diff options
Diffstat (limited to 'docs/libcurl/libcurl.3')
| -rw-r--r-- | docs/libcurl/libcurl.3 | 106 |
1 files changed, 102 insertions, 4 deletions
diff --git a/docs/libcurl/libcurl.3 b/docs/libcurl/libcurl.3 index c9eee4e30..6d18e1fd1 100644 --- a/docs/libcurl/libcurl.3 +++ b/docs/libcurl/libcurl.3 @@ -13,10 +13,11 @@ in-depth understanding on how to program with libcurl. 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 @@ -86,6 +87,10 @@ Never ever call curl-functions simultaneously using the same handle from 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. @@ -103,3 +108,96 @@ libcurl will be closed and forgotten. 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. |