libcurl.md (10556B)
1 --- 2 c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al. 3 SPDX-License-Identifier: curl 4 Title: libcurl 5 Section: 3 6 Source: libcurl 7 See-also: 8 - libcurl-easy (3) 9 - libcurl-multi (3) 10 - libcurl-security (3) 11 - libcurl-thread (3) 12 Protocol: 13 - All 14 Added-in: n/a 15 --- 16 17 # NAME 18 19 libcurl - client-side URL transfers 20 21 # DESCRIPTION 22 23 This is a short overview on how to use libcurl in your C programs. There are 24 specific man pages for each function mentioned in here. See 25 libcurl-easy(3), libcurl-multi(3), libcurl-share(3), 26 libcurl-url(3), libcurl-ws(3) and libcurl-tutorial(3) for 27 in-depth understanding on how to program with libcurl. 28 29 There are many bindings available that bring libcurl access to your favorite 30 language. Look elsewhere for documentation on those. 31 32 # TRANSFERS 33 34 To transfer files, you create an "easy handle" using curl_easy_init(3) 35 for a single individual transfer (in either direction). You then set your 36 desired set of options in that handle with curl_easy_setopt(3). Options 37 you set with curl_easy_setopt(3) stick. They are then used for every 38 repeated use of this handle until you either change the option, or you reset 39 them all with curl_easy_reset(3). 40 41 To actually transfer data you have the option of using the "easy" interface, 42 or the "multi" interface. 43 44 The easy interface is a synchronous interface with which you call 45 curl_easy_perform(3) and let it perform the transfer. When it is 46 completed, the function returns and you can continue. More details are found in 47 the libcurl-easy(3) man page. 48 49 The multi interface on the other hand is an asynchronous interface, that you 50 call and that performs only a little piece of the transfer on each invoke. It 51 is perfect if you want to do things while the transfer is in progress, or 52 similar. The multi interface allows you to select() on libcurl action, and 53 even to easily download multiple files simultaneously using a single 54 thread. See further details in the libcurl-multi(3) man page. 55 56 # SUPPORT INTERFACES 57 58 There is also a series of other helpful functions and interface families to 59 use, including these: 60 61 ## curl_version_info() 62 63 gets detailed libcurl (and other used libraries) version info. See 64 curl_version_info(3) 65 66 ## curl_getdate() 67 68 converts a date string to time_t. See curl_getdate(3) 69 70 ## curl_easy_getinfo() 71 72 get information about a performed transfer. See curl_easy_getinfo(3) 73 74 ## curl_mime_addpart() 75 76 helps building an HTTP form POST. See curl_mime_addpart(3) 77 78 ## curl_slist_append() 79 80 builds a linked list. See curl_slist_append(3) 81 82 ## Sharing data between transfers 83 84 You can have multiple easy handles share certain data, even if they are used 85 in different threads. This magic is setup using the share interface, as 86 described in the libcurl-share(3) man page. 87 88 ## URL Parsing 89 90 URL parsing and manipulations. See libcurl-url(3) 91 92 ## WebSocket communication 93 94 See libcurl-ws(3) 95 96 # LINKING WITH LIBCURL 97 98 On Unix-like machines, there is a tool named curl-config that gets installed 99 with the rest of the curl stuff when 'make install' is performed. 100 101 curl-config is added to make it easier for applications to link with libcurl 102 and developers to learn about libcurl and how to use it. 103 104 Run 'curl-config --libs' to get the (additional) linker options you need to 105 link with the particular version of libcurl you have installed. See the 106 *curl-config(1)* man page for further details. 107 108 Unix-like operating system that ship libcurl as part of their distributions 109 often do not provide the curl-config tool, but simply install the library and 110 headers in the common path for this purpose. 111 112 Many Linux and similar systems use pkg-config to provide build and link 113 options about libraries and libcurl supports that as well. 114 115 # LIBCURL SYMBOL NAMES 116 117 All public functions in the libcurl interface are prefixed with 'curl_' (with 118 a lowercase c). You can find other functions in the library source code, but 119 other prefixes indicate that the functions are private and may change without 120 further notice in the next release. 121 122 Only use documented functions and functionality. 123 124 # PORTABILITY 125 126 libcurl works 127 **exactly** 128 the same, on any of the platforms it compiles and builds on. 129 130 # THREADS 131 132 libcurl is thread safe but there are a few exceptions. Refer to 133 libcurl-thread(3) for more information. 134 135 # PERSISTENT CONNECTIONS 136 137 Persistent connections means that libcurl can reuse the same connection for 138 several transfers, if the conditions are right. 139 140 libcurl always attempts to use persistent connections. Whenever you use 141 curl_easy_perform(3) or curl_multi_perform(3) etc, libcurl 142 attempts to use an existing connection to do the transfer, and if none exists 143 it opens a new one that is subject for reuse on a possible following call to 144 curl_easy_perform(3) or curl_multi_perform(3). 145 146 To allow libcurl to take full advantage of persistent connections, you should 147 do as many of your file transfers as possible using the same handle. 148 149 If you use the easy interface, and you call curl_easy_cleanup(3), all 150 the possibly open connections held by libcurl are closed and forgotten. 