$Id$ _ _ ____ _ ___| | | | _ \| | / __| | | | |_) | | | (__| |_| | _ <| |___ \___|\___/|_| \_\_____| PROGRAMMING WITH LIBCURL About this Document This document will attempt to describe the general principle and some basic approaches to consider when programming with libcurl. The text will focus mainly on the C/C++ interface but might apply fairly well on other interfaces as well as they usually follow the C one pretty closely. This document will refer to 'the user' as the person writing the source code that uses libcurl. That would probably be you or someone in your position. What will be generally refered to as 'the program' will be the collected source code that you write that is using libcurl for transfers. The program is outside libcurl and libcurl is outside of the program. Building There are many different ways to build C programs. This chapter will assume a unix-style build process. If you use a different build system, you can still read this to get general information that may apply to your environment as well. Compiling the Program Your compiler needs to know where the libcurl headers are located. Therefore you must set your compiler's include path to point to the directory where you installed them. The 'curl-config'[3] tool can be used to get this information: $ curl-config --cflags Linking the Program with libcurl When having compiled the program, you need to link your object files to create a single executable. For that to succeed, you need to link with libcurl and possibly also with other libraries that libcurl itself depends on. Like OpenSSL librararies, but even some standard OS libraries may be needed on the command line. To figure out which flags to use, once again the 'curl-config' tool comes to the rescue: $ curl-config --libs SSL or Not libcurl can be built and customized in many ways. One of the things that varies from different libraries and builds is the support for SSL-based transfers, like HTTPS and FTPS. If OpenSSL was detected properly at build-time, libcurl will be built with SSL support. To figure out if an installed libcurl has been built with SSL support enabled, use 'curl-config' like this: $ curl-config --feature And if SSL is supported, the keyword 'SSL' will be written to stdout, possibly together with a few other features that can be on and off on different libcurls. Portable Code in a Portable World The people behind libcurl have put a considerable effort to make libcurl work on a large amount of different operating systems and environments. You program libcurl the same way on all platforms that libcurl runs on. There are only very few minor considerations that differs. If you just make sure to write your code portable enough, you may very well create yourself a very portable program. libcurl shouldn't stop you from that. Global Preparation The program must initialize some of the libcurl functionality globally. That means it should be done exactly once, no matter how many times you intend to use the library. Once for your program's entire life time. This is done using curl_global_init() and it takes one parameter which is a bit pattern that tells libcurl what to intialize. Using CURL_GLOBAL_ALL will make it initialize all known internal sub modules, and might be a good default option. The current two bits that are specified are: CURL_GLOBAL_WIN32 which only does anything on Windows machines. When used on a Windows machine, it'll make libcurl intialize the win32 socket stuff. Without having that initialized properly, your program cannot use sockets properly. You should only do this once for each application, so if your program already does this or of another library in use does it, you should not tell libcurl to do this as well. CURL_GLOBAL_SSL which only does anything on libcurls compiled and built SSL-enabled. On these systems, this will make libcurl init OpenSSL properly for this application. This is only needed to do once for each application so if your program or another library already does this, this bit should not be needed. libcurl has a default protection mechanism that detects if curl_global_init() hasn't been called by the time curl_easy_perform() is called and if that is the case, libcurl runs the function itself with a guessed bit pattern. Please note that depending solely on this is not considered nice nor very good. When the program no longer uses libcurl, it should call curl_global_cleanup(), which is the opposite of the init call. It will then do the reversed operations to cleanup the resources the curl_global_init() call initialized. Repeated calls to curl_global_init() and curl_global_cleanup() should be avoided. They should be called once each. Handle the Easy libcurl libcurl version 7 is oriented around the so called easy interface. All operations in the easy interface are prefixed with 'curl_easy'. Future libcurls will also offer the multi interface. More about that interface, what it is targeted for and how to use it is still only debated on the libcurl mailing list and developer web pages. Join up to discuss and figure out! To use the easy interface, you must first create yourself an easy handle. You need one handle for each easy session you want to perform. Basicly, you should use one handle for every thread you plan to use for transferring. You must never share the same handle in multiple threads. Get an easy handle with easyhandle = curl_easy_init(); It returns an easy handle. Using that you proceed to the next step: setting up your preferred actions. A handle is just a logic entity for the upcoming transfer or series of transfers. One of the most basic properties to set in the handle is the URL. You set your preferred URL to transfer with CURLOPT_URL in a manner similar to: curl_easy_setopt(easyhandle, CURLOPT_URL, "http://curl.haxx.se/"); Let's assume for a while that you want to receive data as the URL indentifies a remote resource you want to get here. Since you write a sort of application that needs this transfer, I assume that you would like to get the data passed to you directly instead of simply getting it passed to stdout. So, you write your own function that matches