- Windows vs Unix
- Persistent Connections
- multi interface/non-blocking
- SSL libraries
- Library Symbols
- Return Codes and Informationals
- Memory Debugging
- Test Suite
- Asynchronous name resolves
- Content Encoding
- Track Down Memory Leaks
- Structs in libcurl
This project is split in two. The library and the client. The client part uses the library, but the library is designed to allow other applications to use it.
The largest amount of code and complexity is in the library part.
All changes to the sources are committed to the git repository as soon as they're somewhat verified to work. Changes shall be committed as independently as possible so that individual changes can be easily spotted and tracked afterwards.
Tagging shall be used extensively, and by the time we release new archives we should tag the sources with a name similar to the released version number.
We write curl and libcurl to compile with C89 compilers. On 32-bit and up machines. Most of libcurl assumes more or less POSIX compliance but that's not a requirement.
We write libcurl to build and work with lots of third party tools, and we want it to remain functional and buildable with these and later versions (older versions may still work but is not what we work hard to maintain):
- OpenSSL 0.9.7
- GnuTLS 2.11.3
- zlib 1.1.4
- libssh2 0.16
- c-ares 1.6.0
- libidn2 2.0.0
- wolfSSL 2.0.0
- openldap 2.0
- MIT Kerberos 1.2.4
- GSKit V5R3M0
- NSS 3.14.x
- PolarSSL 1.3.0
- Heimdal ?
- nghttp2 1.0.0
On systems where configure runs, we aim at working on them all - if they have a suitable C compiler. On systems that don't run configure, we strive to keep curl running correctly on:
- Windows 98
- AS/400 V5R3M0
- Symbian 9.1
- Windows CE ?
- TPF ?
When writing code (mostly for generating stuff included in release tarballs) we use a few "build tools" and we make sure that we remain functional with these versions:
- GNU Libtool 1.4.2
- GNU Autoconf 2.57
- GNU Automake 1.7
- GNU M4 1.4
- perl 5.004
- roffit 0.5
- groff ? (any version that supports
groff -Tps -man [in] [out])
- ps2pdf (gs) ?
Windows vs Unix
There are a few differences in how to program curl the Unix way compared to the Windows way. Perhaps the four most notable details are:
Different function names for socket operations.
In curl, this is solved with defines and macros, so that the source looks the same in all places except for the header file that defines them. The macros in use are
Windows requires a couple of init calls for the socket stuff.
That's taken care of by the
curl_global_init()call, but if other libs also do it etc there might be reasons for applications to alter that behaviour.
The file descriptors for network communication and file operations are not as easily interchangeable as in Unix.
We avoid this by not trying any funny tricks on file descriptors.
When writing data to stdout, Windows makes end-of-lines the DOS way, thus destroying binary data, although you do want that conversion if it is text coming through... (sigh)
We set stdout to binary under windows
Inside the source code, We make an effort to avoid
#ifdef [Your OS]. All conditionals that deal with features should instead be in the format
#ifdef HAVE_THAT_WEIRD_FUNCTION. Since Windows can't run configure scripts, we maintain a
curl_config-win32.hfile in lib directory that is supposed to look exactly like a
curl_config.hfile would have looked like on a Windows machine!
Generally speaking: always remember that this will be compiled on dozens of operating systems. Don't walk on the edge!
(See Structs in libcurl for the separate section describing all major internal structs and their purposes.)
There are plenty of entry points to the library, namely each publicly defined
function that libcurl offers to applications. All of those functions are
rather small and easy-to-follow. All the ones prefixed with
put in the
curl_global_cleanup() should be called by the
application to initialize and clean up global stuff in the library. As of
today, it can handle the global SSL initing if SSL is enabled and it can init
the socket layer on windows machines. libcurl itself has no "global" scope.
