RTSP協(xié)議

RTSP簡(jiǎn)介

RTSP(Real Time Streaming Protocol)是由Real Network和Netscape共同提出的如何有效地在IP網(wǎng)絡(luò)上傳輸流媒體數(shù)據(jù)的應(yīng)用層協(xié)議。RTSP對(duì)流媒體提供了諸如暫停,快進(jìn)等控制,而它本身并不傳輸數(shù)據(jù),RTSP的作用相當(dāng)于流媒體服務(wù)器的遠(yuǎn)程控制。服務(wù)器端可以自行選擇使用TCP或UDP來(lái)傳送串流內(nèi)容,它的語(yǔ)法和運(yùn)作跟HTTP 1.1類(lèi)似,但并不特別強(qiáng)調(diào)時(shí)間同步,所以比較能容忍網(wǎng)絡(luò)延遲。

RTSP和HTTP RTP(RTCP)的關(guān)系

RTSP和HTTP

  • 聯(lián)系:兩者都用純文本來(lái)發(fā)送消息,且rtsp協(xié)議的語(yǔ)法也和HTTP類(lèi)似。Rtsp一開(kāi)始這樣設(shè)計(jì),也是為了能夠兼容使用以前寫(xiě)的HTTP協(xié)議分析代碼 。
  • 區(qū)別:rtsp是有狀態(tài)的,不同的是RTSP的命令需要知道現(xiàn)在正處于一個(gè)什么狀態(tài),也就是說(shuō)rtsp的命令總是按照順序來(lái)發(fā)送,某個(gè)命令總在另外一個(gè)命令之前要發(fā)送。Rtsp不管處于什么狀態(tài)都不會(huì)斷掉連接。而http則不保存狀態(tài),協(xié)議在發(fā)送一個(gè)命令以后,連接就會(huì)斷開(kāi),且命令之間是沒(méi)有依賴(lài)性。rtsp協(xié)議使用554端口,http使用80端口。

RTSP和RTP(RTCP)

  • RTP:Realtime Transport Potocol 實(shí)時(shí)傳輸協(xié)議
    RTP提供時(shí)間標(biāo)志,序列號(hào)以及其他能夠保證在實(shí)時(shí)數(shù)據(jù)傳輸時(shí)處理時(shí)間的方法。
  • RTCP:Realtime Transport Control Potocol 實(shí)時(shí)傳輸控制協(xié)議
    RTCP是RTP的控制部分,用來(lái)保證服務(wù)質(zhì)量和成員管理。RTP和RTCP是一起使用的。
  • RTSP:RealTime Streaming Potocol 實(shí)時(shí)流協(xié)議
    RTSP具體數(shù)據(jù)傳輸交給RTP,提供對(duì)流的遠(yuǎn)程控制

RTP是基于 UDP協(xié)議的, UDP不用建立連接,效率更高;但允許丟包, 這就要求在重新組裝媒體的時(shí)候多做些工作
RTP只是包裹內(nèi)容信息,而RTCP是交換控制信息的,Qos是通過(guò)RTCP實(shí)現(xiàn)的
應(yīng)用程序?qū)?yīng)的是play, seek, pause, stop等命令,RTSP則是處理這些命令,在UDP傳輸時(shí)并使用RTP(RTCP)來(lái)完成。如果是TCP連接則不會(huì)使用RTP(RTCP)。


RTSP structure

RTSP的client連接server通過(guò)SDP(會(huì)話描述協(xié)議)傳遞信息,詳細(xì)請(qǐng)見(jiàn):RTSP消息

RTSP消息

RTSP的消息有兩大類(lèi),一是請(qǐng)求消息(request),一是回應(yīng)消息(response),兩種消息的格式不同。
請(qǐng)求消息格式

方法 URI RTSP版本 CR LF
消息頭 CR LF CR LF
消息體 CR LF

方法包括:OPTIONS、SETUP、PLAY、TEARDOWN DESCRIBE
URI是接收方(服務(wù)端)的地址,例如:rtsp://192.168.22.136:5000/v0
每行后面的CR LF表示回車(chē)換行,需要接收端有相應(yīng)的解析,消息頭需要有兩個(gè)CR LF。

