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229 lines
11 KiB
229 lines
11 KiB
.\" **************************************************************************
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.\" * _ _ ____ _
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.\" * Project ___| | | | _ \| |
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.\" * / __| | | | |_) | |
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.\" * | (__| |_| | _ <| |___
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.\" * \___|\___/|_| \_\_____|
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.\" *
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.\" * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
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.\" *
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.\" * This software is licensed as described in the file COPYING, which
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.\" * you should have received as part of this distribution. The terms
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.\" * are also available at https://curl.haxx.se/docs/copyright.html.
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.\" *
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.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
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.\" * copies of the Software, and permit persons to whom the Software is
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.\" * furnished to do so, under the terms of the COPYING file.
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.\" *
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.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
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.\" * KIND, either express or implied.
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.\" *
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.\" **************************************************************************
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.TH libcurl 3 "March 23, 2020" "libcurl 7.73.0" "libcurl overview"
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.SH NAME
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libcurl \- client-side URL transfers
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.SH DESCRIPTION
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This is a short overview on how to use libcurl in your C programs. There are
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specific man pages for each function mentioned in here. There are also the
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\fIlibcurl-easy(3)\fP man page, the \fIlibcurl-multi(3)\fP man page, the
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\fIlibcurl-share(3)\fP man page and the \fIlibcurl-tutorial(3)\fP man page for
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in-depth understanding on how to program with libcurl.
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There are many bindings available that bring libcurl access to your favourite
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language. Look elsewhere for documentation on those.
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libcurl has a global constant environment that you must set up and maintain
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while using libcurl. This essentially means you call
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\fIcurl_global_init(3)\fP at the start of your program and
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\fIcurl_global_cleanup(3)\fP at the end. See \fBGLOBAL CONSTANTS\fP below for
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details.
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If libcurl was compiled with support for multiple SSL backends, the function
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\fIcurl_global_sslset(3)\fP can be called before \fIcurl_global_init(3)\fP
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to select the active SSL backend.
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To transfer files, you create an "easy handle" using \fIcurl_easy_init(3)\fP
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for a single individual transfer (in either direction). You then set your
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desired set of options in that handle with \fIcurl_easy_setopt(3)\fP. Options
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you set with \fIcurl_easy_setopt(3)\fP stick. They will be used on every
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repeated use of this handle until you either change the option, or you reset
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them all with \fIcurl_easy_reset(3)\fP.
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To actually transfer data you have the option of using the "easy" interface,
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or the "multi" interface.
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The easy interface is a synchronous interface with which you call
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\fIcurl_easy_perform(3)\fP and let it perform the transfer. When it is
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completed, the function returns and you can continue. More details are found in
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the \fIlibcurl-easy(3)\fP man page.
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The multi interface on the other hand is an asynchronous interface, that you
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call and that performs only a little piece of the transfer on each invoke. It
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is perfect if you want to do things while the transfer is in progress, or
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similar. The multi interface allows you to select() on libcurl action, and
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even to easily download multiple files simultaneously using a single
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thread. See further details in the \fIlibcurl-multi(3)\fP man page.
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You can have multiple easy handles share certain data, even if they are used
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in different threads. This magic is setup using the share interface, as
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described in the \fIlibcurl-share(3)\fP man page.
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There is also a series of other helpful functions to use, including these:
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.RS
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.IP curl_version_info()
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gets detailed libcurl (and other used libraries) version info
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.IP curl_getdate()
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converts a date string to time_t
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.IP curl_easy_getinfo()
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get information about a performed transfer
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.IP curl_formadd()
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helps building an HTTP form POST
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.IP curl_formfree()
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free a list built with \fIcurl_formadd(3)\fP
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.IP curl_slist_append()
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builds a linked list
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.IP curl_slist_free_all()
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frees a whole curl_slist
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.RE
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.SH "LINKING WITH LIBCURL"
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On unix-like machines, there's a tool named curl-config that gets installed
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with the rest of the curl stuff when 'make install' is performed.
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curl-config is added to make it easier for applications to link with libcurl
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and developers to learn about libcurl and how to use it.
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Run 'curl-config --libs' to get the (additional) linker options you need to
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link with the particular version of libcurl you've installed. See the
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\fIcurl-config(1)\fP man page for further details.
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Unix-like operating system that ship libcurl as part of their distributions
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often don't provide the curl-config tool, but simply install the library and
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headers in the common path for this purpose.
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Many Linux and similar systems use pkg-config to provide build and link
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options about libraries and libcurl supports that as well.
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.SH "LIBCURL SYMBOL NAMES"
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All public functions in the libcurl interface are prefixed with 'curl_' (with
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a lowercase c). You can find other functions in the library source code, but
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other prefixes indicate that the functions are private and may change without
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further notice in the next release.
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Only use documented functions and functionality!
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.SH "PORTABILITY"
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libcurl works
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.B exactly
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the same, on any of the platforms it compiles and builds on.
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.SH "THREADS"
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libcurl is thread safe but there are a few exceptions. Refer to
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\fIlibcurl-thread(3)\fP for more information.
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.SH "PERSISTENT CONNECTIONS"
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Persistent connections means that libcurl can re-use the same connection for
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several transfers, if the conditions are right.