All printf()-style functions use the supplied clones in
makes sure we stay absolutely platform independent.
curl_easy_init() allocates an internal struct and makes some
initializations. The returned handle does not reveal internals. This is the
Curl_easy struct which works as an "anchor" struct for all
functions. All connections performed will get connect-specific data allocated
that should be used for things related to particular connections/requests.
curl_easy_setopt() takes three arguments, where the option stuff must
be passed in pairs: the parameter-ID and the parameter-value. The list of
options is documented in the man page. This function mainly sets things in
curl_easy_perform() is just a wrapper function that makes use of the multi
API. It basically calls
curl_multi_perform() until the transfer is done
and then returns.
Some of the most important key functions in
url.c are called from
multi.c when certain key steps are to be made in the transfer operation.
Analyzes the URL, it separates the different components and connects to the
remote host. This may involve using a proxy and/or using SSL. The
Curl_resolv() function in
lib/hostip.c is used for looking up host
names (it does then use the proper underlying method, which may vary
between platforms and builds).
Curl_connect is done, we are connected to the remote site. Then it
is time to tell the server to get a document/file.
This function makes sure there's an allocated and initiated
struct that is used for this particular connection only (although there may
be several requests performed on the same connect). A bunch of things are
inited/inherited from the
multi_do() makes sure the proper protocol-specific function is called.
The functions are named after the protocols they handle.
The protocol-specific functions of course deal with protocol-specific
negotiations and setup. They have access to the
lib/sendf.c) function to send printf-style formatted data to the remote
host and when they're ready to make the actual file transfer they call the
Curl_setup_transfer() function (in
lib/transfer.c) to setup the
transfer and returns.
If this DO function fails and the connection is being re-used, libcurl will then close this connection, setup a new connection and re-issue the DO request on that. This is because there is no way to be perfectly sure that we have discovered a dead connection before the DO function and thus we might wrongly be re-using a connection that was closed by the remote peer.
Called during the transfer of the actual protocol payload.
During transfer, the progress functions in
lib/progress.c are called at
frequent intervals (or at the user's choice, a specified callback might get
called). The speedcheck functions in
lib/speedcheck.c are also used to
verify that the transfer is as fast as required.
Called after a transfer is done. This function takes care of everything
that has to be done after a transfer. This function attempts to leave
matters in a state so that
multi_do() should be possible to call again on
the same connection (in a persistent connection case). It might also soon
be closed with
When doing normal connections and transfers, no one ever tries to close any
connections so this is not normally called when
used. This function is only used when we are certain that no more transfers
are going to be made on the connection. It can be also closed by force, or
it can be called to make sure that libcurl doesn't keep too many
connections alive at the same time.
This function cleans up all resources that are associated with a single connection.
HTTP offers a lot and is the protocol in curl that uses the most lines of
code. There is a special file
lib/formdata.c that offers all the
multipart post functions.
base64-functions for user+password stuff (and more) is in
and all functions for parsing and sending cookies are found in
HTTPS uses in almost every case the same procedure as HTTP, with only two
exceptions: the connect procedure is different and the function used to read
or write from the socket is different, although the latter fact is hidden in
the source by the use of
Curl_read() for reading and
writing data to the remote server.
http_chunks.c contains functions that understands HTTP 1.1 chunked transfer
An interesting detail with the HTTP(S) request, is the
series of functions we use. They append data to one single buffer, and when
the building is finished the entire request is sent off in one single write.
This is done this way to overcome problems with flawed firewalls and lame
Curl_if2ip() function can be used for getting the IP number of a
specified network interface, and it resides in
Curl_ftpsendf() is used for sending FTP commands to the remote server. It
was made a separate function to prevent us programmers from forgetting that
they must be CRLF terminated. They must also be sent in one single
to make firewalls and similar happy.
Kerberos support is mainly in
lib/security.c but also
curl_sasl_gssapi.c for the email protocols and
socks_sspi.c for SOCKS5 proxy specifics.
Telnet is implemented in
file:// protocol is dealt with in
smb:// protocol is dealt with in
Everything LDAP is in
The e-mail related source code is in
URL encoding and decoding, called escaping and unescaping in the source code,
is found in
While transferring data in
Transfer() a few functions might get used.
lib/parsedate.c is for HTTP date comparisons (and
curl_getenv() which is for reading environment
variables in a neat platform independent way. That's used in the client, but
lib/url.c when checking the proxy environment variables. Note that
contrary to the normal unix
getenv(), this returns an allocated buffer that
free()ed after use.
lib/netrc.c holds the
lib/timeval.c features replacement functions for systems that don't have
gettimeofday() and a few support functions for timeval conversions.
