您没有明确说明您使用的是multi interface还是easy interface可能值得一提的是您正在使用什么平台以及libcurl您正在使用什么等。
下面是最小卷曲easy和multi针对libcurl/7.21.6.
我很高兴地拔掉了网线，停止了 http 服务器等等~它似乎可以应付。
这些可能会帮助您：
curl_easy_setopt(curl, CURLOPT_LOW_SPEED_LIMIT, dl_lowspeed_bytes); //bytes/sec
curl_easy_setopt(curl, CURLOPT_LOW_SPEED_TIME, dl_lowspeed_time); //seconds
curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
注意：当连接断开时，您必须非常努力地摔倒 curl。这是设计使然，但令一些人感到惊讶。
编辑
我怀疑你会想要使用CURLOPT_TIMEOUT. 这将使传输超时。如果您的下载量很大，那么几乎肯定会花费比您准备等待找出网络连接是否有问题的时间更长的时间 - 超时会受到影响。相比之下，CURLOPT_LOW_SPEED_TIME即使经过数小时的传输时间，超时也可能永远不会受到影响。
curltest_easy.c：
/*----------------------------------------------------
curltest_easy.c
WARNING: for test purposes only ~
*/
#include <stdio.h>
#include <unistd.h>
#include <curl/curl.h>
#include <curl/types.h>
#include <curl/easy.h>
#include <sys/stat.h>
static int dl_progress(void *clientp,double dltotal,double dlnow,double ultotal,double ulnow)
{
if (dlnow && dltotal)
printf("dl:%3.0f%%\r",100*dlnow/dltotal); //shenzi prog-mon
fflush(stdout);
return 0;
}
static size_t dl_write(void *buffer, size_t size, size_t nmemb, void *stream)
{
return fwrite(buffer, size, nmemb, (FILE*)stream);
}
int do_dl(void)
{
CURL *curl;
FILE *fp;
CURLcode curl_retval;
long http_response;
double dl_size;
int retval=0;
long dl_lowspeed_bytes=1000; //1K
long dl_lowspeed_time=10; //sec
/*put something biG here, preferably on a server that you can switch off at will ;) */
char url[] = {"http://fc00.deviantart.net/fs26/f/2008/134/1/a/Dragon_VII_by_NegativeFeedback.swf"};
char filename[]={"blah.dl"};
struct stat st={0};
if (!stat(filename, &st));
printf("st.st_size:[%ld]\n", st.st_size);
if(!(fp=fopen(filename, "ab"))) /*append binary*/
return 1;
curl_global_init(CURL_GLOBAL_DEFAULT);
curl = curl_easy_init();
if (curl)
{
//http://linux.die.net/man/3/curl_easy_setopt
curl_easy_setopt(curl, CURLOPT_URL, url);
/*callbacks*/
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, dl_write);
curl_easy_setopt(curl, CURLOPT_PROGRESSFUNCTION, dl_progress);
curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0);
/*curl will keep running -so you have the freedom to recover
from network disconnects etc in your own way without
distrubing the curl task in hand. ** this is by design :p ** */
//curl_easy_setopt(curl, CURLOPT_TIMEOUT, 60);
//curl_easy_setopt(curl, CURLOPT_CONNECTTIMEOUT, 30);
/*set up min download speed threshold & time endured before aborting*/
curl_easy_setopt(curl, CURLOPT_LOW_SPEED_LIMIT, dl_lowspeed_bytes); //bytes/sec
curl_easy_setopt(curl, CURLOPT_LOW_SPEED_TIME, dl_lowspeed_time); //seconds while below low spped limit before aborting
curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
curl_easy_setopt(curl, CURLOPT_RESUME_FROM,st.st_size);
/*uncomment this to get curl to tell you what its up to*/
//curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
if(CURLE_OK != (curl_retval=curl_easy_perform(curl)))
{
printf("curl_retval:[%d]\n", curl_retval);
switch(curl_retval)
{
//Transferred a partial file
case CURLE_WRITE_ERROR: //can be due to a dropped connection
break;
//all defined in curl/curl.h
default: //suggest quitting on unhandled error
retval=0;
};
curl_easy_getinfo(curl, CURLINFO_CONTENT_LENGTH_DOWNLOAD, &dl_size);
