)
{
    buffer[i].name = strdup(DirEntry->d_name);
    if (buffer[i].name != 0)
        buffer[i].fd = open(buffer[i].name, O_RDONLY);
    i++;
}

where buffer is an array of File and i is a convenient integer:

enum { MAX_FILES = 4096 };
int i;
File buffer[MAX_FILES];

You should also add a condition to the main condition to ensure no overflow (or replace the fixed size buffer with a dynamically allocated one):

if (DirEntry != NULL && i < MAX_FILES)

You could sensibly break the loop if i reaches the limit. You could test whether the name represents a file (as opposed to FIFO, block device, character device, socket, symlink, directory, …); you’d probably use stat() or lstat() for that. The file descriptor would be negative (-1) if the open() call failed. You might conserve entries by not incrementing i if the memory allocation fails, but it is probably not worth worrying about. If the memory allocation for a file name fails, there isn’t going to be much else that works.