A function named
curl_version() that returns the full curl version string
is found in
The persistent connection support in libcurl requires some considerations on how to do things inside of the library.
Curl_easystruct returned in the
curl_easy_init()call must never hold connection-oriented data. It is meant to hold the root data as well as all the options etc that the library-user may choose.
Curl_easystruct holds the "connection cache" (an array of pointers to
This enables the 'curl handle' to be reused on subsequent transfers.
When libcurl is told to perform a transfer, it first checks for an already existing connection in the cache that we can use. Otherwise it creates a new one and adds that to the cache. If the cache is full already when a new connection is added, it will first close the oldest unused one.
When the transfer operation is complete, the connection is left open. Particular options may tell libcurl not to, and protocols may signal closure on connections and then they won't be kept open, of course.
curl_easy_cleanup()is called, we close all still opened connections, unless of course the multi interface "owns" the connections.
The curl handle must be re-used in order for the persistent connections to work.
The multi interface is a non-blocking interface to the library. To make that interface work as well as possible, no low-level functions within libcurl must be written to work in a blocking manner. (There are still a few spots violating this rule.)
One of the primary reasons we introduced c-ares support was to allow the name resolve phase to be perfectly non-blocking as well.
The FTP and the SFTP/SCP protocols are examples of how we adapt and adjust the code to allow non-blocking operations even on multi-stage command- response protocols. They are built around state machines that return when they would otherwise block waiting for data. The DICT, LDAP and TELNET protocols are crappy examples and they are subject for rewrite in the future to better fit the libcurl protocol family.
Originally libcurl supported SSLeay for SSL/TLS transports, but that was then extended to its successor OpenSSL but has since also been extended to several other SSL/TLS libraries and we expect and hope to further extend the support in future libcurl versions.
To deal with this internally in the best way possible, we have a generic SSL
function API as provided by the
vtls/vtls.[ch] system, and they are the only
SSL functions we must use from within libcurl. vtls is then crafted to use
the appropriate lower-level function calls to whatever SSL library that is in
use. For example
vtls/openssl.[ch] for the OpenSSL library.
All symbols used internally in libcurl must use a
Curl_ prefix if they're
used in more than a single file. Single-file symbols must be made static.
Public ("exported") symbols must use a
curl_ prefix. (There are exceptions,
but they are to be changed to follow this pattern in future versions.) Public
API functions are marked with
CURL_EXTERN in the public header files so
that all others can be hidden on platforms where this is possible.
Return Codes and Informationals
I've made things simple. Almost every function in libcurl returns a CURLcode,
that must be
CURLE_OK if everything is OK or otherwise a suitable error
code as the
curl/curl.h include file defines. The very spot that detects an
error must use the
Curl_failf() function to set the human-readable error
In aiding the user to understand what's happening and to debug curl usage, we
must supply a fair number of informational messages by using the
Curl_infof() function. Those messages are only displayed when the user
explicitly asks for them. They are best used when revealing information that
isn't otherwise obvious.
We make an effort to not export or show internals or how internals work, as that makes it easier to keep a solid API/ABI over time. See docs/libcurl/ABI for our promise to users.
main() resides in
src/tool_hugehelp.c is automatically generated by the
script to display the complete "manual" and the
holds the functions used for the URL-"globbing" support. Globbing in the
sense that the
 expansion stuff is there.
The client mostly sets up its
config struct properly, then
it calls the
curl_easy_*() functions of the library and when it gets back
control after the
curl_easy_perform() it cleans up the library, checks
status and exits.
When the operation is done, the
ourWriteOut() function in
may be called to report about the operation. That function is using the
curl_easy_getinfo() function to extract useful information from the curl
It may loop and do all this several times if many URLs were specified on the command line or config file.
lib/memdebug.c contains debug-versions of a few functions.