DESCRIBE rtsp://192.168.1.211 RTSP/1.0
CSeq: 1
Accept: application/sdp
User-Agent: magnus-fc

回應(yīng)消息格式

RTSP版本 狀態(tài)碼 解釋 CR LF
消息頭 CR LF CR LF
消息體 CR LF

其中RTSP版本一般都是RTSP/1.0,狀態(tài)碼是一個(gè)數(shù)值,200表示成功,解釋是與狀態(tài)碼對(duì)應(yīng)的文本解釋?zhuān)敿?xì)請(qǐng)見(jiàn):SDP協(xié)議介紹。

RTSP/1.0 200 OK
CSeq: 1
Server: GrandStream Rtsp Server V100R001
Content-Type: application/sdp
Content-length: 256
Content-Base: rtsp://192.168.1.211/0

v=0
o=StreamingServer 3331435948 1116907222000 IN IP4 192.168.1.211
s=h264.mp4
c=IN IP4 0.0.0.0
t=0 0
a=control:*
m=video 0 RTP/AVP 96
a=control:trackID=0
a=rtpmap:96 H264/90000
m=audio 0 RTP/AVP 97
a=control:trackID=1
a=rtpmap:97 G726-16/8000

簡(jiǎn)單的rtsp交互過(guò)程:

C表示rtsp客戶端, S表示rtsp服務(wù)端

step1:
C->S:OPTION request //詢問(wèn)S有哪些方法可用
S->C:OPTION response //S回應(yīng)信息中包括提供的所有可用方法

step2:
C->S:DESCRIBE request //要求得到S提供的媒體初始化描述信息
S->C:DESCRIBE response //S回應(yīng)媒體初始化描述信息,主要是sdp

step3:
C->S:SETUP request //設(shè)置會(huì)話的屬性,以及傳輸模式,提醒S建立會(huì)話
S->C:SETUP response //S建立會(huì)話,返回會(huì)話標(biāo)識(shí)符,以及會(huì)話相關(guān)信息

step4:
C->S:PLAY request //C請(qǐng)求播放
S->C:PLAY response //S回應(yīng)該請(qǐng)求的信息

S->C:發(fā)送流媒體數(shù)據(jù)

step5:
C->S:TEARDOWN request //C請(qǐng)求關(guān)閉會(huì)話
S->C:TEARDOWN response //S回應(yīng)該請(qǐng)求

RTSP中常用方法

OPTION

得到服務(wù)器提供的可用方法

OPTIONS rtsp://192.168.20.136:5000/xxx666 RTSP/1.0
CSeq: 1 //每個(gè)消息都有序號(hào)來(lái)標(biāo)記,第一個(gè)包通常是option請(qǐng)求消息
User-Agent: VLC media player (LIVE555 Streaming Media v2005.11.10)

服務(wù)器的回應(yīng)信息包括提供的一些方法,例如:

RTSP/1.0 200 OK 
Server: UServer 0.9.7_rc1
Cseq: 1 //每個(gè)回應(yīng)消息的cseq數(shù)值和請(qǐng)求消息的cseq相對(duì)應(yīng)
Public: OPTIONS, DESCRIBE, SETUP, TEARDOWN, PLAY, PAUSE, SCALE,GET_PARAMETER //服務(wù)器提供的可用的方法

DESCRIBE

C向S發(fā)起DESCRIBE請(qǐng)求,為了得到會(huì)話描述信息(SDP):

DESCRIBE rtsp://192.168.20.136:5000/xxx666 RTSP/1.0
CSeq: 2
token: 
Accept: application/sdp
User-Agent: VLC media player (LIVE555 Streaming Media v2005.11.10) 

服務(wù)器回應(yīng)一些對(duì)此會(huì)話的描述信息(sdp):

