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So far, all of the SSL backends' private data has been declared as part of the ssl_connect_data struct, in one big #if .. #elif .. #endif block. This can only work as long as the SSL backend is a compile-time option, something we want to change in the next commits. Therefore, let's encapsulate the exact data needed by each SSL backend into a private struct, and let's avoid bleeding any SSL backend-specific information into urldata.h. This is also necessary to allow multiple SSL backends to be compiled in at the same time, as e.g. OpenSSL's and CyaSSL's headers cannot be included in the same .c file. To avoid too many malloc() calls, we simply append the private structs to the connectdata struct in allocate_conn(). This requires us to take extra care of alignment issues: struct fields often need to be aligned on certain boundaries e.g. 32-bit values need to be stored at addresses that divide evenly by 4 (= 32 bit / 8 bit-per-byte). We do that by assuming that no SSL backend's private data contains any fields that need to be aligned on boundaries larger than `long long` (typically 64-bit) would need. Under this assumption, we simply add a dummy field of type `long long` to the `struct connectdata` struct. This field will never be accessed but acts as a placeholder for the four instances of ssl_backend_data instead. the size of each ssl_backend_data struct is stored in the SSL backend-specific metadata, to allow allocate_conn() to know how much extra space to allocate, and how to initialize the ssl[sockindex]->backend and proxy_ssl[sockindex]->backend pointers. This would appear to be a little complicated at first, but is really necessary to encapsulate the private data of each SSL backend correctly. And we need to encapsulate thusly if we ever want to allow selecting CyaSSL and OpenSSL at runtime, as their headers cannot be included within the same .c file (there are just too many conflicting definitions and declarations for that). Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>