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e3fe020089
Remove the PROTOCOLS section from the source files completely and instead generate them based on the header data in the curldown files. It also generates TLS backend information for options marked for TLS as protocol. Closes #13175
250 lines
10 KiB
Markdown
250 lines
10 KiB
Markdown
---
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c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
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SPDX-License-Identifier: curl
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Title: libcurl
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Section: 3
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Source: libcurl
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See-also:
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- libcurl-easy (3)
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- libcurl-multi (3)
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- libcurl-security (3)
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- libcurl-thread (3)
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Protocol:
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- All
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---
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# NAME
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libcurl - client-side URL transfers
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# 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. See
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libcurl-easy(3), libcurl-multi(3), libcurl-share(3),
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libcurl-url(3), libcurl-ws(3) and libcurl-tutorial(3) 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 favorite
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language. Look elsewhere for documentation on those.
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# TRANSFERS
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To transfer files, you create an "easy handle" using curl_easy_init(3)
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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 curl_easy_setopt(3). Options
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you set with curl_easy_setopt(3) stick. They are then used for 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 curl_easy_reset(3).
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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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curl_easy_perform(3) 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 libcurl-easy(3) 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 libcurl-multi(3) man page.
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# SUPPORT INTERFACES
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There is also a series of other helpful functions and interface families to
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use, including these:
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## curl_version_info()
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gets detailed libcurl (and other used libraries) version info. See
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curl_version_info(3)
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## curl_getdate()
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converts a date string to time_t. See curl_getdate(3)
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## curl_easy_getinfo()
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get information about a performed transfer. See curl_easy_getinfo(3)
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## curl_mime_addpart()
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helps building an HTTP form POST. See curl_mime_addpart(3)
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## curl_slist_append()
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builds a linked list. See curl_slist_append(3)
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## Sharing data between transfers
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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 libcurl-share(3) man page.
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## URL Parsing
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URL parsing and manipulations. See libcurl-url(3)
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## WebSocket communication
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See libcurl-ws(3)
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# LINKING WITH LIBCURL
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On unix-like machines, there is 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 have installed. See the
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*curl-config(1)* 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 do not 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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# 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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# PORTABILITY
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libcurl works
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**exactly**
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the same, on any of the platforms it compiles and builds on.
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# THREADS
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libcurl is thread safe but there are a few exceptions. Refer to
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libcurl-thread(3) for more information.
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# PERSISTENT CONNECTIONS
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Persistent connections means that libcurl can reuse the same connection for
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several transfers, if the conditions are right.
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libcurl always attempts to use persistent connections. Whenever you use
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curl_easy_perform(3) or curl_multi_perform(3) etc, libcurl
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attempts to use an existing connection to do the transfer, and if none exists
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it opens a new one that is subject for reuse on a possible following call to
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curl_easy_perform(3) or curl_multi_perform(3).
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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 curl_easy_cleanup(3), all
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the possibly open connections held by libcurl are closed and forgotten.
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When you have 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 do not affect them. Instead all added easy
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handles can take advantage of the single shared pool.
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# 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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curl_global_init(3) 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 curl_global_cleanup(3) releases them.
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If libcurl was compiled with support for multiple SSL backends, the function
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curl_global_sslset(3) can be called before curl_global_init(3)
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to select the active SSL backend.
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The global constant functions are thread-safe since libcurl 7.84.0 if
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curl_version_info(3) has the CURL_VERSION_THREADSAFE feature bit set
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(most platforms). Read libcurl-thread(3) for thread safety guidelines.
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If the global constant functions are *not thread safe*, then you must
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not call them when any other thread in the program is running. It
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is not 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 cannot
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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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If the global constant functions are *not thread safe*, then the basic rule
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for constructing a program that uses libcurl is this: Call
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curl_global_init(3), with a *CURL_GLOBAL_ALL* 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 curl_global_cleanup(3) 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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It is not actually required that the functions be called at the beginning
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and end of the program -- that is just usually the easiest way to do it.
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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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The global constant situation merits special consideration when the code you
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are writing to use libcurl is not the main program, but rather a modular piece
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of a program, e.g. another library. As a module, your code does not know about
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other parts of the program -- it does not know whether they use libcurl or
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not. Its code does not necessarily run at the start and end of the whole
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program.
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A module like this must have global constant functions of its own, just like
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curl_global_init(3) and curl_global_cleanup(3). 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. If multiple modules in the program use libcurl, they
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all separately call the libcurl functions, and that is OK because only the
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first curl_global_init(3) and the last curl_global_cleanup(3) in a
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program change anything. (libcurl uses a 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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curl_global_init(3) and the destructor call curl_global_cleanup(3)
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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
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the loader lock during that time and it could cause a deadlock.)
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curl_global_init(3) 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 *CURL_GLOBAL_ALL* (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. curl_global_init(3) selects the system default memory
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allocator, but you can use curl_global_init_mem(3) to supply one of your
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own. However, there is no way to use curl_global_init_mem(3) 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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curl_easy_init(3) sets up the environment itself if it has not 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 there
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was a time when the global functions did not exist. Because it is sufficient
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only in the simplest of programs, it is not recommended for any program to
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rely on it.
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