20 Library introduction [library]

20.5 Library-wide requirements [requirements]

20.5.4 Constraints on programs [constraints]

20.5.4.10 Library object access [res.on.objects]

1
#
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
:
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 sharable without data races or the user supplies a locking mechanism.
— end note
 ]
2
#
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
:
This applies even to objects such as mutexes intended for thread synchronization.
— end note
 ]