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diff --git a/docs/INTERNALS.md b/docs/INTERNALS.md new file mode 100644 index 000000000..565d9df6c --- /dev/null +++ b/docs/INTERNALS.md @@ -0,0 +1,1094 @@ +Table of Contents +================= + + - [Intro](#intro) + - [git](#git) + - [Portability](#Portability) + - [Windows vs Unix](#winvsunix) + - [Library](#Library) + - [`Curl_connect`](#Curl_connect) + - [`Curl_do`](#Curl_do) + - [`Curl_readwrite`](#Curl_readwrite) + - [`Curl_done`](#Curl_done) + - [`Curl_disconnect`](#Curl_disconnect) + - [HTTP(S)](#http) + - [FTP](#ftp) + - [Kerberos](#kerberos) + - [TELNET](#telnet) + - [FILE](#file) + - [SMB](#smb) + - [LDAP](#ldap) + - [E-mail](#email) + - [General](#general) + - [Persistent Connections](#persistent) + - [multi interface/non-blocking](#multi) + - [SSL libraries](#ssl) + - [Library Symbols](#symbols) + - [Return Codes and Informationals](#returncodes) + - [AP/ABI](#abi) + - [Client](#client) + - [Memory Debugging](#memorydebug) + - [Test Suite](#test) + - [Asynchronous name resolves](#asyncdns) + - [c-ares](#cares) + - [`curl_off_t`](#curl_off_t) + - [curlx](#curlx) + - [Content Encoding](#contentencoding) + - [hostip.c explained](#hostip) + - [Track Down Memory Leaks](#memoryleak) + - [`multi_socket`](#multi_socket) + - [Structs in libcurl](#structs) + +<a name="intro"></a> +curl internals +============== + + 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. + + +<a name="git"></a> +git +=== + + 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 easier 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. + +<a name="Portability"></a> +Portability +=========== + + We write curl and libcurl to compile with C89 compilers. On 32bit 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): + +Dependencies +------------ + + - OpenSSL 0.9.7 + - GnuTLS 1.2 + - zlib 1.1.4 + - libssh2 0.16 + - c-ares 1.6.0 + - libidn 0.4.1 + - cyassl 2.0.0 + - openldap 2.0 + - MIT Kerberos 1.2.4 + - GSKit V5R3M0 + - NSS 3.14.x + - axTLS 1.2.7 + - PolarSSL 1.3.0 + - Heimdal ? + - nghttp2 1.0.0 + +Operating Systems +----------------- + + 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 fine on: + + - Windows 98 + - AS/400 V5R3M0 + - Symbian 9.1 + - Windows CE ? + - TPF ? + +Build tools +----------- + + 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) ? + +<a name="winvsunix"></a> +Windows vs Unix +=============== + + There are a few differences in how to program curl the unix way compared to + the Windows way. The four perhaps most notable details are: + + 1. Different function names for socket operations. + + In curl, this is solved with defines and macros, so that the source looks + the same at all places except for the header file that defines them. The + macros in use are sclose(), sread() and swrite(). + + 2. 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. + + 3. The file descriptors for network communication and file operations are + not easily interchangeable as in unix. + + We avoid this by not trying any funny tricks on file descriptors. + + 4. 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.h` file in lib directory that is supposed to + look exactly as a `curl_config.h` file 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. + +<a name="Library"></a> +Library +======= + + (See [Structs in libcurl](#structs) 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 `curl_easy` are + put in the lib/easy.c file. + + `curl_global_init_()` and `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 lib/mprintf.c. This + makes sure we stay absolutely platform independent. + + [ `curl_easy_init()`][2] 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 `curl_easy` + functions. All connections performed will get connect-specific data allocated + that should be used for things related to particular connections/requests. + + [`curl_easy_setopt()`][1] 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 + the 'Curl_easy' struct. + + `curl_easy_perform()` is just a wrapper function that makes use of the multi + API. It basically calls `curl_multi_init()`, `curl_multi_add_handle()`, + `curl_multi_wait()`, and `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. + +<a name="Curl_connect"></a> +Curl_connect() +-------------- + + 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). + + When `Curl_connect` is done, we are connected to the remote site. Then it + is time to tell the server to get a document/file. `Curl_do()` arranges + this. + + This function makes sure there's an allocated and initiated 'connectdata' + 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 Curl_easy struct. + +<a name="Curl_do"></a> +Curl_do() +--------- + + `Curl_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 `Curl_sendf()` (from + 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_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. + + Some time during the DO function, the `Curl_setup_transfer()` function must + be called with some basic info about the upcoming transfer: what socket(s) + to read/write and the expected file transfer sizes (if known). + +<a name="Curl_readwrite"></a> +Curl_readwrite() +---------------- + + Called during the transfer of the actual protocol payload. + + During transfer, the progress functions in lib/progress.c are called at a + frequent interval (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. + +<a name="Curl_done"></a> +Curl_done() +----------- + + 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 `Curl_do()` should be possible to call again on + the same connection (in a persistent connection case). It might also soon + be closed with `Curl_disconnect()`. + +<a name="Curl_disconnect"></a> +Curl_disconnect() +----------------- + + When doing normal connections and transfers, no one ever tries to close any + connections so this is