函数 av_bprint_get_buffer 

pub unsafe extern "C" fn av_bprint_get_buffer(
    buf: *mut AVBPrint,
    size: c_uint,
    mem: *mut *mut c_uchar,
    actual_size: *mut c_uint,
)

Allocate bytes in the buffer for external use.

@param[in] buf buffer structure @param[in] size required size @param[out] mem pointer to the memory area @param[out] actual_size size of the memory area after allocation; can be larger or smaller than size