Compiling the libcurl networking library_Android NDK Game Development Cookbook-QQ阅读男生历史网

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Compiling the libcurl networking library

The libcurl library is a de facto standard for native applications, which deal with numerous networking protocols. The libcurl compilation for Android on a Windows host requires some additional steps to be done. We explain them in this recipe.

Getting ready

Download the libcurl source code from the library homepage: http://curl.haxx.se/libcurl/.

How to do it...

Since the libcurl library build process is based on Autoconf, we will need to generate a 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):

>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

The configure script will generate a valid curl_config.h header file. You may find it in the accompanying materials.
Further compilation requires a usual set of Android.mk/Application.mk files, which is also available in the accompanying materials.

How it works…

A simplistic usage example looks like the following:

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. A minimalistic unsafe implementation to dump a network response to the terminal can be as follows:

size_t MemoryCallback( void* P, size_t Size, size_t Num, void* )
{
  printf( (unsigned char*)P) );
}

The retrieved data will be printed on the screen in the Windows application. The same code will work like a dummy in Android, without producing any visible side effects.

There’s more…

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”