What does that mean?

The English word monolith literally means a large single upright block of stone:

Figure 5: Corinthian columns  they're monolithic! 

On the Linux OS, applications run as independent entities called processes. A process may be single-threaded (original Unix) or multithreaded. Regardless, for now, we will consider the process as the unit of execution on Linux; a process is defined as an instance of a program in execution.

When a user-space process issues a library call, the library API, in turn, may or may not issue a system call. For example, issuing the atoi(3) API does not cause glibc to issue a system call as it does ...

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