not normally called when `curl_easy_perform()` is + used. This function is only used when we are certain that no more transfers + is 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. + +<a name="http"></a> +HTTP(S) +======= + + 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 (lib/base64.c) and + all functions for parsing and sending cookies are found in (lib/cookie.c). + + HTTPS uses in almost every means 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 `Curl_write()` for + writing data to the remote server. + + `http_chunks.c` contains functions that understands HTTP 1.1 chunked transfer + encoding. + + An interesting detail with the HTTP(S) request, is the `Curl_add_buffer()` + series of functions we use. They append data to one single buffer, and when + the building is done the entire request is sent off in one single write. This + is done this way to overcome problems with flawed firewalls and lame servers. + +<a name="ftp"></a> +FTP +=== + + The `Curl_if2ip()` function can be used for getting the IP number of a + specified network interface, and it resides in lib/if2ip.c. + + `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 write() to + make firewalls and similar happy. + +<a name="kerberos"></a> +Kerberos +-------- + + Kerberos support is mainly in lib/krb5.c and lib/security.c but also + `curl_sasl_sspi.c` and `curl_sasl_gssapi.c` for the email protocols and + `socks_gssapi.c` and `socks_sspi.c` for SOCKS5 proxy specifics. + +<a name="telnet"></a> +TELNET +====== + + Telnet is implemented in lib/telnet.c. + +<a name="file"></a> +FILE +==== + + The file:// protocol is dealt with in lib/file.c. + +<a name="smb"></a> +SMB +=== + + The smb:// protocol is dealt with in lib/smb.c. + +<a name="ldap"></a> +LDAP +==== + + Everything LDAP is in lib/ldap.c and lib/openldap.c + +<a name="email"></a> +E-mail +====== + + The e-mail related source code is in lib/imap.c, lib/pop3.c and lib/smtp.c. + +<a name="general"></a> +General +======= + + URL encoding and decoding, called escaping and unescaping in the source code, + is found in lib/escape.c. + + While transferring data in Transfer() a few functions might get used. + `curl_getdate()` in lib/parsedate.c is for HTTP date comparisons (and more). + + lib/getenv.c offers `curl_getenv()` which is for reading environment + variables in a neat platform independent way. That's used in the client, but + also in lib/url.c when checking the proxy environment variables. Note that + contrary to the normal unix getenv(), this returns an allocated buffer that + must be free()ed after use. + + lib/netrc.c holds the .netrc parser + + 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 lib/version.c. + +<a name="persistent"></a> +Persistent Connections +====================== + + The persistent connection support in libcurl requires some considerations on + how to do things inside of the library. + + - The 'Curl_easy' struct returned in the [`curl_easy_init()`][2] 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. + + - The 'Curl_easy' struct holds the "connection cache" (an array of + pointers to 'connectdata' structs). + + - 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 the cache. If the cache is full already when a new + connection is added 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. + + - When `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. + +<a name="multi"></a> +multi interface/non-blocking +============================ + + The multi interface is a non-blocking interface to the library. To make that + interface work as good 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. + +<a name="ssl"></a> +SSL libraries +============= + + 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. + +<a name="symbols"></a> +Library Symbols +=============== + + 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. + +<a name="returncodes"></a> +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 + description. + + In aiding the user to understand what's happening and to debug curl usage, we + must supply a fair amount 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. + +<a name="abi"></a> +API/ABI +======= + + 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. + +<a name="client"></a> +Client +====== + + main() resides in `src/tool_main.c`. + + `src/tool_hugehelp.c` is automatically generated by the mkhelp.pl perl script + to display the complete "manual" and the src/tool_urlglob.c file holds the + functions used for the URL-"globbing" support. Globbing in the sense that the + {} and [] expansion stuff is there. + + The client mostly messes around to setup 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 src/writeout.c may + be called to report about the operation. That function is using the + `curl_easy_getinfo()` function to extract useful information from the curl + session. + + It may loop and do all this several times if many URLs were specified on the + command line or config file. + +<a name="memorydebug"></a> +Memory Debugging +================ + + The file lib/memdebug.c contains debug-versions of a few functions. Functions + such as malloc, free, fopen, 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 + management. + + Internally, definition of preprocessor symbol DEBUGBUILD restricts code which + is only compiled for debug enabled builds. And symbol CURLDEBUG is used to + differentiate code which is _only_ used for memory tracking/debugging. + + Use -DCURLDEBUG when compiling to enable memory debugging, this is also + switched on by running configure with --enable-curldebug. Use -DDEBUGBUILD + when compiling to enable a debug build or run configure with --enable-debug. + + 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. + +<a name="test"></a> +Test Suite +========== + + 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 + httpserver.pl and 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 tests/FILEFORMAT file. + + The test suite automatically detects if curl was built with the memory + debugging enabled, and if it was it will detect memory leaks, too. + +<a name="asyncdns"></a> +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. + +<a name="cares"></a> +[c-ares][3] +------ + +### Build libcurl to use a c-ares + +1. ./configure --enable-ares=/path/to/ares/install +2. make + +### 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 (malloc, stricmp, + etc.) + +<a name="curl_off_t"></a> +`curl_off_t` +========== + + curl_off_t is a data type provided by the external libcurl include + headers. It is the type meant to be used for the [`curl_easy_setopt()`][1] + options that end with LARGE. The type is 64bit large on most modern + platforms. + +curlx +===== + + 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: + "curlx.h" + +`curlx_strtoofft()` +------------------- + A macro that converts a string containing a number to a curl_off_t number. + 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 curl_off_t + is only a 32 bit number on your platform, this macro uses strtol(). + +`curlx_tvnow()` +--------------- + returns a struct timeval for the current time. + +`curlx_tvdiff()` +-------------- + returns the difference between two timeval structs, in number of + milliseconds. + +`curlx_tvdiff_secs()` +--------------------- + returns the same as curlx_tvdiff but with full usec resolution (as a + double) + +Future +------ + + Several functions will be removed from the public curl_ name space in a + future libcurl release. They will then only become available as curlx_ + functions instead. To make the transition easier, we already today provide + these functions with the curlx_ prefix to allow sources to get built properly + with the new function names. The functions this concerns are: + + - `curlx_getenv` + - `curlx_strequal` + - `curlx_strnequal` + - `curlx_mvsnprintf` + - `curlx_msnprintf` + - `curlx_maprintf` + - `curlx_mvaprintf` + - `curlx_msprintf` + - `curlx_mprintf` + - `curlx_mfprintf` + - `curlx_mvsprintf` + - `curlx_mvprintf` + - `curlx_mvfprintf` + +<a name="contentencoding"></a> +Content Encoding +================ + +## About content encodings + + [HTTP/1.1][4] specifies that a client may request that a server encode its + response. This is usually used to compress a response using one of a set of + commonly available compression techniques. These schemes are 'deflate' (the + zlib algorithm), 'gzip' and 'compress'. A client requests that the sever + perform an encoding by including an 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 includes a Content-Encoding header in the + response. The value of the Content-Encoding header indicates which scheme was + used to encode the data. + + A client may tell a server that it can understand several different encoding + schemes. In this case the server may choose any one of those and use it to + encode the response (indicating which one using the Content-Encoding header). + 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. + +## Supported content encodings + + The 'deflate' and 'gzip' content encoding are supported by libcurl. Both + regular and chunked transfers work fine. The zlib library is required for + this feature. + +## The libcurl interface + + To cause libcurl to request a content encoding use: + + [`curl_easy_setopt`][1](curl, [`CURLOPT_ACCEPT_ENCODING`][5], string) + + where string is the intended value of the Accept-Encoding header. + + Currently, libcurl only understands how to process responses that use the + "deflate" or "gzip" Content-Encoding, so the only values for + [`CURLOPT_ACCEPT_ENCODING`][5] that will work (besides "identity," which does + nothing) are "deflate" and "gzip" 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 generated. If <string> is a zero-length string, then an Accept-Encoding + header containing all supported encodings will be generated. + + The [`CURLOPT_ACCEPT_ENCODING`][5] 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 + + Use the [--compressed][6] option with curl to cause it to ask servers to + compress responses using any format supported by curl. + +<a name="hostip"></a> +hostip.c explained +================== + + The main compile-time defines to keep in mind when reading the host*.c source + file are these: + +## `CURLRES_IPV6` + + 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 CURLRES_IPV4 defined. + +## `CURLRES_ARES` + + is defined if libcurl is built to use c-ares for asynchronous name + resolves. This can be Windows or *nix. + +## `CURLRES_THREADED` + + 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, `CURLRES_SYNCH` is + defined. + +## host*.c sources + + The 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 + + The hostip.h is the single united header file for all this. It defines the + `CURLRES_*` defines based on the config*.h and curl_setup.h defines. + +<a name="memoryleak"></a> +Track Down Memory Leaks +======================= + +## Single-threaded + + 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. + + +## Build + + Rebuild libcurl with -DCURLDEBUG (usually, rerunning configure with + --enable-debug fixes this). 