Functions such as
fclose(), etc that
somehow deal with resources that might give us problems if we "leak" them.
The functions in the memdebug system do nothing fancy, they do their normal
function and then log information about what they just did. The logged data
can then be analyzed after a complete session,
memanalyze.pl is the perl script present in
tests/ that analyzes a log
file generated by the memory tracking system. It detects if resources are
allocated but never freed and other kinds of errors related to resource
Internally, definition of preprocessor symbol
DEBUGBUILD restricts code
which is only compiled for debug enabled builds. And symbol
used to differentiate code which is only used for memory
-DCURLDEBUG when compiling to enable memory debugging, this is also
switched on by running configure with
-DDEBUGBUILD when compiling to enable a debug build or run configure with
curl --version will list 'Debug' feature for debug enabled builds, and
will list 'TrackMemory' feature for curl debug memory tracking capable
builds. These features are independent and can be controlled when running
the configure script. When
--enable-debug is given both features will be
enabled, unless some restriction prevents memory tracking from being used.
The test suite is placed in its own subdirectory directly off the root in the curl archive tree, and it contains a bunch of scripts and a lot of test case data.
The main test script is
runtests.pl that will invoke test servers like
ftpserver.pl before all the test cases are performed.
The test suite currently only runs on Unix-like platforms.
You'll find a description of the test suite in the
tests/README file, and
the test case data files in the
The test suite automatically detects if curl was built with the memory debugging enabled, and if it was, it will detect memory leaks, too.
Asynchronous name resolves
libcurl can be built to do name resolves asynchronously, using either the normal resolver in a threaded manner or by using c-ares.
Build libcurl to use a c-ares
- ./configure --enable-ares=/path/to/ares/install
c-ares on win32
First I compiled c-ares. I changed the default C runtime library to be the single-threaded rather than the multi-threaded (this seems to be required to prevent linking errors later on). Then I simply build the areslib project (the other projects adig/ahost seem to fail under MSVC).
Next was libcurl. I opened
lib/config-win32.h and I added a:
#define USE_ARES 1
Next thing I did was I added the path for the ares includes to the include path, and the libares.lib to the libraries.
Lastly, I also changed libcurl to be single-threaded rather than
multi-threaded, again this was to prevent some duplicate symbol errors. I'm
not sure why I needed to change everything to single-threaded, but when I
didn't I got redefinition errors for several CRT functions (
curl_off_t is a data type provided by the external libcurl include
headers. It is the type meant to be used for the
options that end with LARGE. The type is 64-bit large on most modern
The libcurl source code offers a few functions by source only. They are not part of the official libcurl API, but the source files might be useful for others so apps can optionally compile/build with these sources to gain additional functions.
We provide them through a single header file for easy access for apps:
A macro that converts a string containing a number to a
This might use the
curlx_strtoll() function which is provided as source
code in strtoofft.c. Note that the function is only provided if no
strtoll() (or equivalent) function exist on your platform. If
is only a 32-bit number on your platform, this macro uses
Several functions will be removed from the public
curl_ name space in a
future libcurl release. They will then only become available as
functions instead. To make the transition easier, we already today provide
these functions with the
curlx_ prefix to allow sources to be built
properly with the new function names. The concerned functions are:
About content encodings
HTTP/1.1 specifies that a client may request that a server encode its
response. This is usually used to compress a response using one (or more)
encodings from a set of commonly available compression techniques. These
deflate (the zlib algorithm),
br (brotli) and
compress. A client requests that the server perform an encoding by including
Accept-Encoding header in the request document. The value of the header
should be one of the recognized tokens
deflate, ... (there's a way to
register new schemes/tokens, see sec 3.5 of the spec). A server MAY honor
the client's encoding request. When a response is encoded, the server
Content-Encoding header in the response. The value of the
Content-Encoding header indicates which encodings were used to encode the
data, in the order in which they were applied.
It's also possible for a client to attach priorities to different schemes so
that the server knows which it prefers. See sec 14.3 of RFC 2616 for more
information on the
Accept-Encoding header. See sec
18.104.22.168 of RFC 7231 for more information on the
Supported content encodings
br content encodings are supported by libcurl.
