Any of the functions defined in the C++ standard library can report a failure by throwing an exception of a type described in its Throws: paragraph, or of a type derived from a type named in the Throws: paragraph that would be caught by a handler ([except.handle]) for the base type.

Functions from the C standard library shall not throw exceptions¹⁴⁵ except when such a function calls a program-supplied function that throws an exception.¹⁴⁶

Destructor operations defined in the C++ standard library shall not throw exceptions.

Every destructor in the C++ standard library shall behave as if it had a non-throwing exception specification ([except.spec]).

Functions defined in the C++ standard library that do not have a Throws: paragraph but do have a potentially-throwing exception specification may throw implementation-defined exceptions.¹⁴⁷

Implementations should report errors by throwing exceptions of or derived from the standard exception classes ([bad.alloc], [support.exception], [std.exceptions]).

An implementation may strengthen the exception specification for a non-virtual function by adding a non-throwing exception specification.

That is, the C standard library functions can all be treated as if they are marked noexcept.

This allows implementations to make performance optimizations based on the absence of exceptions at runtime.

The functions qsort() and bsearch() ([alg.c.library]) meet this condition.

In particular, they can report a failure to allocate storage by throwing an exception of type bad_alloc, or a class derived from bad_alloc ([bad.alloc]).