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Is a POSIX context switch well-defined? Is it the same thing as switching threads in C? Can the C compiler generate everything for a context switch or is assembly programming still needed for a routine that switches the threads or switches the "context"? Is there even defined what is meant by "context" - isn't it the same as a thread?

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POSIX uses the term context switch for at least two different purposes, without attempting to define it rigorously (or even providing a definition):

  • switching between threads, and
  • switching between processes

Rather, POSIX assumes you already know what the term means. For instance,

3.118 CPU Time (Execution Time)

The time spent executing a process or thread, including the time spent executing system services on behalf of that process or thread. If the Threads option is supported, then the value of the CPU-time clock for a process is implementation-defined. With this definition the sum of all the execution times of all the threads in a process might not equal the process execution time, even in a single-threaded process, because implementations may differ in how they account for time during context switches or for other reasons.

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