How not to open a file twice in linux?

Question

I have a linked list with an fd and a string I used to open this file in each entry. I want to open and add files to this list only if this file is not already opened, because I

Answer 1

In situations like this, it's often useful to consider your data structures. Change to a data structure which does not allow duplicates, such as a hash table.

Maintain a set of which data you've seen before. I've used a hash table for this set. As per @RossRidge's answer, use the inode and device as the key. This allows duplicates to be discovered in O(1) time.

Here is an example implementation.

#include 
#include 
#include 
#include 
#include 
#include 
#include 

static int get_fd(GHashTable *fds, const char *filename, int mode) {
    int fd;
    struct stat stat;
    int keysize = 33;
    char key[keysize];  /* Two 64 bit numbers as hex and a separator */

    /* Resolve any symlinks */
    char *real_filename = realpath(filename, NULL);
    if( real_filename == NULL ) {
        printf("%s could not be resolved.\n", filename);
        return -1;
    }

    /* Open and stat */
    fd = open( real_filename, mode );
    if( fd < 0 ) {
        printf("Could not open %s: %s.\n", real_filename, strerror(errno));
        return -1;
    }
    if( fstat(fd, &stat) != 0 ) {
        printf("Could not stat %s: %s.\n", real_filename, strerror(errno));
        return -1;
    }

    /* Make a key for tracking which data we've processed.
       This uses both the inode and the device it's on.
       It could be done more efficiently as a bit field.
     */
    snprintf(key, keysize, "%lx|%lx", (long int)stat.st_ino, (long int)stat.st_dev);

    /*  See if we've already processed that */
    if( g_hash_table_contains(fds, key) ) {
        return 0;
    }
    else {
        /* Note that we've processed it */
        g_hash_table_add(fds, key);
        return fd;
    }
}


int main(int argc, char** argv) {
    int mode = O_RDONLY;
    int fd;
    GHashTable *fds = g_hash_table_new(&g_str_hash, &g_str_equal);

    for(int i = 1; i < argc; i++) {
        char *filename = argv[i];

        fd = get_fd(fds, filename, mode);
        if( fd == 0 ) {
            printf("%s has already been processed.\n", filename);
        }
        else if( fd < 0 ) {
            printf("%s could not be processed.\n", filename);
        }
        else {
            printf("%s: %d\n", filename, fd);
        }
    }
}

And here's a sample result.

$ touch one two three
$ ln one one_link
$ ln -s two two_sym
$ ./test one* two* three*
one: 3
one_link has already been processed.
two: 5
two_sym has already been processed.
three: 7