Use open's flag to separate readers and writers, handler multiple ops at a same time

[hakancelikdev]

GDBM allows an application to have multiple databases open at the same time. When an application opens a GDBM database, it is designated as a reader or a writer. A GDBM database can be opened by at most one writer at a time. However, many readers may open the database simultaneously. Readers and writers can not open the GDBM database at the same time.

Speaking about application we usually mean a separate process. However, it is entirely normal for a multi-thread program to operate as a GDBM reader in one thread and writer in another, provided, of course, that the two threads don’t operate on the same database simultaneously.

https://www.gnu.org.ua/software/gdbm/manual/Intro.html