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Use of errno by the library
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## Use of errno by the library

The C and POSIX standards guarantee that `errno` is never set to zero by any library function. The C++ standard has less to say about when `errno` is or isn't set, but libstdc++ follows the same rule and never sets it to zero.

On the other hand, there are few guarantees about when the C++ library sets `errno` on error, beyond what is specified for functions that come from the C library. For example, when `std::stoi` throws an exception of type `std::out_of_range`, `errno` may or may not have been set to `ERANGE`.

Parts of the C++ library may be implemented in terms of C library functions, which may result in `errno` being set with no explicit call to a C function. For example, on a target where `operator new` uses `malloc` a failed memory allocation with `operator new` might set `errno` to `ENOMEM`. Which C++ library functions can set `errno` in this way is unspecified because it may vary between platforms and between releases.

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