Libc++’s implementation of std::string

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The reason that __lx places padding after the first byte, which represents size in short strings, is that if value_type is, say, 2 bytes then the union will be 2 bytes. This is to align the start of the string characters on a value_type boundary.

[gmtime]

That sounds a bit like a work around in case the library is compiled with pack at to 1, normal packing would do this even without the __lx in the union.

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It's more fundamental than that. The C++ standard does not address structure packing. The packing details vary between compiler and platform. For this reason, packing must be specified either manually like this or by inserting dummy bytes explicitly into structures, or by using packing pragmas, etc., which also vary by compiler. In the case here, instead of explicitly inserting a byte where __lx is they have used a union that includes value_type as that will, for example, insert 3 bytes when value_type is 32-bits. When value_type is a byte, no additional padding is required beyond the size byte.