#include <mongoc/mongoc.h>
typedef struct _mongoc_gridfs_t mongoc_gridfs_t;

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

#include <assert.h>
#include <mongoc/mongoc.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
int
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;


   const char *uri_string = "mongodb://127.0.0.1:27017/?appname=gridfs-example";


   mongoc_uri_t *uri;


   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 EXIT_FAILURE;


   }


   mongoc_init ();


   iov.iov_base = (void *) buf;


   iov.iov_len = sizeof buf;


   /* connect to localhost client */


   uri = mongoc_uri_new_with_error (uri_string, &error);


   if (!uri) {


      fprintf (stderr,


               "failed to parse URI: %s\n"


               "error message:       %s\n",


               uri_string,


               error.message);


      return EXIT_FAILURE;


   }


   client = mongoc_client_new_from_uri (uri);


   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 EXIT_FAILURE;


      }


      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) {


            break;


         }


         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 EXIT_FAILURE;


      }


      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 EXIT_FAILURE;


      }


      if (!mongoc_gridfs_file_save (file)) {


         mongoc_gridfs_file_error (file, &error);


         fprintf (stderr, "Could not save: %s\n", error.message);


         return EXIT_FAILURE;


      }


      mongoc_gridfs_file_destroy (file);


   } else {


      fprintf (stderr, "Unknown command");


      return EXIT_FAILURE;


   }


   mongoc_gridfs_destroy (gridfs);


   mongoc_uri_destroy (uri);


   mongoc_client_destroy (client);


   mongoc_cleanup ();


   return EXIT_SUCCESS;
}

The MongoDB GridFS specification.

The spec-compliant mongoc_gridfs_bucket_t.