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 fa‐
16 vorite 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 global constant functions are thread-safe since libcurl 7.84.0 if
157 curl_version_info(3) has the CURL_VERSION_THREADSAFE feature bit set
158 (most platforms). Read libcurl-thread(3) for thread safety guidelines.
159 If the global constant functions are not thread safe, then you must not
161 call them when any other thread in the program is running. It is not
162 good enough that no other thread is using libcurl at the time, because
163 these functions internally call similar functions of other libraries,
164 and those functions are similarly thread-unsafe. You cannot generally
165 know what these libraries are, or whether other threads are using them.
166 If the global constant functions are not thread safe, then the basic
168 rule for constructing a program that uses libcurl is this: Call
169 curl_global_init(3), with a CURL_GLOBAL_ALL argument, immediately after
170 the program starts, while it is still only one thread and before it
171 uses libcurl at all. Call curl_global_cleanup(3) immediately before the
172 program exits, when the program is again only one thread and after its
173 last use of libcurl.
174 It is not actually required that the functions be called at the begin‐
176 ning and end of the program -- that is just usually the easiest way to
177 do it.
178 You can call both of these multiple times, as long as all calls meet
180 these requirements and the number of calls to each is the same.
181 The global constant situation merits special consideration when the
183 code you are writing to use libcurl is not the main program, but rather
184 a modular piece of a program, e.g. another library. As a module, your
185 code does not know about other parts of the program -- it does not know
186 whether they use libcurl or not. And its code does not necessarily run
187 at the start and end of the whole program.
188 A module like this must have global constant functions of its own, just
190 like curl_global_init(3) and curl_global_cleanup(3). The module thus
191 has control at the beginning and end of the program and has a place to
192 call the libcurl functions. If multiple modules in the program use
193 libcurl, they all will separately call the libcurl functions, and that
194 is OK because only the first curl_global_init(3) and the last
195 curl_global_cleanup(3) in a program change anything. (libcurl uses a
196 reference count in static memory).
197 In a C++ module, it is common to deal with the global constant situa‐
199 tion by defining a special class that represents the global constant
200 environment of the module. A program always has exactly one object of
201 the class, in static storage. That way, the program automatically calls
202 the constructor of the object as the program starts up and the destruc‐
203 tor as it terminates. As the author of this libcurl-using module, you
204 can make the constructor call curl_global_init(3) and the destructor
205 call curl_global_cleanup(3) and satisfy libcurl's requirements without
206 your user having to think about it. (Caveat: If you are initializing
207 libcurl from a Windows DLL you should not initialize it from DllMain or
208 a static initializer because Windows holds the loader lock during that
209 time and it could cause a deadlock.)
210 curl_global_init(3) has an argument that tells what particular parts of
212 the global constant environment to set up. In order to successfully use
213 any value except CURL_GLOBAL_ALL (which says to set up the whole
214 thing), you must have specific knowledge of internal workings of
215 libcurl and all other parts of the program of which it is part.
216 A special part of the global constant environment is the identity of
218 the memory allocator. curl_global_init(3) selects the system default
219 memory allocator, but you can use curl_global_init_mem(3) to supply one
220 of your own. However, there is no way to use curl_global_init_mem(3) in
221 a modular program -- all modules in the program that might use libcurl
222 would have to agree on one allocator.
223 There is a failsafe in libcurl that makes it usable in simple situa‐
225 tions without you having to worry about the global constant environment
226 at all: curl_easy_init(3) sets up the environment itself if it has not
227 been done yet. The resources it acquires to do so get released by the
228 operating system automatically when the program exits.
229 This failsafe feature exists mainly for backward compatibility because
231 there was a time when the global functions did not exist. Because it is
232 sufficient only in the simplest of programs, it is not recommended for
233 any program to rely on it.
234libcurl 8.0.1 January 02, 2023 libcurl(3)