Why does the standard C++ library use all lower case?

Main reason : To keep compatibility with the existing code, since they have done it with C also.

Also have a look at these C++ Coding standards, which presents some generic reasoning regarding the importance of convention.

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These links discusses about the naming conventions of C/C++ Standard Library.

• Naming Convention for C API
• C/C++ Library Naming Convention
• C Library Naming Convention
• Why does Java use CamelCase, while C++ doesn't ?

Convention. They've done it that way all along, since the C days and before...and didn't see a good reason to break from that convention (not to mention potentially break compatibility with tons of existing code) just to add caps to a bunch of function names.