I did this

`
curl_multi_setopt(multi_handle, CURLMOPT_MAX_TOTAL_CONNECTIONS, 0);
curl_multi_setopt(multi_handle, CURLMOPT_MAX_HOST_CONNECTIONS, 6);

while (...) {
..
curl_multi_wait(multi_handle, efd, nefd, 1000, &numfds);
curl_multi_perform(multi_handle, &pending);
...
}
`

sometime crashed on:

#0 0x00007fb9f89e9780 in __write_nocancel () from /lib64/libpthread.so.0
#1 0x00007fb9f9492076 in sock_write () from /lib64/libcrypto.so.10
#2 0x00007fb9f949013b in BIO_write () from /lib64/libcrypto.so.10
#3 0x00007fb9f858b742 in ssl3_write_pending () from /lib64/libssl.so.10
#4 0x00007fb9f858d9a3 in ssl3_dispatch_alert () from /lib64/libssl.so.10
#5 0x00007fb9f85894e2 in ssl3_shutdown () from /lib64/libssl.so.10
#6 0x0000000000460945 in ossl_close.isra.3 ()
#7 0x00000000004609b1 in Curl_ossl_close ()
#8 0x000000000044f3e7 in Curl_disconnect ()
#9 0x0000000000452f24 in Curl_connect ()
#10 0x00000000004342c3 in multi_runsingle ()
#11 0x0000000000435203 in curl_multi_perform ()

curl/libcurl version

custom build:

./bin/curl -V
curl 7.61.1-DEV (x86_64-unknown-linux-gnu) libcurl/7.61.1-DEV OpenSSL/1.0.2k zlib/1.2.7 brotli/1.0.5
Release-Date: [unreleased]
Protocols: file http https
Features: IPv6 Largefile NTLM SSL libz brotli UnixSockets HTTPS-proxy

operating system

CentOS Linux release 7.4.1708 (Core)
Linux 3.10.0-693.el7.x86_64 #1 SMP