1libcurl(3) libcurl overview libcurl(3)
2
6 libcurl - client-side URL transfers
7
9 This is a short overview on how to use libcurl in your C programs.
10 There are specific man pages for each function mentioned in here. See
11 libcurl-easy(3), libcurl-multi(3), libcurl-share(3), libcurl-url(3) and
12 libcurl-tutorial(3) for in-depth understanding on how to program with
13 libcurl.
14 There are many bindings available that bring libcurl access to your
16 favourite language. Look elsewhere for documentation on those.
17 libcurl has a global constant environment that you must set up and
19 maintain while using libcurl. This essentially means you call
20 curl_global_init(3) at the start of your program and
21 curl_global_cleanup(3) at the end. See GLOBAL CONSTANTS below for de‐
22 tails.
23 If libcurl was compiled with support for multiple SSL backends, the
25 function curl_global_sslset(3) can be called before curl_global_init(3)
26 to select the active SSL backend.
27 To transfer files, you create an "easy handle" using curl_easy_init(3)
29 for a single individual transfer (in either direction). You then set
30 your desired set of options in that handle with curl_easy_setopt(3).
31 Options you set with curl_easy_setopt(3) stick. They will be used on
32 every repeated use of this handle until you either change the option,
33 or you reset them all with curl_easy_reset(3).
34 To actually transfer data you have the option of using the "easy" in‐
36 terface, or the "multi" interface.
37 The easy interface is a synchronous interface with which you call
39 curl_easy_perform(3) and let it perform the transfer. When it is com‐
40 pleted, the function returns and you can continue. More details are
41 found in the libcurl-easy(3) man page.
42 The multi interface on the other hand is an asynchronous interface,
44 that you call and that performs only a little piece of the transfer on
45 each invoke. It is perfect if you want to do things while the transfer
46 is in progress, or similar. The multi interface allows you to select()
47 on libcurl action, and even to easily download multiple files simulta‐
48 neously using a single thread. See further details in the libcurl-
49 multi(3) man page.
50 You can have multiple easy handles share certain data, even if they are
52 used in different threads. This magic is setup using the share inter‐
53 face, as described in the libcurl-share(3) man page.
54 There is also a series of other helpful functions to use, including
56 these:
57 curl_version_info()
59 gets detailed libcurl (and other used libraries) version
60 info
61 curl_getdate()
63 converts a date string to time_t
64 curl_easy_getinfo()
66 get information about a performed transfer
67 curl_formadd()
69 helps building an HTTP form POST
70 curl_formfree()
72 free a list built with curl_formadd(3)
73 curl_slist_append()
75 builds a linked list
76 curl_slist_free_all()
78 frees a whole curl_slist
79 curl_url_set()
81 parses a URL
82
85 On unix-like machines, there's a tool named curl-config that gets in‐
86 stalled with the rest of the curl stuff when 'make install' is per‐
87 formed.
88 curl-config is added to make it easier for applications to link with
90 libcurl and developers to learn about libcurl and how to use it.
91 Run 'curl-config --libs' to get the (additional) linker options you
93 need to link with the particular version of libcurl you have installed.
94 See the curl-config(1) man page for further details.
95 Unix-like operating system that ship libcurl as part of their distribu‐
97 tions often do not provide the curl-config tool, but simply install the
98 library and headers in the common path for this purpose.
99 Many Linux and similar systems use pkg-config to provide build and link
101 options about libraries and libcurl supports that as well.
102
104 All public functions in the libcurl interface are prefixed with 'curl_'
105 (with a lowercase c). You can find other functions in the library
106 source code, but other prefixes indicate that the functions are private
107 and may change without further notice in the next release.
108 Only use documented functions and functionality!
110
112 libcurl works exactly the same, on any of the platforms it compiles and
113 builds on.
114
116 libcurl is thread safe but there are a few exceptions. Refer to
117 libcurl-thread(3) for more information.
118
121 Persistent connections means that libcurl can re-use the same connec‐
122 tion for several transfers, if the conditions are right.
123 libcurl will always attempt to use persistent connections. Whenever you
125 use curl_easy_perform(3) or curl_multi_perform(3) etc, libcurl will at‐
126 tempt to use an existing connection to do the transfer, and if none ex‐
127 ists it will open a new one that will be subject for re-use on a possi‐
128 ble following call to curl_easy_perform(3) or curl_multi_perform(3).
129 To allow libcurl to take full advantage of persistent connections, you
131 should do as many of your file transfers as possible using the same
132 handle.
