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ADD: MuPDF v1.26.7: the MuPDF source as downloaded by a default build of PyMuPDF 1.26.4. The directory name has changed: no version number in the expanded directory now.
author Franz Glasner <fzglas.hg@dom66.de>
date Mon, 15 Sep 2025 11:43:07 +0200
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21 .\" **************************************************************************
22 .TH libcurl 3 "19 March 2002" "libcurl 7.9.6" "libcurl overview"
23 .SH NAME
24 libcurl \- client-side URL transfers
25 .SH DESCRIPTION
26 This is a short overview on how to use libcurl in your C programs. There are
27 specific man pages for each function mentioned in here. There are also the
28 \fIlibcurl-easy(3)\fP man page, the \fIlibcurl-multi(3)\fP man page, the
29 \fIlibcurl-share(3)\fP man page and the \fIlibcurl-tutorial(3)\fP man page for
30 in-depth understanding on how to program with libcurl.
31
32 There are many bindings available that bring libcurl access to your favourite
33 language. Look elsewhere for documentation on those.
34
35 libcurl has a global constant environment that you must set up and maintain
36 while using libcurl. This essentially means you call
37 \fIcurl_global_init(3)\fP at the start of your program and
38 \fIcurl_global_cleanup(3)\fP at the end. See \fBGLOBAL CONSTANTS\fP below for
39 details.
40
41 If libcurl was compiled with support for multiple SSL backends, the function
42 \fIcurl_global_sslset(3)\fP can be called before \fIcurl_global_init(3)\fP
43 to select the active SSL backend.
44
45 To transfer files, you create an "easy handle" using \fIcurl_easy_init(3)\fP
46 for a single individual transfer (in either direction). You then set your
47 desired set of options in that handle with \fIcurl_easy_setopt(3)\fP. Options
48 you set with \fIcurl_easy_setopt(3)\fP stick. They will be used on every
49 repeated use of this handle until you either change the option, or you reset
50 them all with \fIcurl_easy_reset(3)\fP.
51
52 To actually transfer data you have the option of using the "easy" interface,
53 or the "multi" interface.
54
55 The easy interface is a synchronous interface with which you call
56 \fIcurl_easy_perform(3)\fP and let it perform the transfer. When it is
57 completed, the function returns and you can continue. More details are found in
58 the \fIlibcurl-easy(3)\fP man page.
59
60 The multi interface on the other hand is an asynchronous interface, that you
61 call and that performs only a little piece of the transfer on each invoke. It
62 is perfect if you want to do things while the transfer is in progress, or
63 similar. The multi interface allows you to select() on libcurl action, and
64 even to easily download multiple files simultaneously using a single
65 thread. See further details in the \fIlibcurl-multi(3)\fP man page.
66
67 You can have multiple easy handles share certain data, even if they are used
68 in different threads. This magic is setup using the share interface, as
69 described in the \fIlibcurl-share(3)\fP man page.
70
71 There is also a series of other helpful functions to use, including these:
72 .RS
73 .IP curl_version_info()
74 gets detailed libcurl (and other used libraries) version info
75 .IP curl_getdate()
76 converts a date string to time_t
77 .IP curl_easy_getinfo()
78 get information about a performed transfer
79 .IP curl_formadd()
80 helps building an HTTP form POST
81 .IP curl_formfree()
82 free a list built with \fIcurl_formadd(3)\fP
83 .IP curl_slist_append()
84 builds a linked list
85 .IP curl_slist_free_all()
86 frees a whole curl_slist
87 .RE
88
89 .SH "LINKING WITH LIBCURL"
90 On unix-like machines, there's a tool named curl-config that gets installed
91 with the rest of the curl stuff when 'make install' is performed.
92
93 curl-config is added to make it easier for applications to link with libcurl
94 and developers to learn about libcurl and how to use it.
95
96 Run 'curl-config --libs' to get the (additional) linker options you need to
97 link with the particular version of libcurl you've installed. See the
98 \fIcurl-config(1)\fP man page for further details.
99
100 Unix-like operating system that ship libcurl as part of their distributions
101 often don't provide the curl-config tool, but simply install the library and
102 headers in the common path for this purpose.
103
104 Many Linux and similar systems use pkg-config to provide build and link
105 options about libraries and libcurl supports that as well.
106 .SH "LIBCURL SYMBOL NAMES"
107 All public functions in the libcurl interface are prefixed with 'curl_' (with
108 a lowercase c). You can find other functions in the library source code, but
109 other prefixes indicate that the functions are private and may change without
110 further notice in the next release.
111
112 Only use documented functions and functionality!
113 .SH "PORTABILITY"
114 libcurl works
115 .B exactly
116 the same, on any of the platforms it compiles and builds on.
117 .SH "THREADS"
118 libcurl is thread safe but there are a few exceptions. Refer to
119 \fIlibcurl-thread(3)\fP for more information.
120
121 .SH "PERSISTENT CONNECTIONS"
122 Persistent connections means that libcurl can re-use the same connection for
123 several transfers, if the conditions are right.