RTSP/1.0 200 OK 
Server: UServer 0.9.7_rc1 
Cseq: 2 
x-prev-url: rtsp://192.168.20.136:5000 
x-next-url: rtsp://192.168.20.136:5000 
x-Accept-Retransmit: our-retransmit 
x-Accept-Dynamic-Rate: 1 
Cache-Control: must-revalidate 
Last-Modified: Fri, 10 Nov 2006 12:34:38 GMT 
Date: Fri, 10 Nov 2006 12:34:38 GMT 
Expires: Fri, 10 Nov 2006 12:34:38 GMT 
Content-Base: rtsp://192.168.20.136:5000/xxx666/ 
Content-Length: 344 
Content-Type: application/sdp 

v=0 //以下都是sdp信息  
o=OnewaveUServerNG 1451516402 1025358037 IN IP4 192.168.20.136 
s=/xxx666 
u=http:/// 
e=admin@ 
c=IN IP4 0.0.0.0 
t=0 0 
a=isma-compliance:1,1.0,1 

a=range:npt=0- 
m=video 0 RTP/AVP 96 //m表示媒體描述,下面是對(duì)會(huì)話中視頻通道的媒體描述
a=rtpmap:96 MP4V-ES/90000 
a=fmtp:96 profile-level-id=245;config=000001B0F5000001B509000001000000012000C888B0E0E0FA62D089028307 a=control:trackID=0 //trackID=0表示視頻流用的是通道0

SETUP

客戶端提醒服務(wù)器建立會(huì)話,并確定傳輸模式:

SETUP rtsp://192.168.20.136:5000/xxx666/trackID=0 RTSP/1.0 
CSeq: 3 
Transport: RTP/AVP/TCP;unicast;interleaved=0-1 
User-Agent: VLC media player (LIVE555 Streaming Media v2005.11.10)
 //uri中 帶有trackID=0,表示對(duì)該通道進(jìn)行設(shè)置。Transport參數(shù)設(shè)置了傳輸模式,包的結(jié)構(gòu)。接下來(lái)的數(shù)據(jù)包頭部第二個(gè)字節(jié)位置就是 interleaved,它的值是每個(gè)通道都不同的,trackID=0的interleaved值有兩個(gè)0或1,0表示rtp包,1表示rtcp包,接收端根據(jù)interleaved的值來(lái)區(qū)別是哪種數(shù)據(jù)包。

服務(wù)器回應(yīng)信息:

RTSP/1.0 200 OK 
Server: UServer 0.9.7_rc1 
Cseq: 3 
Session: 6310936469860791894 //服務(wù)器回應(yīng)的會(huì)話標(biāo)識(shí)符
Cache-Control: no-cache 
Transport: RTP/AVP/TCP;unicast;interleaved=0-1;ssrc=6B8B4567

PLAY

客戶端發(fā)送播放請(qǐng)求:

PLAY rtsp://192.168.20.136:5000/xxx666 RTSP/1.0 
CSeq: 4 
Session: 6310936469860791894 
Range: npt=0.000- //設(shè)置播放時(shí)間的范圍
User-Agent: VLC media player (LIVE555 Streaming Media v2005.11.10)

服務(wù)器回應(yīng)信息:

RTSP/1.0 200 OK 
Server: UServer 0.9.7_rc1 
Cseq: 4 
Session: 6310936469860791894 
Range: npt=0.000000- 
RTP-Info: url=trackID=0;seq=17040;rtptime=1467265309 
 //seq和rtptime都是rtp包中的信息

TEARDOWN

客戶端發(fā)起關(guān)閉請(qǐng)求:

TEARDOWN rtsp://192.168.20.136:5000/xxx666 RTSP/1.0 
CSeq: 5 
Session: 6310936469860791894 
User-Agent: VLC media player (LIVE555 Streaming Media v2005.11.10) 

服務(wù)器回應(yīng):

RTSP/1.0 200 OK 
Server: UServer 0.9.7_rc1 
Cseq: 5 
Session: 6310936469860791894 

