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curl_easy hanging with threads

From: Aaron Meriwether <>
Date: Tue, 30 Jun 2015 15:36:15 -0600

Process hangs (sometimes) when making sequential requests from multiple threads. Any idea what's going wrong?
Simplest test case is to use the "multithread.c" example ( <>) with this alternate thread body:

static void *pull_two_urls(void *url)
  CURL *curl;

  curl = curl_easy_init();
  curl_easy_setopt(curl, CURLOPT_URL, url);
  curl_easy_perform(curl); /* ignores error */

  curl = curl_easy_init();
  curl_easy_setopt(curl, CURLOPT_URL, url);
  curl_easy_perform(curl); /* ignores error */

  return NULL;

My environment:
# cat /proc/version
Linux version 4.0.4-x86_64-linode57 (maker_at_build) (gcc version 4.7.2 (Debian 4.7.2-5) ) #1 SMP Thu May 21 11:01:47 EDT 2015

# curl -V
curl 7.26.0 (x86_64-pc-linux-gnu) libcurl/7.26.0 OpenSSL/1.0.1e zlib/1.2.7 libidn/1.25 libssh2/1.4.2 librtmp/2.3
Protocols: dict file ftp ftps gopher http https imap imaps ldap pop3 pop3s rtmp rtsp scp sftp smtp smtps telnet tftp
Features: Debug GSS-Negotiate IDN IPv6 Largefile NTLM NTLM_WB SSL libz TLS-SRP

# gcc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.7.2-5' --with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs --enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.7 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.7 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin --enable-objc-gc --with-arch-32=i586 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.7.2 (Debian 4.7.2-5)

Compiled with:
# gcc -lcurl multithread.c

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Received on 2015-06-30