最近在Linux嵌入式平台上使用curl出现卡死的情况。
1.第一种情况
在发送的时候不加上链接超时和发送超时,这样子很容易造成在发送的时候出现卡死的现象,导致线程阻塞
curl_easy_setopt(curl, CURLOPT_CONNECTTIMEOUT, 5);
curl_easy_setopt(curl, CURLOPT_TIMEOUT, 10);
2.第二种情况是加上链接超时和发送超时
curl_easy_setopt(curl, CURLOPT_CONNECTTIMEOUT, 5);
curl_easy_setopt(curl, CURLOPT_TIMEOUT, 10);
这种情况,在发送的时候不会出现卡死的状况,但是如果在发送的时候,设备的断网后,再链接网以后,会出现程序崩溃的现象
3.第三种情况加上
既然第二种情况会出现崩溃,我们加上
//Setting CURLOPT_NOSIGNAL to 1 makes libcurl NOT ask the system to ignore SIGPIPE signals
curl_easy_setopt(curl, CURLOPT_NOSIGNAL, 1)
为什么加上这个呢,先看下下面这段代码:
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int Curl_resolv_timeout(struct connectdata *conn,
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const char *hostname,
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int port,
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struct Curl_dns_entry **entry,
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time_t timeoutms)
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{
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#ifdef USE_ALARM_TIMEOUT
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#ifdef HAVE_SIGACTION
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struct sigaction keep_sigact; /* store the old struct here */
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volatile bool keep_copysig = FALSE; /* wether old sigact has been saved */
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struct sigaction sigact;
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#else
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#ifdef HAVE_SIGNAL
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void (*keep_sigact)(int); /* store the old handler here */
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#endif /* HAVE_SIGNAL */
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#endif /* HAVE_SIGACTION */
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volatile long timeout;
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volatile unsigned int prev_alarm = 0;
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struct Curl_easy *data = conn->data;
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#endif /* USE_ALARM_TIMEOUT */
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int rc;
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*entry = NULL;
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if(timeoutms < 0)
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/* got an already expired timeout */
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return CURLRESOLV_TIMEDOUT;
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#ifdef USE_ALARM_TIMEOUT
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if(data->set.no_signal)// 注意
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/* Ignore the timeout when signals are disabled */
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timeout = 0;
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else
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timeout = timeoutms;
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if(!timeout)
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/* USE_ALARM_TIMEOUT defined, but no timeout actually requested */
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return Curl_resolv(conn, hostname, port, entry);
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if(timeout < 1000) {
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/* The alarm() function only provides integer second resolution, so if
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we want to wait less than one second we must bail out already now. */
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failf(data,
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"remaining timeout of %ld too small to resolve via SIGALRM method",
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timeout);
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return CURLRESOLV_TIMEDOUT;
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}
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/* This allows us to time-out from the name resolver, as the timeout
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will generate a signal and we will siglongjmp() from that here.
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This technique has problems (see alarmfunc).
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This should be the last thing we do before calling Curl_resolv(),
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as otherwise we'd have to worry about variables that get modified
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before we invoke Curl_resolv() (and thus use "volatile"). */
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if(sigsetjmp(curl_jmpenv, 1)) {
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/* this is coming from a siglongjmp() after an alarm signal */
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failf(data, "name lookup timed out");
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rc = CURLRESOLV_ERROR;
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goto clean_up;
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}
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else {
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/*************************************************************
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* Set signal handler to catch SIGALRM
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* Store the old value to be able to set it back later!