SDP協(xié)議

sdp的格式:

v=<version>
o=<username> <session id> <version> <network type> <address type> <address>
s=<session name>
i=<session description>
u=<URI>
e=<email address>
p=<phone number>
c=<network type> <address type> <connection address>
b=<modifier>:<bandwidth-value>
t=<start time> <stop time>
r=<repeat interval> <active duration> <list of offsets from start-time>
z=<adjustment time> <offset> <adjustment time> <offset> ....
k=<method>
k=<method>:<encryption key>
a=<attribute>
a=<attribute>:<value>
m=<media> <port> <transport> <fmt list>

v = (協(xié)議版本)
o = (所有者/創(chuàng)建者和會(huì)話標(biāo)識(shí)符)
s = (會(huì)話名稱(chēng))
i = * (會(huì)話信息)
u = * (URI 描述)
e = * (Email 地址)
p = * (電話號(hào)碼)
c = * (連接信息)
b = * (帶寬信息)
z = * (時(shí)間區(qū)域調(diào)整)
k = * (加密密鑰)
a = * (0 個(gè)或多個(gè)會(huì)話屬性行)

  • 時(shí)間描述:
    t = (會(huì)話活動(dòng)時(shí)間)
    r = * (0或多次重復(fù)次數(shù))

  • 媒體描述:
    m = (媒體名稱(chēng)和傳輸?shù)刂罚?br> i = * (媒體標(biāo)題)
    c = * (連接信息 — 如果包含在會(huì)話層則該字段可選)
    b = * (帶寬信息)
    k = * (加密密鑰)
    a = * (0 個(gè)或多個(gè)媒體屬性行)

SDP一會(huì)話描述協(xié)議一描述SAP、SIP和RTSR會(huì)話的協(xié)議,是一種文件描述協(xié)議,是由服務(wù)器生成的描述媒體文件編碼信息以及所在服務(wù)器的鏈接等的信息。在多媒體會(huì)話 中sDP傳送有關(guān)媒體流的信息,使會(huì)話描述的參人方加人會(huì)話。SDP主要用于Intemet網(wǎng)中,但也可以在其它網(wǎng)絡(luò)環(huán)境下使用。SDP十分通用,可描述其它網(wǎng)絡(luò)環(huán)境中的會(huì)話,但主要用 于Intemet中。在Intemet環(huán)境下,SDP有兩個(gè)主要目的:一是表明會(huì)話存在,二是傳送足夠信息給接收方,以便能加人、參加該會(huì)話。SDP所傳達(dá)的信息包括:會(huì)話名稱(chēng)和目的,會(huì)話 活動(dòng)時(shí)間,組成會(huì)話媒體種類(lèi),接收這些媒體的控制信息(如地址、端口、格式、帶寬和會(huì)議管理人員資料等)。

總結(jié):在RTSP交互過(guò)程中,只要在客戶端發(fā)出Describe請(qǐng)求的時(shí)候,服務(wù)端回應(yīng)的時(shí)候會(huì)有SDP消息發(fā)出,用SDP來(lái)描述會(huì)話情況和內(nèi)容,方便客戶端能夠加入該會(huì)話。

RTSP基于libcurl代碼實(shí)現(xiàn)

/*
 * Copyright (c) 2011, Jim Hollinger
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *   * Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of Jim Hollinger nor the names of its contributors
 *     may be used to endorse or promote products derived from this
 *     software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 */
/* <DESC>
 * A basic RTSP transfer
 * </DESC>
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <curl/curl.h>

#if defined (WIN32)
#include <conio.h>  /* _getch() */
#else
#include <termios.h>
#include <unistd.h>

#define VERSION_STR  "V1.0"

/* error handling macros */
#define my_curl_easy_setopt(A, B, C)                             \
  res = curl_easy_setopt((A), (B), (C));                         \
  if(!res)                                                       \
    fprintf(stderr, "curl_easy_setopt(%s, %s, %s) failed: %d\n", \
            #A, #B, #C, res);

