函数 av_strndup 

pub unsafe extern "C" fn av_strndup(
    s: *const c_char,
    len: usize,
) -> *mut c_char

Duplicate a substring of a string.

@param s String to be duplicated @param len Maximum length of the resulting string (not counting the terminating byte) @return Pointer to a newly-allocated string containing a substring of s or NULL if the string cannot be allocated