Both regular and chunked transfers work fine. The zlib library is required
gzip encodings, while the brotli decoding library is
The libcurl interface
To cause libcurl to request a content encoding use:
where string is the intended value of the
Currently, libcurl does support multiple encodings but only
understands how to process responses that use the
br content encodings, so the only values for
that will work (besides
identity, which does nothing) are
br. If a response is encoded using the
compress or methods,
libcurl will return an error indicating that the response could
not be decoded. If
<string> is NULL no
Accept-Encoding header is
<string> is a zero-length string, then an
header containing all supported encodings will be generated.
CURLOPT_ACCEPT_ENCODING must be set to any non-NULL value for
content to be automatically decoded. If it is not set and the server still
sends encoded content (despite not having been asked), the data is returned
in its raw form and the
Content-Encoding type is not checked.
The curl interface
--compressed option with curl to cause it to ask servers to
compress responses using any format supported by curl.
The main compile-time defines to keep in mind when reading the
source file are these:
this host has
getaddrinfo() and family, and thus we use that. The host may
not be able to resolve IPv6, but we don't really have to take that into
account. Hosts that aren't IPv6-enabled have
is defined if libcurl is built to use c-ares for asynchronous name resolves. This can be Windows or *nix.
is defined if libcurl is built to use threading for asynchronous name resolves. The name resolve will be done in a new thread, and the supported asynch API will be the same as for ares-builds. This is the default under (native) Windows.
If any of the two previous are defined,
CURLRES_ASYNCH is defined too. If
libcurl is not built to use an asynchronous resolver,
host*.c sources files are split up like this:
hostip.c- method-independent resolver functions and utility functions
hostasyn.c- functions for asynchronous name resolves
hostsyn.c- functions for synchronous name resolves
asyn-ares.c- functions for asynchronous name resolves using c-ares
asyn-thread.c- functions for asynchronous name resolves using threads
hostip4.c- IPv4 specific functions
hostip6.c- IPv6 specific functions
hostip.his the single united header file for all this. It defines the
CURLRES_*defines based on the
Track Down Memory Leaks
Please note that this memory leak system is not adjusted to work in more than one thread. If you want/need to use it in a multi-threaded app. Please adjust accordingly.
Rebuild libcurl with
-DCURLDEBUG (usually, rerunning configure with
--enable-debug fixes this).
make clean first, then
make so that all
files are actually rebuilt properly. It will also make sense to build
libcurl with the debug option (usually
-g to the compiler) so that
debugging it will be easier if you actually do find a leak in the library.
This will create a library that has memory debugging enabled.
Modify Your Application
Add a line in your application code:
This will make the malloc debug system output a full trace of all resource using functions to the given file name. Make sure you rebuild your program and that you link with the same libcurl you built for this purpose as described above.
Run Your Application
Run your program as usual. Watch the specified memory trace file grow.
Make your program exit and use the proper libcurl cleanup functions etc. So that all non-leaks are returned/freed properly.
Analyze the Flow
tests/memanalyze.pl perl script to analyze the dump file:
This now outputs a report on what resources that were allocated but never freed etc. This report is very fine for posting to the list!
If this doesn't produce any output, no leak was detected in libcurl. Then the leak is mostly likely to be in your code.
Implementation of the
The main ideas of this API are simply:
The application can use whatever event system it likes as it gets info from libcurl about what file descriptors libcurl waits for what action on. (The previous API returns
fd_setswhich is very
When the application discovers action on a single socket, it calls libcurl and informs that there was action on this particular socket and libcurl can then act on that socket/transfer only and not care about any other transfers. (The previous API always had to scan through all the existing transfers.)
The idea is that
curl_multi_socket_action()calls a given callback with information about what socket to wait for what action on, and the callback only gets called if the status of that socket has changed.