#define my_curl_easy_perform(A)                                     \
  res = curl_easy_perform(A);                                       \
  if(!res)                                                          \
    fprintf(stderr, "curl_easy_perform(%s) failed: %d\n", #A, res);

static int _getch(void)
{
  struct termios oldt, newt;
  int ch;
  tcgetattr(STDIN_FILENO, &oldt);
  newt = oldt;
  newt.c_lflag &= ~( ICANON | ECHO);
  tcsetattr(STDIN_FILENO, TCSANOW, &newt);
  ch = getchar();
  tcsetattr(STDIN_FILENO, TCSANOW, &oldt);
  return ch;
}
#endif

/* send RTSP OPTIONS request */
static void rtsp_options(CURL *curl, const char *uri)
{
  CURLcode res = CURLE_OK;
  printf("\nRTSP: OPTIONS %s\n", uri);
  my_curl_easy_setopt(curl, CURLOPT_RTSP_STREAM_URI, uri);
  my_curl_easy_setopt(curl, CURLOPT_RTSP_REQUEST, (long)CURL_RTSPREQ_OPTIONS);
  my_curl_easy_perform(curl);
}

/* send RTSP DESCRIBE request and write sdp response to a file */
static void rtsp_describe(CURL *curl, const char *uri,
                          const char *sdp_filename)
{
  CURLcode res = CURLE_OK;
  FILE *sdp_fp = fopen(sdp_filename, "wb");
  printf("\nRTSP: DESCRIBE %s\n", uri);
  if(sdp_fp == NULL) {
    fprintf(stderr, "Could not open '%s' for writing\n", sdp_filename);
    sdp_fp = stdout;
  }
  else {
    printf("Writing SDP to '%s'\n", sdp_filename);
  }
  my_curl_easy_setopt(curl, CURLOPT_WRITEDATA, sdp_fp);
  my_curl_easy_setopt(curl, CURLOPT_RTSP_REQUEST, (long)CURL_RTSPREQ_DESCRIBE);
  my_curl_easy_perform(curl);
  my_curl_easy_setopt(curl, CURLOPT_WRITEDATA, stdout);
  if(sdp_fp != stdout) {
    fclose(sdp_fp);
  }
}

/* send RTSP SETUP request */
static void rtsp_setup(CURL *curl, const char *uri, const char *transport)
{
  CURLcode res = CURLE_OK;
  printf("\nRTSP: SETUP %s\n", uri);
  printf("      TRANSPORT %s\n", transport);
  my_curl_easy_setopt(curl, CURLOPT_RTSP_STREAM_URI, uri);
  my_curl_easy_setopt(curl, CURLOPT_RTSP_TRANSPORT, transport);
  my_curl_easy_setopt(curl, CURLOPT_RTSP_REQUEST, (long)CURL_RTSPREQ_SETUP);
  my_curl_easy_perform(curl);
}

/* send RTSP PLAY request */
static void rtsp_play(CURL *curl, const char *uri, const char *range)
{
  CURLcode res = CURLE_OK;
  printf("\nRTSP: PLAY %s\n", uri);
  my_curl_easy_setopt(curl, CURLOPT_RTSP_STREAM_URI, uri);
  my_curl_easy_setopt(curl, CURLOPT_RANGE, range);
  my_curl_easy_setopt(curl, CURLOPT_RTSP_REQUEST, (long)CURL_RTSPREQ_PLAY);
  my_curl_easy_perform(curl);
}

/* send RTSP TEARDOWN request */
static void rtsp_teardown(CURL *curl, const char *uri)
{
  CURLcode res = CURLE_OK;
  printf("\nRTSP: TEARDOWN %s\n", uri);
  my_curl_easy_setopt(curl, CURLOPT_RTSP_REQUEST, (long)CURL_RTSPREQ_TEARDOWN);
  my_curl_easy_perform(curl);
}

/* convert url into an sdp filename */
static void get_sdp_filename(const char *url, char *sdp_filename,
                             size_t namelen)
{
  const char *s = strrchr(url, '/');
  strcpy(sdp_filename, "video.sdp");
  if(s != NULL) {
    s++;
    if(s[0] != '\0') {
      snprintf(sdp_filename, namelen, "%s.sdp", s);
    }
  }
}