this prototype: size_t write_data(void *buffer, size_t size, size_t nmemb, void *userp); You tell libcurl to pass all data to this function by issuing a function similar to this: curl_easy_setopt(easyhandle, CURLOPT_WRITEFUNCTION, write_data); You can control what data your function get in the forth argument by setting another property: curl_easy_setopt(easyhandle, CURLOPT_FILE, &internal_struct); Using that property, you can easily pass local data between your application and the function that gets invoked by libcurl. libcurl itself won't touch the data you pass with CURLOPT_FILE. libcurl offers its own default internal callback that'll take care of the data if you don't set the callback with CURLOPT_WRITEFUNCTION. It will then simply output the received data to stdout. You can have the default callback write the data to a different file handle by passing a 'FILE *' to a file opened for writing with the CURLOPT_FILE option. Now, we need to take a step back and have a deep breath. Here's one of those rare platform-dependent nitpicks. Did you spot it? On some platforms[2], libcurl won't be able to operate on files opened by the program. Thus, if you use the default callback and pass in a an open file with CURLOPT_FILE, it will crash. You should therefore avoid this to make your program run fine virtually everywhere. There are of course many more options you can set, and we'll get back to a few of them later. Let's instead continue to the actual transfer: success = curl_easy_perform(easyhandle); The curl_easy_perform() will connect to the remote site, do the necessary commands and receive the transfer. Whenever it receives data, it calls the callback function we previously set. The function may get one byte at a time, or it may get many kilobytes at once. libcurl delivers as much as possible as often as possible. Your callback function should return the number of bytes it "took care of". If that is not the exact same amount of bytes that was passed to it, libcurl will abort the operation and return with an error code. When the transfer is complete, the function returns a return code that informs you if it succeeded in its mission or not. If a return code isn't enough for you, you can use the CURLOPT_ERRORBUFFER to point libcurl to a buffer of yours where it'll store a human readable error message as well. If you then want to transfer another file, the handle is ready to be used again. Mind you, it is even preferred that you re-use an existing handle if you intend to make another transfer. libcurl will then attempt to re-use the previous When It Doesn't Work There will always be times when the transfer fails for some reason. You might have set the wrong libcurl option or misunderstood what the libcurl option actually does, or the remote server might return non-standard replies that confuse the library which then confuses your program. There's one golden rule when these things occur: set the CURLOPT_VERBOSE option to TRUE. It'll cause the library to spew out the entire protocol details it sends, some internal info and some received protcol data as well (especially when using FTP). If you're using HTTP, adding the headers in the received output to study is also a clever way to get a better understanding wht the server behaves the way it does. Include headers in the normal body output with CURLOPT_HEADER set TRUE. Of course there are bugs left. We need to get to know about them to be able to fix them, so we're quite dependent on your bug reports! When you do report suspected bugs in libcurl, please include as much details you possibly can: a protocol dump that CURLOPT_VERBOSE produces, library version, as much as possible of your code that uses libcurl, operating system name and version, compiler name and version etc. Upload Data to a Remote Site libcurl tries to keep a protocol independent approach to most transfers, thus uploading to a remote FTP site is very similar to uploading data to a HTTP server with a PUT request. Of course, first you either create an easy handle or you re-use one existing one. Then you set the URL to operate on just like before. This is the remote URL, that we now will upload. Since we write an application, we most likely want libcurl to get the upload data by asking us for it. To make it do that, we set the read callback and the custom pointer libcurl will pass to our read callback. The read callback should have a prototype similar to: size_t function(char *bufptr, size_t size, size_t nitems, void *userp); Where bufptr is the pointer to a buffer we fill in with data to upload and size*nitems is the size of the buffer and therefore also the maximum amount of data we can return to libcurl in this call. The 'userp' pointer is the custom pointer we set to point to a struct of ours to pass private data between the application and the callback. curl_easy_setopt(easyhandle, CURLOPT_READFUNCTION, read_function); curl_easy_setopt(easyhandle, CURLOPT_INFILE, &filedata); Tell libcurl that we want to upload: curl_easy_setopt(easyhandle, CURLOPT_UPLOAD, TRUE); A few protocols won't behave properly when uploads are done without any prior knowledge of the expected file size. HTTP PUT is one example [1]. So, set the upload file size using the CURLOPT_INFILESIZE like this: curl_easy_setopt(easyhandle, CURLOPT_INFILESIZE, file_size); When you call curl_easy_perform() this time, it'll perform all the necessary operations and when it has invoked the upload it'll call your supplied callback to get the data to upload. The program should return as much data as possible in every invoke, as that is likely to make the upload perform as fast as possible. The callback should return the number of bytes it wrote in the buffer. Returning 0 will signal the end of the upload. Passwords Many protocols use or even require that user name and password are provided to be able to download or upload the data of your choice. libcurl offers several ways to specify them. Most protocols support that you specify the name and password in the URL itself. libcurl will detect this and use them accordingly. This is written like this: protocol://user:password@example.com/path/ If you need any odd letters in your user name or password, you should enter them URL encoded, as %XX where XX is a two-digit hexadecimal number. libcurl also provides options to set various passwords. The user name and password as shown embedded in the URL can instead get set with the CURLOPT_USERPWD option. The argument passed to libcurl should be a char * to a string in the format "user:password:". In a manner like this: curl_easy_setopt(easyhandle, CURLOPT_USERPWD, "myname:thesecret"); Another case where name and password might be needed at times, is for those users who need to athenticate themselves to a proxy they use. libcurl offers another option for this, the CURLOPT_PROXYUSERPWD. It is used quite similar to the CURLOPT_USERPWD option like this: curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "myname:thesecret"); [ more options, setting passsword callback ] HTTP POSTing We get many questions regarding how to issue HTTP POSTs with libcurl the proper way. This chapter will thus include examples using both different versions of HTTP POST that libcurl supports. The first version is the simple POST, the most common version, that most HTML pages using the
tag uses. We provide a pointer to the data and tell libcurl to post it all to the remote site: char *data="name=daniel&project=curl"; curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, data); curl_easy_setopt(easyhandle, CURLOPT_URL, "http://posthere.com/"); curl_easy_perform(easyhandle); /* post away! */ Simple enough, huh? Ok, so what if you want to post binary data that also requires you to set the Content-Type: header of the post? Well, binary posts prevents libcurl from being able to do strlen() on the data to figure out the size, so therefore we must tell libcurl the size of the post data. Setting headers in libcurl requests are done in a generic way, by building a list of our own headers and then passing that list to libcurl. struct curl_slist *headers=NULL; headers = curl_slist_append(headers, "Content-Type: text/xml"); /* post binary data */ curl_easy_setopt(easyhandle, CURLOPT_POSTFIELD, binaryptr); /* set the size of the postfields data */ curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDSIZE, 23); /* pass our list of custom made headers */ curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers); curl_easy_perform(easyhandle); /* post away! */ curl_slist_free_all(headers); /* free the header list */ While the simple examples above cover the majority of all cases where HTTP POST operations are required, they don't do multipart formposts. Multipart formposts were introduced as a better way to post (possibly large) binary data and was first documented in the RFC1867. They're called multipart because they're built by a chain of parts, each being a single unit. Each part has its own name and contents. You can in fact create and post a multipart formpost with the regular libcurl POST support described above, but that would require that you build a formpost yourself and provide to libcurl. To make that easier, libcurl provides curl_formadd(). Using this function, you add parts to the form. When you're done adding parts, you post the whole form. The following example sets two simple text parts with plain textual contents, and then a file with binary contents and upload the whole thing. struct HttpPost *post=NULL; struct HttpPost *last=NULL; curl_formadd(&post, &last, CURLFORM_COPYNAME, "name", CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END); curl_formadd(&post, &last, CURLFORM_COPYNAME, "project", CURLFORM_COPYCONTENTS, "curl", CURLFORM_END); curl_formadd(&post, &last, CURLFORM_COPYNAME, "logotype-image", CURLFORM_FILECONTENT, "curl.png", CURLFORM_END); /* Set the form info */ curl_easy_setopt(easyhandle, CURLOPT_HTTPPOST, post); curl_easy_perform(easyhandle); /* post away! */ /* free the post data again */ curl_formfree(post); The multipart formposts are a chain of parts using MIME-style separators and headers. That means that each of these separate parts get a few headers set that describes its individual content-type, size etc. Now, to enable your application to handicraft this formpost even more, libcurl allows you to supply your own custom headers to an individual form part. You can of course supply headers to as many parts you like, but this little example will show how you have set headers to one specific part when you add that to post handle: struct curl_slist *headers=NULL; headers = curl_slist_append(headers, "Content-Type: text/xml"); curl_formadd(&post, &last, CURLFORM_COPYNAME, "logotype-image", CURLFORM_FILECONTENT, "curl.xml", CURLFORM_CONTENTHEADER, headers, CURLFORM_END); curl_easy_perform(easyhandle); /* post away! */ curl_formfree(post); /* free post */ curl_slist_free_all(post); /* free custom header list */ Showing Progress libcurl with C++ There's basicly only one thing to keep in mind when using C++ instead of C when interfacing libcurl: "The Callbacks Must Be Plain C" So if you want a write callback set in libcurl, you should put it within 'extern'. Similar to this: extern "C" { size_t write_data(void *ptr, size_t size, size_t nmemb, void *ourpointer) { /* do what you want with the data */ } } This will of course effectively turn the callback code into C. There won't be any "this" pointer available etc. Proxies [ regular http, authorization, ftp => http, SSL, tunneling ] Security Considerations [ ps output, netrc plain text, plain text protocols / base64 ] Certificates and Other SSL Tricks Future ----- Footnotes: [1] = HTTP PUT without knowing the size prior to transfer is indeed possible, but libcurl does not support the chunked transfers on uploading that is necessary for this feature to work. We'd gratefully appreciate patches that bring this functionality... [2] = This happens on Windows machines when libcurl is built and used as a DLL. However, you can still do this on Windows if you link with a static library. [3] = The curl-config tool is generated at build-time (on unix-like systems) and should be installed with the 'make install' or similar instruction that installs the library, header files, man pages etc.