How to use const char pointer returned from a function in this instance? (mongodb related)

I'm having a bit of trouble with a a function that the mongodb c driver uses. The function in question looks like this:

gridfile_get_field (gridfile *gfile, const char *name) (returns const char *)

I'm attempting to use it in the following manner:

const char * field = "file";
char * filename;
filename = (char *)gridfile_get_field(&gfile, field);
FILE * file;
file = fopen("test.txt", "a+");
fprintf(file, "file contains: %s\n", filename);
fclose(file);

However, after execution, I see this in test.txt:

file contains: ^A
file contains: ^A
file contains: ^A

I'm not sure what I'm doing wrong. The field that I specify exists in all of my files that I store in gridfs, so I don't think that's the case (specifying something that doesn't exist just files in "file contains: " with no character afterwards). I guess I'm doing something wrong in regards to the pointers. If anyone has any suggestions, that'd be awesome.

EDIT:: The real declaration is

const char * gridfile_get_field (gridfile *gfile, const char *name);

Solution

Turned out to be an issue with the MongoDB C driver- it wasn't returning the correct information. I think that issue will be addressed in an upcoming release.