/* scan sdp file for media control attribute */
static void get_media_control_attribute(const char *sdp_filename,
                                        char *control)
{
  int max_len = 256;
  char *s = malloc(max_len);
  FILE *sdp_fp = fopen(sdp_filename, "rb");
  control[0] = '\0';
  if(sdp_fp != NULL) {
    while(fgets(s, max_len - 2, sdp_fp) != NULL) {
      sscanf(s, " a = control: %s", control);
    }
    fclose(sdp_fp);
  }
  free(s);
}

/* main app */
int main(int argc, char * const argv[])
{
#if 1
  const char *transport = "RTP/AVP;unicast;client_port=1234-1235";  /* UDP */
#else
  /* TCP */
  const char *transport = "RTP/AVP/TCP;unicast;client_port=1234-1235";
#endif
  const char *range = "0.000-";
  int rc = EXIT_SUCCESS;
  char *base_name = NULL;

  printf("\nRTSP request %s\n", VERSION_STR);
  printf("    Project web site: http://code.google.com/p/rtsprequest/\n");
  printf("    Requires curl V7.20 or greater\n\n");

  /* check command line */
  if((argc != 2) && (argc != 3)) {
    base_name = strrchr(argv[0], '/');
    if(base_name == NULL) {
      base_name = strrchr(argv[0], '\\');
    }
    if(base_name == NULL) {
      base_name = argv[0];
    }
    else {
      base_name++;
    }
    printf("Usage:   %s url [transport]\n", base_name);
    printf("         url of video server\n");
    printf("         transport (optional) specifier for media stream"
           " protocol\n");
    printf("         default transport: %s\n", transport);
    printf("Example: %s rtsp://192.168.0.2/media/video1\n\n", base_name);
    rc = EXIT_FAILURE;
  }
  else {
    const char *url = argv[1];
    char *uri = malloc(strlen(url) + 32);
    char *sdp_filename = malloc(strlen(url) + 32);
    char *control = malloc(strlen(url) + 32);
    CURLcode res;
    get_sdp_filename(url, sdp_filename, strlen(url) + 32);
    if(argc == 3) {
      transport = argv[2];
    }

    /* initialize curl */
    res = curl_global_init(CURL_GLOBAL_ALL);
    if(res == CURLE_OK) {
      curl_version_info_data *data = curl_version_info(CURLVERSION_NOW);
      CURL *curl;
      fprintf(stderr, "    curl V%s loaded\n", data->version);

      /* initialize this curl session */
      curl = curl_easy_init();
      if(curl != NULL) {
        my_curl_easy_setopt(curl, CURLOPT_VERBOSE, 0L);
        my_curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 1L);
        my_curl_easy_setopt(curl, CURLOPT_HEADERDATA, stdout);
        my_curl_easy_setopt(curl, CURLOPT_URL, url);

        /* request server options */
        snprintf(uri, strlen(url) + 32, "%s", url);
        rtsp_options(curl, uri);

        /* request session description and write response to sdp file */
        rtsp_describe(curl, uri, sdp_filename);

        /* get media control attribute from sdp file */
        get_media_control_attribute(sdp_filename, control);

        /* setup media stream */
        snprintf(uri, strlen(url) + 32, "%s/%s", url, control);
        rtsp_setup(curl, uri, transport);

        /* start playing media stream */
        snprintf(uri, strlen(url) + 32, "%s/", url);
        rtsp_play(curl, uri, range);
        printf("Playing video, press any key to stop ...");
        _getch();
        printf("\n");

        /* teardown session */
        rtsp_teardown(curl, uri);

        /* cleanup */
        curl_easy_cleanup(curl);
        curl = NULL;
      }
      else {
        fprintf(stderr, "curl_easy_init() failed\n");
      }
      curl_global_cleanup();
    }
    else {
      fprintf(stderr, "curl_global_init(%s) failed: %d\n",
              "CURL_GLOBAL_ALL", res);
    }
    free(control);
    free(sdp_filename);
    free(uri);
  }

  return rc;
}

參考資料

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