We also added a timer callback that makes libcurl call the application when the timeout value changes, and you set that with
CURLMOPT_TIMERFUNCTIONoption. To get this to work, Internally, there's an added struct to each easy handle in which we store an "expire time" (if any). The structs are then "splay sorted" so that we can add and remove times from the linked list and yet somewhat swiftly figure out both how long there is until the next nearest timer expires and which timer (handle) we should take care of now. Of course, the upside of all this is that we get a
curl_multi_timeout()that should also work with old-style applications that use
We created an internal "socket to easy handles" hash table that given a socket (file descriptor) returns the easy handle that waits for action on that socket. This hash is made using the already existing hash code (previously only used for the DNS cache).
To make libcurl able to report plain sockets in the socket callback, we had to re-organize the internals of the
curl_multi_fdset()etc so that the conversion from sockets to
fd_setsfor that function is only done in the last step before the data is returned. I also had to extend c-ares to get a function that can return plain sockets, as that library too returned only
fd_setsand that is no longer good enough. The changes done to c-ares are available in c-ares 1.3.1 and later.
Structs in libcurl
This section should cover 7.32.0 pretty accurately, but will make sense even for older and later versions as things don't change drastically that often.
Curl_easy struct is the one returned to the outside in the external API
CURL *. This is usually known as an easy handle in API documentations
Information and state that is related to the actual connection is in the
connectdata struct. When a transfer is about to be made, libcurl will
either create a new connection or re-use an existing one. The particular
connectdata that is used by this handle is pointed out by
Data and information that regard this particular single transfer is put in
Curl_easy struct is added to a multi handle, as it must be in
order to do any transfer, the
->multi member will point to the
struct it belongs to. The
->next members will then be used by
the multi code to keep a linked list of
Curl_easy structs that are added to
that same multi handle. libcurl always uses multi so
->multi will point
Curl_multi when a transfer is in progress.
->mstate is the multi state of this particular
multi_runsingle() is called, it will act on this handle according to which
state it is in. The mstate is also what tells which sockets to return for a
curl_multi_fdset() is called etc.
The libcurl source code generally use the name
data for the variable that
points to the
When doing multiplexed HTTP/2 transfers, each
Curl_easy is associated with
an individual stream, sharing the same connectdata struct. Multiplexing
makes it even more important to keep things associated with the right thing!
A general idea in libcurl is to keep connections around in a connection "cache" after they have been used in case they will be used again and then re-use an existing one instead of creating a new as it creates a significant performance boost.
connectdata identifies a single physical connection to a server. If
the connection can't be kept alive, the connection will be closed after use
and then this struct can be removed from the cache and freed.
Thus, the same
Curl_easy can be used multiple times and each time select
connectdata struct to use for the connection. Keep this in mind,
as it is then important to consider if options or choices are based on the
connection or the
Functions in libcurl will assume that
connectdata->data points to the
Curl_easy that uses this connection (for the moment).
As a special complexity, some protocols supported by libcurl require a
special disconnect procedure that is more than just shutting down the
socket. It can involve sending one or more commands to the server before
doing so. Since connections are kept in the connection cache after use, the
Curl_easy may no longer be around when the time comes to shut down
a particular connection. For this purpose, libcurl holds a special dummy
Curl_easy in the
Curl_multi struct to use when needed.
FTP uses two TCP connections for a typical transfer but it keeps both in this single struct and thus can be considered a single connection for most internal concerns.
The libcurl source code generally use the name
conn for the variable that
points to the connectdata.
Internally, the easy interface is implemented as a wrapper around multi interface functions. This makes everything multi interface.
Curl_multi is the multi handle struct exposed as
CURLM * in external
This struct holds a list of
Curl_easy structs that have been added to this
curl_multi_add_handle(). The start of the list is
->num_easy is a counter of added
->msglist is a linked list of messages to send back when
curl_multi_info_read() is called. Basically a node is added to that
list when an individual
Curl_easy's transfer has completed.
->hostcache points to the name cache. It is a hash table for looking up
name to IP. The nodes have a limited life time in there and this cache is
meant to reduce the time for when the same name is wanted within a short
period of time.
->timetree points to a tree of
Curl_easys, sorted by the remaining time
until it should be checked - normally some sort of timeout. Each
has one node in the tree.
