binding an external C library with I/O methods

[Pierre Lindenbaum]

(cross posted on SO: http://stackoverflow.com/questions/24486926)
I'd like to use an external library that is able to write files using a 
custom format. The API for this library looks like:

    MyFormatPtr format= myformat_openfd(int filedescriptor,const char* 
mode);
    MyFormatPtr format= myformat_open(const char* filename,const char* 
mode);
    (...)
    void myformat_close(MyFormatPtr format);

while the Apache API writes data using methods like `ap_rwrite`

How can I bind my library with Apache ? Something like:

    (...)
    ap_set_content_type(r,"data");
    int fd= ap_r_get_file_descriptor(r);/* <- this is what I need */
    MyFormatPtr format = myformat_openfd( fd , "w");
    (...)
    myformat_close(format);
    return OK;
    (...)

Thanks.