#include <mongoc.h>

typedef struct _mongoc_gridfs_t mongoc_gridfs_t;


mongoc_gridfs_t provides a MongoDB gridfs implementation. The system as a whole is made up of gridfs objects, which contain gridfs_files and gridfs_file_lists. Essentially, a basic file system API.

There are extensive caveats about the kind of use cases gridfs is practical for. In particular, any writing after initial file creation is likely to both break any concurrent readers and be quite expensive. That said, this implementation does allow for arbitrary writes to existing gridfs object, just use them with caution.

mongoc_gridfs also integrates tightly with the mongoc_stream_t abstraction, which provides some convenient wrapping for file creation and reading/writing. It can be used without, but its worth looking to see if your problem can fit that model.


mongoc_gridfs_t does not support read preferences. In a replica set, GridFS queries are always routed to the primary.

Thread Safety

mongoc_gridfs_t is NOT thread-safe and should only be used in the same thread as the owning mongoc_client_t.


It is an error to free a mongoc_gridfs_t before freeing all related instances of mongoc_gridfs_file_t and mongoc_gridfs_file_list_t.


#include <assert.h>
#include <mongoc.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

main (int argc, char *argv[])
   mongoc_gridfs_t *gridfs;
   mongoc_gridfs_file_t *file;
   mongoc_gridfs_file_list_t *list;
   mongoc_gridfs_file_opt_t opt = {0};
   mongoc_client_t *client;
   mongoc_stream_t *stream;
   bson_t filter;
   bson_t opts;
   bson_t child;
   bson_error_t error;
   ssize_t r;
   char buf[4096];
   mongoc_iovec_t iov;
   const char *filename;
   const char *command;
   bson_value_t id;

   if (argc < 2) {
      fprintf (stderr, "usage - %s command ...\n", argv[0]);
      return 1;

   mongoc_init ();

   iov.iov_base = (void *) buf;
   iov.iov_len = sizeof buf;

   /* connect to localhost client */
   client =
      mongoc_client_new ("mongodb://");
   assert (client);
   mongoc_client_set_error_api (client, 2);

   /* grab a gridfs handle in test prefixed by fs */
   gridfs = mongoc_client_get_gridfs (client, "test", "fs", &error);
   assert (gridfs);

   command = argv[1];
   filename = argv[2];

   if (strcmp (command, "read") == 0) {
      if (argc != 3) {
         fprintf (stderr, "usage - %s read filename\n", argv[0]);
         return 1;
      file = mongoc_gridfs_find_one_by_filename (gridfs, filename, &error);
      assert (file);

      stream = mongoc_stream_gridfs_new (file);
      assert (stream);

      for (;;) {
         r = mongoc_stream_readv (stream, &iov, 1, -1, 0);

         assert (r >= 0);

         if (r == 0) {

         if (fwrite (iov.iov_base, 1, r, stdout) != r) {
            MONGOC_ERROR ("Failed to write to stdout. Exiting.\n");
            exit (1);

      mongoc_stream_destroy (stream);
      mongoc_gridfs_file_destroy (file);
   } else if (strcmp (command, "list") == 0) {
      bson_init (&filter);

      bson_init (&opts);
      bson_append_document_begin (&opts, "sort", -1, &child);
      BSON_APPEND_INT32 (&child, "filename", 1);
      bson_append_document_end (&opts, &child);

      list = mongoc_gridfs_find_with_opts (gridfs, &filter, &opts);

      bson_destroy (&filter);
      bson_destroy (&opts);

      while ((file = mongoc_gridfs_file_list_next (list))) {
         const char *name = mongoc_gridfs_file_get_filename (file);
         printf ("%s\n", name ? name : "?");

         mongoc_gridfs_file_destroy (file);

      mongoc_gridfs_file_list_destroy (list);
   } else if (strcmp (command, "write") == 0) {
      if (argc != 4) {
         fprintf (stderr, "usage - %s write filename input_file\n", argv[0]);
         return 1;

      stream = mongoc_stream_file_new_for_path (argv[3], O_RDONLY, 0);
      assert (stream);

      opt.filename = filename;

      /* the driver generates a file_id for you */
      file = mongoc_gridfs_create_file_from_stream (gridfs, stream, &opt);
      assert (file);

      id.value_type = BSON_TYPE_INT32;
      id.value.v_int32 = 1;

      /* optional: the following method specifies a file_id of any
         BSON type */
      if (!mongoc_gridfs_file_set_id (file, &id, &error)) {
         fprintf (stderr, "%s\n", error.message);
         return 1;

      mongoc_gridfs_file_save (file);
      mongoc_gridfs_file_destroy (file);
   } else {
      fprintf (stderr, "Unknown command");
      return 1;

   mongoc_gridfs_destroy (gridfs);
   mongoc_client_destroy (client);

   mongoc_cleanup ();

   return 0;