This function must be called at least once within a program (a program is all the code that shares a memory space) before the program calls any other function in libcurl. The environment it sets up is constant for the life of the program and is the same for every program, so multiple calls have the same effect as one call.
The flags option is a bit pattern that tells libcurl exactly what features to init, as described below. Set the desired bits by ORing the values together. In normal operation, you must specify CURL_GLOBAL_ALL. Don't use any other value unless you are familiar with it and mean to control internal operations of libcurl.
This function is not thread safe. You must not call it when any other thread in the program (i.e. a thread sharing the same memory) is running. This doesn't just mean no other thread that is using libcurl. Because curl_global_init(3) calls functions of other libraries that are similarly thread unsafe, it could conflict with any other thread that uses these other libraries.
If you are initializing libcurl from a Windows DLL you should not initialize it from DllMain or a static initializer because Windows holds the loader lock during that time and it could cause a deadlock.
See the description in libcurl(3) of global environment requirements for details of how to use this function.
The implication here is that if this bit is not set, the initialization of the SSL layer needs to be done by the application or at least outside of libcurl. The exact procedure how to do SSL initialization depends on the TLS backend libcurl uses.
Doing TLS based transfers without having the TLS layer initialized may lead to unexpected behaviors.
The implication here is that if this bit is not set, the initialization of winsock has to be done by the application or you risk getting undefined behaviors. This option exists for when the initialization is handled outside of libcurl so there's no need for libcurl to do it again.
Before 7.69.0: when this flag is set, curl will acknowledge EINTR condition when connecting or when waiting for data. Otherwise, curl waits until full timeout elapses. (Added in 7.30.0)