151 152 When you have created a multi handle and are using the multi interface, the 153 connection pool is instead kept in the multi handle so closing and creating 154 new easy handles to do transfers do not affect them. Instead all added easy 155 handles can take advantage of the single shared pool. 156 157 # GLOBAL CONSTANTS 158 159 There are a variety of constants that libcurl uses, mainly through its 160 internal use of other libraries, which are too complicated for the 161 library loader to set up. Therefore, a program must call a library 162 function after the program is loaded and running to finish setting up 163 the library code. For example, when libcurl is built for SSL 164 capability via the GNU TLS library, there is an elaborate tree inside 165 that library that describes the SSL protocol. 166 167 curl_global_init(3) is the function that you must call. This may 168 allocate resources (e.g. the memory for the GNU TLS tree mentioned above), so 169 the companion function curl_global_cleanup(3) releases them. 170 171 If libcurl was compiled with support for multiple SSL backends, the function 172 curl_global_sslset(3) can be called before curl_global_init(3) 173 to select the active SSL backend. 174 175 The global constant functions are thread-safe since libcurl 7.84.0 if 176 curl_version_info(3) has the CURL_VERSION_THREADSAFE feature bit set 177 (most platforms). Read libcurl-thread(3) for thread safety guidelines. 178 179 If the global constant functions are *not thread safe*, then you must 180 not call them when any other thread in the program is running. It 181 is not good enough that no other thread is using libcurl at the time, 182 because these functions internally call similar functions of other 183 libraries, and those functions are similarly thread-unsafe. You cannot 184 generally know what these libraries are, or whether other threads are 185 using them. 186 187 If the global constant functions are *not thread safe*, then the basic rule 188 for constructing a program that uses libcurl is this: Call 189 curl_global_init(3), with a *CURL_GLOBAL_ALL* argument, immediately 190 after the program starts, while it is still only one thread and before it uses 191 libcurl at all. Call curl_global_cleanup(3) immediately before the 192 program exits, when the program is again only one thread and after its last 193 use of libcurl. 194 195 It is not actually required that the functions be called at the beginning 196 and end of the program -- that is just usually the easiest way to do it. 197 198 You can call both of these multiple times, as long as all calls meet 199 these requirements and the number of calls to each is the same. 200 201 The global constant situation merits special consideration when the code you 202 are writing to use libcurl is not the main program, but rather a modular piece 203 of a program, e.g. another library. As a module, your code does not know about 204 other parts of the program -- it does not know whether they use libcurl or 205 not. Its code does not necessarily run at the start and end of the whole 206 program. 207 208 A module like this must have global constant functions of its own, just like 209 curl_global_init(3) and curl_global_cleanup(3). The module thus 210 has control at the beginning and end of the program and has a place to call 211 the libcurl functions. If multiple modules in the program use libcurl, they 212 all separately call the libcurl functions, and that is OK because only the 213 first curl_global_init(3) and the last curl_global_cleanup(3) in a 214 program change anything. (libcurl uses a reference count in static memory). 215 216 In a C++ module, it is common to deal with the global constant situation by 217 defining a special class that represents the global constant environment of 218 the module. A program always has exactly one object of the class, in static 219 storage. That way, the program automatically calls the constructor of the 220 object as the program starts up and the destructor as it terminates. As the 221 author of this libcurl-using module, you can make the constructor call 222 curl_global_init(3) and the destructor call curl_global_cleanup(3) 223 and satisfy libcurl's requirements without your user having to think about it. 224 (Caveat: If you are initializing libcurl from a Windows DLL you should not 225 initialize it from *DllMain* or a static initializer because Windows holds 226 the loader lock during that time and it could cause a deadlock.) 227 228 curl_global_init(3) has an argument that tells what particular parts of 229 the global constant environment to set up. In order to successfully use any 230 value except *CURL_GLOBAL_ALL* (which says to set up the whole thing), you 231 must have specific knowledge of internal workings of libcurl and all other 232 parts of the program of which it is part. 233 234 A special part of the global constant environment is the identity of the 235 memory allocator. curl_global_init(3) selects the system default memory 236 allocator, but you can use curl_global_init_mem(3) to supply one of your 237 own. However, there is no way to use curl_global_init_mem(3) in a 238 modular program -- all modules in the program that might use libcurl would 239 have to agree on one allocator. 240 241 There is a failsafe in libcurl that makes it usable in simple situations 242 without you having to worry about the global constant environment at all: 243 curl_easy_init(3) sets up the environment itself if it has not been done 244 yet. The resources it acquires to do so get released by the operating system 245 automatically when the program exits. 246 247 This failsafe feature exists mainly for backward compatibility because there 248 was a time when the global functions did not exist. Because it is sufficient 249 only in the simplest of programs, it is not recommended for any program to 250 rely on it.