文档不完整。在ffmpeg/libavformat/unix.c中，

static const AVOption unix_options[] = {
    { "listen",    "Open socket for listening",             OFFSET(listen),  AV_OPT_TYPE_BOOL,  { .i64 = 0 },                    0,       1, ED },
    { "timeout",   "Timeout in ms",                         OFFSET(timeout), AV_OPT_TYPE_INT,   { .i64 = -1 },                  -1, INT_MAX, ED },
    { "type",      "Socket type",                           OFFSET(type),    AV_OPT_TYPE_INT,   { .i64 = SOCK_STREAM },    INT_MIN, INT_MAX, ED, "type" },
    { "stream",    "Stream (reliable stream-oriented)",     0,               AV_OPT_TYPE_CONST, { .i64 = SOCK_STREAM },    INT_MIN, INT_MAX, ED, "type" },
    { "datagram",  "Datagram (unreliable packet-oriented)", 0,               AV_OPT_TYPE_CONST, { .i64 = SOCK_DGRAM },     INT_MIN, INT_MAX, ED, "type" },
    { "seqpacket", "Seqpacket (reliable packet-oriented",   0,               AV_OPT_TYPE_CONST, { .i64 = SOCK_SEQPACKET }, INT_MIN, INT_MAX, ED, "type" },
    { NULL }
};

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它处理stream=1、datagram=1、seqpacket=1或type=#，其中#是所需套接字类型的整数值（SOCK_STREAM=1、SOCK_DGRAM=2、SOCK_SEQPACKET=5）。