.BI "CURLcode curl_global_init(long " flags ");"
.ad
.SH DESCRIPTION
-This function should only be called once (no matter how many threads or
-libcurl sessions that'll be used) by every application that uses libcurl.
+This function sets up the program environment that libcurl needs. Think
+of it as an extension of the library loader.
-If this function hasn't been invoked when \fIcurl_easy_init(3)\fP is called,
-it will be done automatically by libcurl. It is adviced that you do not rely
-on this automatic call, but instead call \fIcurl_global_init(3)\fP properly.
+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 exact what features to
+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 and mean to control internal operations
+of libcurl.
-You must however \fBalways\fP use the \fIcurl_global_cleanup(3)\fP function,
-as that cannot be called automatically for you by libcurl.
+\fBThis function is not thread safe.\fP 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 \fIcurl_global_init()\fP 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 \fBlibcurl\fP(3) of global environment
+requirements for details of how to use this function.
-Calling this function more than once will cause unpredictable results. If that
-is not enough, calling this function from more than one thread may also cause
-unpredictable results.
.SH FLAGS
.TP 5
.B CURL_GLOBAL_ALL
.SH "SEE ALSO"
.BR curl_global_init_mem "(3), "
.BR curl_global_cleanup "(3), "
-.BR curl_easy_init "(3) "
\ No newline at end of file
+.BR curl_easy_init "(3) "
+.BR libcurl "(3) "
There are more than a twenty custom bindings available that bring libcurl
access to your favourite language. Look elsewhere for documentation on those.
-All applications that use libcurl should call \fIcurl_global_init(3)\fP
-exactly once before any libcurl function can be used. After all usage of
-libcurl is complete, it \fBmust\fP call \fIcurl_global_cleanup(3)\fP. In
-between those two calls, you can use libcurl as described below.
+libcurl has a global constant environment that you must set up and
+maintain while using libcurl. This essentially means you call
+\fIcurl_global_init(3)\fP at the start of your program and
+\fIcurl_global_cleanup(3)\fP at the end. See GLOBAL CONSTANTS below
+for details.
To transfer files, you always set up an "easy handle" using
\fIcurl_easy_init(3)\fP, but when you want the file(s) transferred you have
several threads. libcurl is thread-safe and can be used in any number of
threads, but you must use separate curl handles if you want to use libcurl in
more than one thread simultaneously.
+
+The global environment functions are not thread-safe. See GLOBAL CONSTANTS
+below for details.
+
.SH "PERSISTENT CONNECTIONS"
Persistent connections means that libcurl can re-use the same connection for
several transfers, if the conditions are right.
Note that the options set with \fIcurl_easy_setopt(3)\fP will be used in on
every repeated \fIcurl_easy_perform(3)\fP call.
+
+.SH "GLOBAL CONSTANTS"
+There are a variety of constants that libcurl uses, mainly through its
+internal use of other libraries, which are too complicated for the
+library loader to set up. Therefore, a program must call a library
+function after the program is loaded and running to finish setting up
+the library code. For example, when libcurl is built for SSL
+capability via the GNU TLS library, there is an elaborate tree inside
+that library that describes the SSL protocol.
+
+\fIcurl_global_init()\fP is the function that you must call. This may
+allocate resources (e.g. the memory for the GNU TLS tree mentioned
+above), so the companion function \fIcurl_global_cleanup()\fP releases
+them.
+
+The basic rule for constructing a program that uses libcurl is this:
+Call \fIcurl_global_init()\fP, with a \fICURL_GLOBAL_ALL\fP argument,
+immediately after the program starts, while it is still only one
+thread and before it uses libcurl at all. Call
+\fIcurl_global_cleanup()\fP immediately before the program exits, when
+the program is again only one thread and after its last use of
+libcurl.
+
+You can call both of these multiple times, as long as all calls meet
+these requirements and the number of calls to each is the same.
+
+It isn't actually required that the functions be called at the beginning
+and end of the program -- that's just usually the easiest way to do it.
+It \fIis\fP required that the functions be called when no other thread
+in the program is running.
+
+These global constant functions are \fInot thread safe\fP, so you must
+not call them when any other thread in the program is running. It
+isn't good enough that no other thread is using libcurl at the time,
+because these functions internally call similar functions of other
+libraries, and those functions are similarly thread-unsafe. You can't
+generally know what these libraries are, or whether other threads are
+using them.
+
+The global constant situation merits special consideration when the
+code you are writing to use libcurl is not the main program, but rather
+a modular piece of a program, e.g. another library. As a module,
+your code doesn't know about other parts of the program -- it doesn't
+know whether they use libcurl or not. And its code doesn't necessarily
+run at the start and end of the whole program.
+
+A module like this must have global constant functions of its own,
+just like \fIcurl_global_init()\fP and \fIcurl_global_cleanup()\fP.
+The module thus has control at the beginning and end of the program
+and has a place to call the libcurl functions. Note that if multiple
+modules in the program use libcurl, they all will separately call the
+libcurl functions, and that's OK because only the first
+\fIcurl_global_init()\fP and the last \fIcurl_global_cleanup()\fP in a
+program changes anything. (libcurl uses a reference count in static
+memory).
+
+In a C++ module, it is common to deal with the global constant
+situation by defining a special class that represents the global
+constant environment of the module. A program always has exactly one
+object of the class, in static storage. That way, the program
+automatically calls the constructor of the object as the program
+starts up and the destructor as it terminates. As the author of this
+libcurl-using module, you can make the constructor call
+\fIcurl_global_init()\fP and the destructor call
+\fIcurl_global_cleanup()\fP and satisfy libcurl's requirements without
+your user having to think about it.
+
+\fIcurl_global_init()\fP has an argument that tells what particular
+parts of the global constant environment to set up. In order to
+successfully use any value except \fICURL_GLOBAL_ALL\fP (which says to
+set up the whole thing), you must have specific knowledge of internal
+workings of libcurl and all other parts of the program of which it is
+part.
+
+A special part of the global constant environment is the identity of
+the memory allocator. \fIcurl_global_init()\fP selects the system
+default memory allocator, but you can use \fIcurl_global_init_mem()\fP
+to supply one of your own. However, there is no way to use
+\fIcurl_global_init_mem()\fP in a modular program -- all modules in
+the program that might use libcurl would have to agree on one
+allocator.
+
+There is a failsafe in libcurl that makes it usable in simple
+situations without you having to worry about the global constant
+environment at all: \fIcurl_easy_init()\fP sets up the environment
+itself if it hasn't been done yet. The resources it acquires to do so
+get released by the operating system automatically when the program
+exits.
+
+This failsafe feature exists mainly for backward compatibility because
+there was a time when the global functions didn't exist. Because it
+is sufficient only in the simplest of programs, it is not recommended
+for any program to rely on it.
projects / platform / upstream / curl.git / commitdiff