->sockhash is a hash table to allow fast lookups of socket descriptor for
Curl_easy uses that descriptor. This is necessary for the
->conn_cache points to the connection cache. It keeps track of all
connections that are kept after use. The cache has a maximum size.
->closure_handle is described in the
The libcurl source code generally use the name
multi for the variable that
points to the
Each unique protocol that is supported by libcurl needs to provide at least
Curl_handler struct. It defines what the protocol is called and what
functions the main code should call to deal with protocol specific issues.
In general, there's a source file named
[protocol].c in which there's a
struct Curl_handler Curl_handler_[protocol] declared. In
then the main array with all individual
Curl_handler structs pointed to
from a single array which is scanned through when a URL is given to libcurl
to work with.
->scheme is the URL scheme name, usually spelled out in uppercase. That's
"HTTP" or "FTP" etc. SSL versions of the protocol need their own
Curl_handler setup so HTTPS separate from HTTP.
->setup_connection is called to allow the protocol code to allocate
protocol specific data that then gets associated with that
the rest of this transfer. It gets freed again at the end of the transfer.
It will be called before the
connectdata for the transfer has been
selected/created. Most protocols will allocate its private
struct [PROTOCOL] here and assign
Curl_easy->req.protop to point to it.
->connect_it allows a protocol to do some specific actions after the TCP
connect is done, that can still be considered part of the connection phase.
Some protocols will alter the
connectdata->send function pointers in this function.
->connecting is similarly a function that keeps getting called as long as
the protocol considers itself still in the connecting phase.
->do_it is the function called to issue the transfer request. What we call
the DO action internally. If the DO is not enough and things need to be kept
getting done for the entire DO sequence to complete,
->doing is then
usually also provided. Each protocol that needs to do multiple commands or
similar for do/doing need to implement their own state machines (see SCP,
SFTP, FTP). Some protocols (only FTP and only due to historical reasons) has
a separate piece of the DO state called
->doing keeps getting called while issuing the transfer request command(s)
->done gets called when the transfer is complete and DONE. That's after the
main data has been transferred.
->do_more gets called during the
DO_MORE state. The FTP protocol uses
this state when setting up the second connection.
Functions that return socket information. Which socket(s) to wait for which
action(s) during the particular multi state.
->disconnect is called immediately before the TCP connection is shutdown.
->readwrite gets called during transfer to allow the protocol to do extra
->defport is the default report TCP or UDP port this protocol uses
->protocol is one or more bits in the
CURLPROTO_* set. The SSL versions
have their "base" protocol set and then the SSL variation. Like
->flags is a bitmask with additional information about the protocol that will
make it get treated differently by the generic engine:
PROTOPT_SSL- will make it connect and negotiate SSL
PROTOPT_DUAL- this protocol uses two connections
PROTOPT_CLOSEACTION- this protocol has actions to do before closing the connection. This flag is no longer used by code, yet still set for a bunch of protocol handlers.
PROTOPT_DIRLOCK- "direction lock". The SSH protocols set this bit to limit which "direction" of socket actions that the main engine will concern itself with.
PROTOPT_NONETWORK- a protocol that doesn't use network (read
PROTOPT_NEEDSPWD- this protocol needs a password and will use a default one unless one is provided
PROTOPT_NOURLQUERY- this protocol can't handle a query part on the URL (?foo=bar)
Is a hash table with connections for later re-use. Each
Curl_easy has a
pointer to its connection cache. Each multi handle sets up a connection
cache that all added
Curl_easys share by default.
The libcurl share API allocates a
Curl_share struct, exposed to the
external API as
The idea is that the struct can have a set of its own versions of caches and
pools and then by providing this struct in the
CURLOPT_SHARE option, those
Curl_easys will use the caches/pools that this share handle
Curl_easy structs can be made to share specific things
that they otherwise wouldn't, such as cookies.
Curl_share struct can currently hold cookies, DNS cache and the SSL
This is the main cookie struct. It holds all known cookies and related
Curl_easy has its own private
CookieInfo even when
they are added to a multi handle. They can be made to share cookies by using
the share API.