printf("CURLINFO_CONTENT_LENGTH_DOWNLOAD:%f\n", dl_size);
curl_retval=curl_easy_getinfo(curl, CURLINFO_RESPONSE_CODE, &http_response);
//see: http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html
printf("CURLINFO_RESPONSE_CODE:%ld\n", http_response);
switch(http_response)
{
case 0: //eg connection down from kick-off ~suggest retrying till some max limit
break;
case 200: //yay we at least got to our url
break;
case 206:
case 416: //http://www.checkupdown.com/status/E416.html
printf("ouch! you might want to handle this & others\n");
default: //suggest quitting on an unhandled error
retval=0;
};
}
else
{
printf("our work here is done ;)\n");
retval=2;
}
if (fp)
fclose(fp);
if (curl)
curl_easy_cleanup(curl);
}
printf("retval [%d]\n", retval);
return retval;
}
int main(void)
{
while (!do_dl())
{
usleep(5000);
}
return 0;
}
/* notes ----
$sudo apt-get install libcurl4-gnutls-dev
$ curl-config --libs
-L/usr/lib/i386-linux-gnu -lcurl -Wl,-Bsymbolic-functions
#oook. lets do it:
$ gcc -o curltest_easy curltest_easy.c -L/usr/lib/i386-linux-gnu -lcurl -Wl,-Bsymbolic-functions
$ ./curltest
*/
curltest_multi.c：
/*----------------------------------------------------
curltest_mult1.c
WARNING: for test purposes only ~
*/
#include <stdio.h>
#include <unistd.h>
#include <curl/curl.h>
#include <curl/types.h>
#include <curl/easy.h>
#include <sys/stat.h>
typedef struct S_dl_byte_data
{
double new_bytes_received; //from the latest request
double existing_filesize;
} dl_byte_data, *pdl_byte_data;
static int dl_progress(pdl_byte_data pdata,double dltotal,double dlnow,double ultotal,double ulnow)
{
/*dltotal := hacky way of getting the Content-Length ~ less hacky would be to first
do a HEAD request & then curl_easy_getinfo with CURLINFO_CONTENT_LENGTH_DOWNLOAD*/
if (dltotal && dlnow)
{
pdata->new_bytes_received=dlnow;
dltotal+=pdata->existing_filesize;
dlnow+=pdata->existing_filesize;
printf(" dl:%3.0f%% total:%.0f received:%.0f\r",100*dlnow/dltotal, dltotal, dlnow); //shenzi prog-mon
fflush(stdout);
}
return 0;
}
static size_t dl_write(void *buffer, size_t size, size_t nmemb, void *stream)
{
return fwrite(buffer, size, nmemb, (FILE*)stream);
}
////////////////////////
int do_dl(void)
{
CURLM *multi_handle;
CURL *curl;
FILE *fp;
CURLcode curl_retval;
int retval=0;
int handle_count=0;
double dl_bytes_remaining, dl_bytes_received;
dl_byte_data st_dldata={0};
char curl_error_buf[CURL_ERROR_SIZE]={"meh"};
long dl_lowspeed_bytes=1000, dl_lowspeed_time=10; /* 1KBs for 10 secs*/
/*put something biG here, preferably on a server that you can switch off at will ;) */
char url[] = {"http://fc00.deviantart.net/fs26/f/2008/134/1/a/Dragon_VII_by_NegativeFeedback.swf"};
char outfilename[]={"blah.swf"}, filename[]={"blah.dl"};
struct stat st={0};
if (!(fp=fopen(filename, "ab")) || -1==fstat(fileno(fp), &st)) //append binary
return -1;
if (curl_global_init(CURL_GLOBAL_DEFAULT))
return -2;
if (!(multi_handle = curl_multi_init()))
return -3;
if (!(curl = curl_easy_init()))
return -4;
st_dldata.new_bytes_received=st_dldata.existing_filesize=st.st_size;
//http://curl.haxx.se/libcurl/c/curl_easy_setopt.html
curl_easy_setopt(curl, CURLOPT_URL, url);
/*callbacks*/
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, dl_write);
