函数 av_asprintf 

pub unsafe extern "C" fn av_asprintf(
    fmt: *const c_char,
    ...
) -> *mut c_char

Print arguments following specified format into a large enough auto allocated buffer. It is similar to GNU asprintf(). @param fmt printf-compatible format string, specifying how the following parameters are used. @return the allocated string @note You have to free the string yourself with av_free().