Reentrant and thread security

Reentrant: A function can be called at the same time without any impact.
Thread Security: A function can be called by multiple threads at the same time without any impact.
Reentrant> thread security, which requires stronger conditions than thread security.

 

A function is reentrant and must be thread-safe.

A function is thread-safe and may not be reentrant.

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For example:

Malloc is not asynchronous-signal security, but thread security, so it is not Reentrant

Malloc is thread-safe because it adds a thread lock to protect the critical section, but it is not reentrant because there are static variables in it. If it is entered at the same time, this will lead to confusion.
Generally, a function that uses static variables cannot be reentrant.