133 If you use the easy interface, and you call curl_easy_cleanup(3), all
135 the possibly open connections held by libcurl will be closed and for‐
136 gotten.
137 When you have created a multi handle and are using the multi interface,
139 the connection pool is instead kept in the multi handle so closing and
140 creating new easy handles to do transfers will not affect them. Instead
141 all added easy handles can take advantage of the single shared pool.
142
144 There are a variety of constants that libcurl uses, mainly through its
145 internal use of other libraries, which are too complicated for the li‐
146 brary loader to set up. Therefore, a program must call a library func‐
147 tion after the program is loaded and running to finish setting up the
148 library code. For example, when libcurl is built for SSL capability via
149 the GNU TLS library, there is an elaborate tree inside that library
150 that describes the SSL protocol.
151 curl_global_init(3) is the function that you must call. This may allo‐
153 cate resources (e.g. the memory for the GNU TLS tree mentioned above),
154 so the companion function curl_global_cleanup(3) releases them.
155 The basic rule for constructing a program that uses libcurl is this:
157 Call curl_global_init(3), with a CURL_GLOBAL_ALL argument, immediately
158 after the program starts, while it is still only one thread and before
159 it uses libcurl at all. Call curl_global_cleanup(3) immediately before
160 the program exits, when the program is again only one thread and after
161 its last use of libcurl.
162 You can call both of these multiple times, as long as all calls meet
164 these requirements and the number of calls to each is the same.
165 It is not actually required that the functions be called at the begin‐
167 ning and end of the program -- that is just usually the easiest way to
168 do it. It is required that the functions be called when no other
169 thread in the program is running.
170 These global constant functions are not thread safe, so you must not
172 call them when any other thread in the program is running. It is not
173 good enough that no other thread is using libcurl at the time, because
174 these functions internally call similar functions of other libraries,
175 and those functions are similarly thread-unsafe. You cannot generally
176 know what these libraries are, or whether other threads are using them.
177 The global constant situation merits special consideration when the
179 code you are writing to use libcurl is not the main program, but rather
180 a modular piece of a program, e.g. another library. As a module, your
181 code does not know about other parts of the program -- it does not know
182 whether they use libcurl or not. And its code does not necessarily run
183 at the start and end of the whole program.
184 A module like this must have global constant functions of its own, just
186 like curl_global_init(3) and curl_global_cleanup(3). The module thus
187 has control at the beginning and end of the program and has a place to
188 call the libcurl functions. If multiple modules in the program use
189 libcurl, they all will separately call the libcurl functions, and that
190 is OK because only the first curl_global_init(3) and the last
191 curl_global_cleanup(3) in a program change anything. (libcurl uses a
192 reference count in static memory).
193 In a C++ module, it is common to deal with the global constant situa‐
195 tion by defining a special class that represents the global constant
196 environment of the module. A program always has exactly one object of
197 the class, in static storage. That way, the program automatically calls
198 the constructor of the object as the program starts up and the destruc‐
199 tor as it terminates. As the author of this libcurl-using module, you
200 can make the constructor call curl_global_init(3) and the destructor
201 call curl_global_cleanup(3) and satisfy libcurl's requirements without
202 your user having to think about it. (Caveat: If you are initializing
203 libcurl from a Windows DLL you should not initialize it from DllMain or
204 a static initializer because Windows holds the loader lock during that
205 time and it could cause a deadlock.)
206 curl_global_init(3) has an argument that tells what particular parts of
208 the global constant environment to set up. In order to successfully use
209 any value except CURL_GLOBAL_ALL (which says to set up the whole
210 thing), you must have specific knowledge of internal workings of
211 libcurl and all other parts of the program of which it is part.
212 A special part of the global constant environment is the identity of
214 the memory allocator. curl_global_init(3) selects the system default
215 memory allocator, but you can use curl_global_init_mem(3) to supply one
216 of your own. However, there is no way to use curl_global_init_mem(3) in
217 a modular program -- all modules in the program that might use libcurl
218 would have to agree on one allocator.
219 There is a failsafe in libcurl that makes it usable in simple situa‐
221 tions without you having to worry about the global constant environment
222 at all: curl_easy_init(3) sets up the environment itself if it has not
223 been done yet. The resources it acquires to do so get released by the
224 operating system automatically when the program exits.
225 This failsafe feature exists mainly for backward compatibility because
227 there was a time when the global functions did not exist. Because it is
228 sufficient only in the simplest of programs, it is not recommended for
229 any program to rely on it.
230libcurl 7.82.0 January 05, 2022 libcurl(3)