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libcurl will \fBalways\fP attempt to use persistent connections. Whenever you
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use \fIcurl_easy_perform(3)\fP or \fIcurl_multi_perform(3)\fP etc, libcurl
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will attempt to use an existing connection to do the transfer, and if none
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exists it'll open a new one that will be subject for re-use on a possible
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following call to \fIcurl_easy_perform(3)\fP or \fIcurl_multi_perform(3)\fP.
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To allow libcurl to take full advantage of persistent connections, you should
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do as many of your file transfers as possible using the same handle.
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If you use the easy interface, and you call \fIcurl_easy_cleanup(3)\fP, all
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the possibly open connections held by libcurl will be closed and forgotten.
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When you've created a multi handle and are using the multi interface, the
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connection pool is instead kept in the multi handle so closing and creating
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new easy handles to do transfers will not affect them. Instead all added easy
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handles can take advantage of the single shared pool.
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.SH "GLOBAL CONSTANTS"
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There are a variety of constants that libcurl uses, mainly through its
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internal use of other libraries, which are too complicated for the
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library loader to set up. Therefore, a program must call a library
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function after the program is loaded and running to finish setting up
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the library code. For example, when libcurl is built for SSL
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capability via the GNU TLS library, there is an elaborate tree inside
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that library that describes the SSL protocol.
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\fIcurl_global_init(3)\fP is the function that you must call. This may
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allocate resources (e.g. the memory for the GNU TLS tree mentioned above), so
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the companion function \fIcurl_global_cleanup(3)\fP releases them.
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The basic rule for constructing a program that uses libcurl is this: Call
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\fIcurl_global_init(3)\fP, with a \fICURL_GLOBAL_ALL\fP argument, immediately
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after the program starts, while it is still only one thread and before it uses
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libcurl at all. Call \fIcurl_global_cleanup(3)\fP immediately before the
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program exits, when the program is again only one thread and after its last
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use of libcurl.
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You can call both of these multiple times, as long as all calls meet
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these requirements and the number of calls to each is the same.
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It isn't actually required that the functions be called at the beginning
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and end of the program -- that's just usually the easiest way to do it.
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It \fIis\fP required that the functions be called when no other thread
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in the program is running.
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These global constant functions are \fInot thread safe\fP, so you must
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not call them when any other thread in the program is running. It
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isn't good enough that no other thread is using libcurl at the time,
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because these functions internally call similar functions of other
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libraries, and those functions are similarly thread-unsafe. You can't
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generally know what these libraries are, or whether other threads are
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using them.
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The global constant situation merits special consideration when the
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code you are writing to use libcurl is not the main program, but rather
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a modular piece of a program, e.g. another library. As a module,
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your code doesn't know about other parts of the program -- it doesn't
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know whether they use libcurl or not. And its code doesn't necessarily
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run at the start and end of the whole program.
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A module like this must have global constant functions of its own, just like
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\fIcurl_global_init(3)\fP and \fIcurl_global_cleanup(3)\fP. The module thus
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has control at the beginning and end of the program and has a place to call
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the libcurl functions. Note that if multiple modules in the program use
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libcurl, they all will separately call the libcurl functions, and that's OK
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because only the first \fIcurl_global_init(3)\fP and the last
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\fIcurl_global_cleanup(3)\fP in a program change anything. (libcurl uses a
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reference count in static memory).
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In a C++ module, it is common to deal with the global constant situation by
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defining a special class that represents the global constant environment of
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the module. A program always has exactly one object of the class, in static
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storage. That way, the program automatically calls the constructor of the
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object as the program starts up and the destructor as it terminates. As the
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author of this libcurl-using module, you can make the constructor call
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\fIcurl_global_init(3)\fP and the destructor call \fIcurl_global_cleanup(3)\fP
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and satisfy libcurl's requirements without your user having to think about it.
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(Caveat: If you are initializing libcurl from a Windows DLL you should not
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initialize it from DllMain or a static initializer because Windows holds the
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loader lock during that time and it could cause a deadlock.)
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\fIcurl_global_init(3)\fP has an argument that tells what particular parts of
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the global constant environment to set up. In order to successfully use any
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value except \fICURL_GLOBAL_ALL\fP (which says to set up the whole thing), you
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must have specific knowledge of internal workings of libcurl and all other
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parts of the program of which it is part.
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A special part of the global constant environment is the identity of the
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memory allocator. \fIcurl_global_init(3)\fP selects the system default memory
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allocator, but you can use \fIcurl_global_init_mem(3)\fP to supply one of your
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own. However, there is no way to use \fIcurl_global_init_mem(3)\fP in a
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modular program -- all modules in the program that might use libcurl would
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have to agree on one allocator.
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There is a failsafe in libcurl that makes it usable in simple situations
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without you having to worry about the global constant environment at all:
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\fIcurl_easy_init(3)\fP sets up the environment itself if it hasn't been done
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yet. The resources it acquires to do so get released by the operating system
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automatically when the program exits.
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This failsafe feature exists mainly for backward compatibility because
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there was a time when the global functions didn't exist. Because it
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is sufficient only in the simplest of programs, it is not recommended
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for any program to rely on it.
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