'make clean' first, then 'make' so that all + files actually are 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: + + `curl_memdebug("dump");` + + 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 + + Use the tests/memanalyze.pl perl script to analyze the dump file: + + tests/memanalyze.pl dump + + 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. + +<a name="multi_socket"></a> +`multi_socket` +============== + + Implementation of the `curl_multi_socket` API + + The main ideas of this API are simply: + + 1 - 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_sets` which is very select()-centric). + + 2 - 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()`][7] 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 [`curl_multi_setopt()`][9] + and the [`CURLMOPT_TIMERFUNCTION`][10] option. To get this to work, + Internally, there's an added a 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 time 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()`][8] that should also + work with old-style applications that use [`curl_multi_perform()`][11]. + + We created an internal "socket to easy handles" hash table that given + a socket (file descriptor) return 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()`][12] etc so that + the conversion from sockets to `fd_sets` for 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_sets` and that is no longer good enough. The changes done to c-ares + are available in c-ares 1.3.1 and later. + +<a name="structs"></a> +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 + + The Curl_easy struct is the one returned to the outside in the external API + as a "CURL *". This is usually known as an easy handle in API documentations + and examples. + + 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 + Curl_easy->easy_conn. + + Data and information that regard this particular single transfer is put in + the SingleRequest sub-struct. + + When the 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 `Curl_multi` struct + it belongs to. The ->prev and ->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 to a + `Curl_multi` when a transfer is in progress. + + ->mstate is the multi state of this particular Curl_easy. When + `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 + specific Curl_easy when [`curl_multi_fdset()`][12] is called etc. + + The libcurl source code generally use the name 'data' for the variable that + points to the Curl_easy. + + 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! + +## connectdata + + 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. + + Each '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 + another 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 Curl_easy. + + 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 + original 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 + `closure_handle` 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. + +## Curl_multi + + 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 APIs. + + This struct holds a list of Curl_easy structs that have been added to this + handle with [`curl_multi_add_handle()`][13]. The start of the list is + ->easyp and ->num_easy is a counter of added Curl_easys. + + ->msglist is a linked list of messages to send back when + [`curl_multi_info_read()`][14] 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 Curl_easy + has one node in the tree. + + ->sockhash is a hash table to allow fast lookups of socket descriptor to + which Curl_easy that uses that descriptor. This is necessary for the + `multi_socket` API. + + ->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 'connectdata' section. + + The libcurl source code generally use the name 'multi' for the variable that + points to the Curl_multi struct. + +## Curl_handler + + Each unique protocol that is supported by libcurl needs to provide at least + one `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 url.c there's + 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 protcol need its 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 Curl_easy for 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->recv[] and 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 `DO_MORE`. + + ->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. + + ->`proto_getsock` + ->`doing_getsock` + ->`domore_getsock` + ->`perform_getsock` + 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 + reads/writes + + ->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 + "HTTP|HTTPS". + + ->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 + 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 about. + + - `PROTOPT_NONETWORK` - a protocol that doesn't use network (read file:) + + - `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) + +## conncache + + 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. + +## Curl_share + + The libcurl share API allocates a `Curl_share` struct, exposed to the + external API as "CURLSH *". + + The idea is that the struct can have a set of own versions of caches and + pools and then by providing this struct in the `CURLOPT_SHARE` option, those + specific Curl_easys will use the caches/pools that this share handle + holds. + + Then individual Curl_easy structs can be made to share specific things + that they otherwise wouldn't, such as cookies. + + The `Curl_share` struct can currently hold cookies, DNS cache and the SSL + session cache. + +## CookieInfo + + This is the main cookie struct. It holds all known cookies and related + information. Each 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. + + +[1]: https://curl.haxx.se/libcurl/c/curl_easy_setopt.html +[2]: https://curl.haxx.se/libcurl/c/curl_easy_init.html +[3]: http://c-ares.haxx.se/ +[4]: https://tools.ietf.org/html/rfc7230 "RFC 7230" +[5]: https://curl.haxx.se/libcurl/c/CURLOPT_ACCEPT_ENCODING.html +[6]: https://curl.haxx.se/docs/manpage.html#--compressed +[7]: https://curl.haxx.se/libcurl/c/curl_multi_socket_action.html +[8]: https://curl.haxx.se/libcurl/c/curl_multi_timeout.html +[9]: https://curl.haxx.se/libcurl/c/curl_multi_setopt.html +[10]: https://curl.haxx.se/libcurl/c/CURLMOPT_TIMERFUNCTION.html +[11]: https://curl.haxx.se/libcurl/c/curl_multi_perform.html +[12]: https://curl.haxx.se/libcurl/c/curl_multi_fdset.html +[13]: https://curl.haxx.se/libcurl/c/curl_multi_add_handle.html +[14]: https://curl.haxx.se/libcurl/c/curl_multi_info_read.html |