16 Library introduction [library]

16.4 Library-wide requirements [requirements]

16.4.5 Constraints on programs [constraints]

16.4.5.10 Library object access [res.on.objects]

The behavior of a program is undefined if calls to standard library functions from different threads may introduce a data race.

The conditions under which this may occur are specified in [res.on.data.races].

[Note 1:

Modifying an object of a standard library type that is shared between threads risks undefined behavior unless objects of that type are explicitly specified as being shareable without data races or the user supplies a locking mechanism.

— end note]

If an object of a standard library type is accessed, and the beginning of the object's lifetime ([basic.life]) does not happen before the access, or the access does not happen before the end of the object's lifetime, the behavior is undefined unless otherwise specified.

[Note 2:

This applies even to objects such as mutexes intended for thread synchronization.

— end note]