Curl

The libcurl library http://curl.haxx.se/libcurl is a free and easy to use client-side URL transfer library. It is a de facto standard for native applications, which deal with numerous networking protocols. Linux and OS X users enjoy having the library on their system, and a possibility to link against it using the -lcurl switch. Compilation of libcurl for Android on a Windows host requires some additional steps to be done. We explain them here.

The libcurl library build process is based on autoconf; we will need to generate the curl_config.h file before actually building the library. Run the configure script from the folder containing the unpacked libcurl distribution package. Cross-compilation command-line flags should be set to:

--host=arm-linux CC=arm-eabi-gcc

The -I parameter of the CPPFLAGS variable should point to the /system/core/include subfolder of your NDK folder, in our case:

CPPFLAGS="-I D:/NDK/system/core/include"

The libcurl library can be customized in many ways. We use this set of parameters (disable all protocols except HTTP and HTTPS):

>configure CC=arm-eabi-gcc --host=arm-linux --disable-tftp --disable-sspi --disable-ipv6 --disable-ldaps --disable-ldap --disable-telnet --disable-pop3 --disable-ftp --without-ssl --disable-imap --disable-smtp --disable-pop3 --disable-rtsp --disable-ares --without-ca-bundle --disable-warnings --disable-manual --without-nss --enable-shared --without-zlib --without-random --enable-threaded-resolver --with-ssl

The --with-ssl parameter enables the usage of OpenSSL library to provide secure HTTPS transfers. This library will be discussed further in this chapter. However, in order to work with SSL-encrypted connections, we need to tell libcurl where our system certificates are located. This can be done with CURL_CA_BUNDLE defined in the beginning of the curl_config.h file:

#define CURL_CA_BUNDLE "/etc/ssl/certs/ca-certificates.crt"

The configure script will generate a valid curl_config.h header file. You may find it in the book's source code bundle. Compilation of an Android static library requires a usual set of Android.mk and Application.mk files, which is also within the 1_Curl example. In the next chapter, we will learn how to use the libcurl library to download the actual content from Internet over HTTPS. However, here is a simplistic usage example to retrieve a HTTP page:

CURL* Curl = curl_easy_init();
curl_easy_setopt( Curl, CURLOPT_URL, "http://www.google.com" );
curl_easy_setopt( Curl, CURLOPT_FOLLOWLOCATION, 1 );
curl_easy_setopt( Curl, CURLOPT_FAILONERROR, true );
curl_easy_setopt( Curl, CURLOPT_WRITEFUNCTION, &MemoryCallback );
curl_easy_setopt( Curl, CURLOPT_WRITEDATA, 0 );
curl_easy_perform( Curl );
curl_easy_cleanup( Curl );

Here MemoryCallback() is a function that handles the received data. It can be as tiny as the following code fragment:

size_t MemoryCallback( void* P, size_t Size, size_t Num, void* )
{
  if ( !P ) return 0;
  printf( "%s\n", P );
}

The retrieved data will be printed on the screen in your desktop application. The same code will work like a dummy in Android, without producing any visible side effects, since the printf() function is just a dummy there.