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Jay Satiro 7860f575fe dllmain: Call OpenSSL thread cleanup for Windows and Cygwin

- Call OPENSSL_thread_stop on thread termination (DLL_THREAD_DETACH)
  to prevent a memory leak in case OpenSSL is linked statically.

- Warn in libcurl-thread.3 that if OpenSSL is linked statically then it
  may require thread cleanup.

OpenSSL may need per-thread cleanup to stop a memory leak. For Windows
and Cygwin if libcurl was built as a DLL then we can do that for the
user by calling OPENSSL_thread_stop on thread termination. However, if
libcurl was built statically then we do not have notification of thread
termination and cannot do that for the user.

Also, there are several other unusual cases where it may be necessary
for the user to call OPENSSL_thread_stop, so in the libcurl-thread
warning I added a link to the OpenSSL documentation.

Co-authored-by: Viktor Szakats

Reported-by: southernedge@users.noreply.github.com
Reported-by: zmcx16@users.noreply.github.com

Ref: https://www.openssl.org/docs/man3.0/man3/OPENSSL_thread_stop.html#NOTES

Fixes https://github.com/curl/curl/issues/12327
Closes https://github.com/curl/curl/pull/12408

2024-04-24 04:04:25 -04:00

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SPDX-License-Identifier

Title

Section

Source

See-also

Protocol

curl

curl_global_cleanup

libcurl

curl_global_init (3)

libcurl (3)

libcurl-thread (3)

All

NAME

curl_global_cleanup - global libcurl cleanup

SYNOPSIS

#include <curl/curl.h>

void curl_global_cleanup(void);

DESCRIPTION

This function releases resources acquired by curl_global_init(3).

You should call curl_global_cleanup(3) once for each call you make to curl_global_init(3), after you are done using libcurl.

This function is thread-safe since libcurl 7.84.0 if curl_version_info(3) has the CURL_VERSION_THREADSAFE feature bit set (most platforms).

If this is not thread-safe, you must not call this function when any other thread in the program (i.e. a thread sharing the same memory) is running. This does not just mean no other thread that is using libcurl. Because curl_global_cleanup(3) calls functions of other libraries that are similarly thread unsafe, it could conflict with any other thread that uses these other libraries.

See the description in libcurl(3) of global environment requirements for details of how to use this function.

CAUTION

curl_global_cleanup(3) does not block waiting for any libcurl-created threads to terminate (such as threads used for name resolving). If a module containing libcurl is dynamically unloaded while libcurl-created threads are still running then your program may crash or other corruption may occur. We recommend you do not run libcurl from any module that may be unloaded dynamically. This behavior may be addressed in the future.

libcurl may not be able to fully clean up after multi-threaded OpenSSL depending on how OpenSSL was built and loaded as a library. It is possible in some rare circumstances a memory leak could occur unless you implement your own OpenSSL thread cleanup. Refer to libcurl-thread(3).

EXAMPLE

int main(void)
{
  curl_global_init(CURL_GLOBAL_DEFAULT);

  /* use libcurl, then before exiting... */

  curl_global_cleanup();
}

AVAILABILITY

Added in 7.8

RETURN VALUE

None

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