curl_easy_setopt(curl, CURLOPT_PROGRESSFUNCTION, dl_progress);
curl_easy_setopt(curl, CURLOPT_PROGRESSDATA, &st_dldata);
curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0);
/*curl will keep running -so you have the freedom to recover from network disconnects etc
in your own way without distrubing the curl task in hand. ** this is by design :p **
The follwoing sets up min download speed threshold & time endured before aborting*/
curl_easy_setopt(curl, CURLOPT_LOW_SPEED_LIMIT, dl_lowspeed_bytes); //bytes/sec
curl_easy_setopt(curl, CURLOPT_LOW_SPEED_TIME, dl_lowspeed_time); //seconds while below low spped limit before aborting
//alternatively these are available in libcurl 7.25
//curl_easy_setopt(curl, CURLOPT_TCP_KEEPALIVE,1L);
//curl_easy_setopt(curl, CURLOPT_TCP_KEEPIDLE,10);
//curl_easy_setopt(curl, CURLOPT_TCP_KEEPINTVL,10);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
/*uncomment this to get curl to tell you what its up to*/
//curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
curl_easy_setopt(curl, CURLOPT_ERRORBUFFER, curl_error_buf);
do
{
if (st_dldata.new_bytes_received) //set the new range for the partial transfer if we have previously received some bytes
{
printf("resuming d/l..\n");
fflush(fp);
//get the new filesize & sanity check for file; on error quit outer do-loop & return to main
if (-1==(retval=fstat(fileno(fp), &st)) || !(st_dldata.existing_filesize=st.st_size)) break;
//see also: CURLOPT_RANGE for passing a string with our own X-Y range
curl_easy_setopt(curl, CURLOPT_RESUME_FROM, st.st_size);
st_dldata.new_bytes_received=0;
}
printf("\n\nbytes already received:[%.0f]\n", st_dldata.existing_filesize);
//re-use the curl handle again & again & again & again... lol
curl_multi_add_handle(multi_handle, curl);
do //curl_multi_perform event-loop
{
CURLMsg *pMsg;
int msgs_in_queue;
while (CURLM_CALL_MULTI_PERFORM == curl_multi_perform(multi_handle, &handle_count));
//check for any mesages regardless of handle count
while(pMsg=curl_multi_info_read(multi_handle, &msgs_in_queue))
{
long http_response;
printf("\nmsgs_in_queue:[%d]\n",msgs_in_queue);
if (CURLMSG_DONE != pMsg->msg)
{
fprintf(stderr,"CURLMSG_DONE != pMsg->msg:[%d]\n", pMsg->msg);
}
else
{
printf("pMsg->data.result:[%d] meaning:[%s]\n",pMsg->data.result,curl_easy_strerror(pMsg->data.result));
if (CURLE_OK != pMsg->data.result) printf("curl_error_buf:[%s]\n", curl_error_buf);
switch(pMsg->data.result)
{
case CURLE_OK: ///////////////////////////////////////////////////////////////////////////////////////
printf("CURLE_OK: ");
curl_easy_getinfo(pMsg->easy_handle, CURLINFO_CONTENT_LENGTH_DOWNLOAD, &dl_bytes_remaining);
curl_easy_getinfo(pMsg->easy_handle, CURLINFO_SIZE_DOWNLOAD, &dl_bytes_received);
if (dl_bytes_remaining == dl_bytes_received)
{
printf("our work here is done ;)\n");
rename(filename, outfilename);
retval=1;
}
else
{
printf("ouch! st_dldata.new_bytes_received[%f]\n",st_dldata.new_bytes_received);
printf("ouch! dl_bytes_received[%f] dl_bytes_remaining[%f]\n",dl_bytes_received,dl_bytes_remaining);
retval=dl_bytes_received < dl_bytes_remaining ? 0 : -5;
}
break; /////////////////////////////////////////////////////////////////////////////////////////////////
case CURLE_COULDNT_CONNECT: //no network connectivity ?