124
125 libcurl will \fBalways\fP attempt to use persistent connections. Whenever you
126 use \fIcurl_easy_perform(3)\fP or \fIcurl_multi_perform(3)\fP etc, libcurl
127 will attempt to use an existing connection to do the transfer, and if none
128 exists it'll open a new one that will be subject for re-use on a possible
129 following call to \fIcurl_easy_perform(3)\fP or \fIcurl_multi_perform(3)\fP.
130
131 To allow libcurl to take full advantage of persistent connections, you should
132 do as many of your file transfers as possible using the same handle.
133
134 If you use the easy interface, and you call \fIcurl_easy_cleanup(3)\fP, all
135 the possibly open connections held by libcurl will be closed and forgotten.
136
137 When you've created a multi handle and are using the multi interface, the
138 connection pool is instead kept in the multi handle so closing and creating
139 new easy handles to do transfers will not affect them. Instead all added easy
140 handles can take advantage of the single shared pool.
141 .SH "GLOBAL CONSTANTS"
142 There are a variety of constants that libcurl uses, mainly through its
143 internal use of other libraries, which are too complicated for the
144 library loader to set up. Therefore, a program must call a library
145 function after the program is loaded and running to finish setting up
146 the library code. For example, when libcurl is built for SSL
147 capability via the GNU TLS library, there is an elaborate tree inside
148 that library that describes the SSL protocol.
149
150 \fIcurl_global_init(3)\fP is the function that you must call. This may
151 allocate resources (e.g. the memory for the GNU TLS tree mentioned above), so
152 the companion function \fIcurl_global_cleanup(3)\fP releases them.
153
154 The basic rule for constructing a program that uses libcurl is this: Call
155 \fIcurl_global_init(3)\fP, with a \fICURL_GLOBAL_ALL\fP argument, immediately
156 after the program starts, while it is still only one thread and before it uses
157 libcurl at all. Call \fIcurl_global_cleanup(3)\fP immediately before the
158 program exits, when the program is again only one thread and after its last
159 use of libcurl.
160
161 You can call both of these multiple times, as long as all calls meet
162 these requirements and the number of calls to each is the same.
163
164 It isn't actually required that the functions be called at the beginning
165 and end of the program -- that's just usually the easiest way to do it.
166 It \fIis\fP required that the functions be called when no other thread
167 in the program is running.
168
169 These global constant functions are \fInot thread safe\fP, so you must
170 not call them when any other thread in the program is running. It
171 isn't good enough that no other thread is using libcurl at the time,
172 because these functions internally call similar functions of other
173 libraries, and those functions are similarly thread-unsafe. You can't
174 generally know what these libraries are, or whether other threads are
175 using them.
176
177 The global constant situation merits special consideration when the
178 code you are writing to use libcurl is not the main program, but rather
179 a modular piece of a program, e.g. another library. As a module,
180 your code doesn't know about other parts of the program -- it doesn't
181 know whether they use libcurl or not. And its code doesn't necessarily
182 run at the start and end of the whole program.
183
184 A module like this must have global constant functions of its own, just like
185 \fIcurl_global_init(3)\fP and \fIcurl_global_cleanup(3)\fP. The module thus
186 has control at the beginning and end of the program and has a place to call
187 the libcurl functions. Note that if multiple modules in the program use
188 libcurl, they all will separately call the libcurl functions, and that's OK
189 because only the first \fIcurl_global_init(3)\fP and the last
190 \fIcurl_global_cleanup(3)\fP in a program change anything. (libcurl uses a
191 reference count in static memory).
192
193 In a C++ module, it is common to deal with the global constant situation by
194 defining a special class that represents the global constant environment of
195 the module. A program always has exactly one object of the class, in static
196 storage. That way, the program automatically calls the constructor of the
197 object as the program starts up and the destructor as it terminates. As the
198 author of this libcurl-using module, you can make the constructor call
199 \fIcurl_global_init(3)\fP and the destructor call \fIcurl_global_cleanup(3)\fP
200 and satisfy libcurl's requirements without your user having to think about it.
201 (Caveat: If you are initializing libcurl from a Windows DLL you should not
202 initialize it from DllMain or a static initializer because Windows holds the
203 loader lock during that time and it could cause a deadlock.)
204
205 \fIcurl_global_init(3)\fP has an argument that tells what particular parts of
206 the global constant environment to set up. In order to successfully use any
207 value except \fICURL_GLOBAL_ALL\fP (which says to set up the whole thing), you
208 must have specific knowledge of internal workings of libcurl and all other
209 parts of the program of which it is part.
210
211 A special part of the global constant environment is the identity of the
212 memory allocator. \fIcurl_global_init(3)\fP selects the system default memory
213 allocator, but you can use \fIcurl_global_init_mem(3)\fP to supply one of your
214 own. However, there is no way to use \fIcurl_global_init_mem(3)\fP in a
215 modular program -- all modules in the program that might use libcurl would
216 have to agree on one allocator.
217
218 There is a failsafe in libcurl that makes it usable in simple situations
219 without you having to worry about the global constant environment at all:
220 \fIcurl_easy_init(3)\fP sets up the environment itself if it hasn't been done
221 yet. The resources it acquires to do so get released by the operating system
222 automatically when the program exits.
223
224 This failsafe feature exists mainly for backward compatibility because
225 there was a time when the global functions didn't exist. Because it
226 is sufficient only in the simplest of programs, it is not recommended
227 for any program to rely on it.