#include <mongoc/mongoc.h>
typedef struct _mongoc_gridfs_bucket_t mongoc_gridfs_bucket_t;

#include <mongoc/mongoc.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{


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


   mongoc_client_t *client;


   mongoc_database_t *db;


   mongoc_stream_t *file_stream;


   mongoc_gridfs_bucket_t *bucket;


   mongoc_cursor_t *cursor;


   bson_t filter;


   bool res;


   bson_value_t file_id;


   bson_error_t error;


   const bson_t *doc;


   char *str;


   mongoc_init();


   if (argc != 3) {


      fprintf(stderr, "usage: %s SOURCE_FILE_PATH FILE_COPY_PATH\n", argv[0]);


      return EXIT_FAILURE;


   }


   /* 1. Make a bucket. */


   client = mongoc_client_new(uri_string);


   db = mongoc_client_get_database(client, "test");


   bucket = mongoc_gridfs_bucket_new(db, NULL, NULL, &error);


   if (!bucket) {


      printf("Error creating gridfs bucket: %s\n", error.message);


      return EXIT_FAILURE;


   }


   /* 2. Insert a file.  */


   file_stream = mongoc_stream_file_new_for_path(argv[1], O_RDONLY, 0);


   res = mongoc_gridfs_bucket_upload_from_stream(bucket, "my-file", file_stream, NULL, &file_id, &error);


   if (!res) {


      printf("Error uploading file: %s\n", error.message);


      return EXIT_FAILURE;


   }


   mongoc_stream_close(file_stream);


   mongoc_stream_destroy(file_stream);


   /* 3. Download the file in GridFS to a local file. */


   file_stream = mongoc_stream_file_new_for_path(argv[2], O_CREAT | O_RDWR, 0);


   if (!file_stream) {


      perror("Error opening file stream");


      return EXIT_FAILURE;


   }


   res = mongoc_gridfs_bucket_download_to_stream(bucket, &file_id, file_stream, &error);


   if (!res) {


      printf("Error downloading file to stream: %s\n", error.message);


      return EXIT_FAILURE;


   }


   mongoc_stream_close(file_stream);


   mongoc_stream_destroy(file_stream);


   /* 4. List what files are available in GridFS. */


   bson_init(&filter);


   cursor = mongoc_gridfs_bucket_find(bucket, &filter, NULL);


   while (mongoc_cursor_next(cursor, &doc)) {


      str = bson_as_canonical_extended_json(doc, NULL);


      printf("%s\n", str);


      bson_free(str);


   }


   /* 5. Delete the file that we added. */


   res = mongoc_gridfs_bucket_delete_by_id(bucket, &file_id, &error);


   if (!res) {


      printf("Error deleting the file: %s\n", error.message);


      return EXIT_FAILURE;


   }


   /* 6. Cleanup. */


   mongoc_stream_close(file_stream);


   mongoc_stream_destroy(file_stream);


   mongoc_cursor_destroy(cursor);


   bson_destroy(&filter);


   mongoc_gridfs_bucket_destroy(bucket);


   mongoc_database_destroy(db);


   mongoc_client_destroy(client);


   mongoc_cleanup();


   return EXIT_SUCCESS;
}

The MongoDB GridFS specification <https://github.com/mongodb/specifications/blob/master/source/gridfs/gridfs-spec.md>.

The non spec-compliant mongoc_gridfs_t <>.