case CURLE_OPERATION_TIMEDOUT: //cos of CURLOPT_LOW_SPEED_TIME
case CURLE_COULDNT_RESOLVE_HOST: //host/DNS down ?
printf("CURMESSAGE switch handle_count:[%d]\n",handle_count);
break; //we'll keep trying
default://see: http://curl.haxx.se/libcurl/c/libcurl-errors.html
handle_count=0;
retval=-5;
};
//see: http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html
curl_retval=curl_easy_getinfo(pMsg->easy_handle, CURLINFO_RESPONSE_CODE, &http_response);
printf("CURLINFO_RESPONSE_CODE HTTP:[%ld]\n", http_response);
switch(http_response)
{
case 0: //eg connection down from kick-off ~suggest retrying till some max limit
case 200: //yay we at least got to our url
case 206: //Partial Content
break;
case 416:
//cannot d/l range ~ either cos no server support
//or cos we're asking for an invalid range ~ie: we already d/ld the file
printf("HTTP416: either the d/l is already complete or the http server cannot d/l a range\n");
retval=2;
default: //suggest quitting on an unhandled error
handle_count=0;
retval=-6;
};
}
}
if (handle_count) //select on any active handles
{
fd_set fd_read={0}, fd_write={0}, fd_excep={0};
struct timeval timeout={5,0};
int select_retval;
int fd_max;
curl_multi_fdset(multi_handle, &fd_read, &fd_write, &fd_excep, &fd_max);
if (-1 == (select_retval=select(fd_max+1, &fd_read, &fd_write, &fd_excep, &timeout)))
{
//errno shall be set to indicate the error
fprintf(stderr, "yikes! select error :(\n");
handle_count=0;
retval=-7;
break;
}
else{/*check whatever*/}
}
} while (handle_count);
curl_multi_remove_handle(multi_handle,curl);
printf("continue from here?");
getchar();
}
while(retval==0);
curl_multi_cleanup(multi_handle);
curl_easy_cleanup(curl);
curl_global_cleanup();
if (fp) fclose(fp);
return retval;
}
////////////////////////
int main(void)
{
int retval;
printf("\n\ncurl_multi d/l test ~curl version:[%s]\n", curl_version());
while (1!=(retval=do_dl()))
{
printf("retval [%d] continue?\n\n", retval);
printf("continue?");
getchar();
}
printf("\nend of test!\n\n", retval);
return retval;
}
/* notes ----
$sudo apt-get install libcurl4-gnutls-dev
$curl-config --libs
-L/usr/lib/i386-linux-gnu -lcurl -Wl,-Bsymbolic-functions
#oook. lets do it:
$gcc -o curltest_multi curltest_multi.c -L/usr/lib/i386-linux-gnu -lcurl -Wl,-Bsymbolic-functions
$./curltest_multi
*/
您可能要记住blah.dl在开始全新测试之前删除该文件。该程序故意不这样做，因此您可以预先截断现有文件以进行测试；）
注意：对于这样的事情，您可能不应该*只是依赖CURLE_COULDNT_CONNECT〜您的代码应该主要是错误处理（如果您的程序严格供个人使用，则可能更少）
编辑
我更新了 curtest_multi.c 来演示easy_handle重用。
请注意文档中的以下引用：
单次传输完成后，简单句柄仍会添加到多堆栈中。您需要先使用 curl_multi_remove_handle(3) 删除简单句柄，然后使用 curl_easy_cleanup(3) 将其关闭，或者可能为其设置新选项并使用 curl_multi_add_handle(3) 再次添加以开始另一次传输。