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*************************************************************/
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#ifdef HAVE_SIGACTION
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sigaction(SIGALRM, NULL, &sigact);
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keep_sigact = sigact;
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keep_copysig = TRUE; /* yes, we have a copy */
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sigact.sa_handler = alarmfunc;
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#ifdef SA_RESTART
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/* HPUX doesn't have SA_RESTART but defaults to that behaviour! */
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sigact.sa_flags &= ~SA_RESTART;
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#endif
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/* now set the new struct */
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sigaction(SIGALRM, &sigact, NULL);
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#else /* HAVE_SIGACTION */
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/* no sigaction(), revert to the much lamer signal() */
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#ifdef HAVE_SIGNAL
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keep_sigact = signal(SIGALRM, alarmfunc);
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#endif
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#endif /* HAVE_SIGACTION */
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/* alarm() makes a signal get sent when the timeout fires off, and that
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will abort system calls */
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prev_alarm = alarm(curlx_sltoui(timeout/1000L));
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}
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#else
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#ifndef CURLRES_ASYNCH
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if(timeoutms)
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infof(conn->data, "timeout on name lookup is not supported\n");
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#else
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(void)timeoutms; /* timeoutms not used with an async resolver */
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#endif
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#endif /* USE_ALARM_TIMEOUT */
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/* Perform the actual name resolution. This might be interrupted by an
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* alarm if it takes too long.
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*/
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rc = Curl_resolv(conn, hostname, port, entry);
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#ifdef USE_ALARM_TIMEOUT
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clean_up:
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if(!prev_alarm)
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/* deactivate a possibly active alarm before uninstalling the handler */
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alarm(0);
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#ifdef HAVE_SIGACTION
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if(keep_copysig) {
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/* we got a struct as it looked before, now put that one back nice
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and clean */
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sigaction(SIGALRM, &keep_sigact, NULL); /* put it back */
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}
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#else
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#ifdef HAVE_SIGNAL
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/* restore the previous SIGALRM handler */
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signal(SIGALRM, keep_sigact);
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#endif
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#endif /* HAVE_SIGACTION */
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/* switch back the alarm() to either zero or to what it was before minus
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the time we spent until now! */
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if(prev_alarm) {
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/* there was an alarm() set before us, now put it back */
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unsigned long elapsed_ms = Curl_tvdiff(Curl_tvnow(), conn->created);
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/* the alarm period is counted in even number of seconds */
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unsigned long alarm_set = prev_alarm - elapsed_ms/1000;
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if(!alarm_set ||
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((alarm_set >= 0x80000000) && (prev_alarm < 0x80000000)) ) {
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/* if the alarm time-left reached zero or turned "negative" (counted
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with unsigned values), we should fire off a SIGALRM here, but we
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won't, and zero would be to switch it off so we never set it to
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less than 1! */
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alarm(1);
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rc = CURLRESOLV_TIMEDOUT;
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failf(data, "Previous alarm fired off!");
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}
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else
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alarm((unsigned int)alarm_set);
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}
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#endif /* USE_ALARM_TIMEOUT */
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return rc;
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}
if(data->set.no_signal)如果设置了这个,那么在DNS超时的时候会立即返回。
另外:curl如果不加curl_easy_setopt(curl, CURLOPT_NOSIGNAL, 1),是不支持小于1000ms的超时的,
原因
if(timeout < 1000) {
/* The alarm() function only provides integer second resolution, so if
we want to wait less than one second we must bail out already now. */
failf(data,
"remaining timeout of %ld too small to resolve via SIGALRM method",
timeout);
return CURLRESOLV_TIMEDOUT;
}
超时时间小于1000ms的时候, name解析会直接返回CURLRESOLV_TIMEOUT, 最后会导致CURLE_OPERATION_TIMEDOUT
4.第四钟情况
第四种情况是,curl能在超时的时候正常返回,但是还是会出现curl卡死的情况
那是因为curl正常是只用系统的DNS进行解析,那就是DNS解析将不受超时限制了, 万一DNS服务器 卡住了话, 那就可能会造成curl卡死的情况。
那么使用了curl_easy_setopt(curl, CURLOPT_NOSIGNAL, 1)还是有问题,无奈之下,只能libcurl使用c-ares(C library for asynchronous DNS requests)来做名字解析,编编译curl的时候加上编译选项./configure --enable-ares这样子就可以不使用curl_easy_setopt(curl, CURLOPT_NOSIGNAL, 1)了。