c - 构建具有可扩展的不同参数的字符串

Question

gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1
    c89

只是想知道是否有更好的方法使用我在下面提供的代码来执行此操作。

我正在根据一些参数构建一个 sdp( session 描述协议(protocol))字符串。

但是，我可能需要扩展 sdp 以包含其他参数，例如视频编解码器。

但是，我真的不想用另一个 if else 来构建我在下面完成的完整字符串。

我只是想知道这种可扩展性是否足够？有没有比我所做的更好的技术我可以使用。

我刚刚复制了构建 sdp 的函数:

void create_sdp_string(char *sdp_string, char reinvite)
{
    char session_id[MAX_STRING_LEN];
    char session_version[MAX_STRING_LEN];
    const char *local_ip_addr = "10.10.10.244";
    apr_time_t time_usec = 0;
    char session_identifier[MAX_STRING_LEN];
    char media_transport[MAX_STRING_LEN];
    char connection_info[MAX_STRING_LEN];
    const char *audio_port = "49152";

    /* Required sdp attributes */
#define V_PROTOCOL_VERSION "0"
#define USERNAME "JOEBLOGGS"
#define NETTYPE "IN"
#define ADDR_TYPE "IP4"
#define S_SESSION_NAME "SIP_CALL"
#define T_TIME_DESCRIPTION "0 0"
#define M_MEDIA_NAME_TRANSPORT_ADDR "RTP/AVP 0 8 101"
#define A_PCMU "rtpmap:0 PCMU/8000"
#define A_PCMA "rtpmap:8 PCMA/8000"
#define A_TELEPHONE "rtpmap:101 telephone-event/8000"

    /* Get the time in micro seconds to create an unique session id */
    time_usec = apr_time_usec(apr_time_now());
    apr_snprintf(session_id, MAX_STRING_LEN, "%lu", (unsigned long)time_usec);

    /* Get the time in micro seconds to create an unique session version */
    time_usec = apr_time_usec(apr_time_now());
    apr_snprintf(session_version, MAX_STRING_LEN, "%lu", (unsigned long)time_usec);

    /* Build session identifier */
    apr_snprintf(session_identifier,
                 MAX_STRING_LEN,
                 "o="USERNAME" "NETTYPE" %s %s "ADDR_TYPE" %s\n",
                 session_id,
                 session_version,
                 local_ip_addr);

    /* Build media transport */
    apr_snprintf(media_transport,
                 MAX_STRING_LEN,
                 "m=audio %s "M_MEDIA_NAME_TRANSPORT_ADDR"\n",
                 audio_port);

    /* Build connection information */
    apr_snprintf(connection_info,
                 MAX_STRING_LEN,
                 "c="NETTYPE" "ADDR_TYPE" %s\n",
                 local_ip_addr);

    if(reinvite == FALSE) {
        /* Build complete sdp string for cases that are not reinvite */
        apr_snprintf(sdp_string,
                     MAX_SDP_STRING_LEN,
                     "v="V_PROTOCOL_VERSION"\n"
                     "%s"
                     "s="S_SESSION_NAME"\n"
                     "%s"
                     "t="T_TIME_DESCRIPTION"\n"
                     "%s"
                     "a="A_PCMU"\n"
                     "a="A_PCMA"\n"
                     "a="A_TELEPHONE"\n",
                     session_identifier,
                     connection_info,
                     media_transport);
    }
    else if(reinvite == TRUE) {
        /* Build complete sdp string for cases that are for reinvite 'inactive' */
#define INACTIVE "inactive"
        apr_snprintf(sdp_string,
                     MAX_SDP_STRING_LEN,
                     "v="V_PROTOCOL_VERSION"\n"
                     "%s"
                     "s="S_SESSION_NAME"\n"
                     "%s"
                     "t="T_TIME_DESCRIPTION"\n"
                     "%s"
                     "a="A_PCMU"\n"
                     "a="A_PCMA"\n"
                     "a="A_TELEPHONE"\n"
                     "a="INACTIVE"\n",
                     session_identifier,
                     connection_info,
                     media_transport);
    }
}

Answer 1

如果你想要更通用的东西，请看这个:

void append_to_sdp_string(char *sdp_string, char *param)
{
  int size = strlen(sdp_string);
  apr_snprintf(sdp_string + size, MAX_SDP_STRING_LEN - size, "%s\n", param);
}

void create_sdp_string(char *sdp_string, char reinvite)
{
  /* Defines and variables here */

  apr_snprintf(session_id, MAX_STRING_LEN, "%lu", (unsigned long) apr_time_usec(apr_time_now());
  apr_snprintf(session_version, MAX_STRING_LEN, "%lu", (unsigned long) apr_time_usec(apr_time_now());
  apr_snprintf(session_identifier, MAX_STRING_LEN, "o="USERNAME" "NETTYPE" %s %s "ADDR_TYPE" %s", session_id, session_version, local_ip_addr);
  apr_snprintf(media_transport, MAX_STRING_LEN, "m=audio %s "M_MEDIA_NAME_TRANSPORT_ADDR"", audio_port);
  apr_snprintf(connection_info, MAX_STRING_LEN, "c="NETTYPE" "ADDR_TYPE" %s", local_ip_addr);

  append_to_sdp_string(sdp_string, "v="V_PROTOCOL_VERSION);
  append_to_sdp_string(sdp_string, session_identifier);
  append_to_sdp_string(sdp_string, "s="S_SESSION_NAME);
  append_to_sdp_string(sdp_string, connection_info);
  append_to_sdp_string(sdp_string, "t="T_TIME_DESCRIPTION);
  append_to_sdp_string(sdp_string, media_transport);
  append_to_sdp_string(sdp_string, "a="A_PCMU);
  append_to_sdp_string(sdp_string, "a="A_PCMA);
  append_to_sdp_string(sdp_string, "a="A_TELEPHONE);

  if(reinvite == TRUE)
    append_to_sdp_string(sdp_string, "a="INACTIVE);

  /* Here you can add as many simple conditional appends as you need */
}

这样可以很容易地向您已经创建的字符串附加任何内容。