### Blocking System Calls

DCE Threads provides jacket routines that make certain system calls thread-synchronous. If calling one of these jacketed system calls would normally block the process, the jacket routine ensures that only the calling thread is blocked and that the process remains available to execute other threads. Examples of jacketed system calls include read( ), write( ), open( ), socket( ), send( ), and recv( ).

If a thread makes a call to any of the other nonjacketed blocking system calls (or if it calls one of the jacketed system calls without going through the jacket), then when the system call blocks the thread, it blocks the whole process, preventing any other threads in the process from executing. Examples of nonjacketed system calls include wait( ), sigpause( ), msgsnd( ), msgrcv( ), and semop( ).

Some care must be used when calling nonjacketed blocking system calls from a multithreaded program. Other threads in the program may not be able to tolerate not running for an extended period of time while the process blocks for the system call. If your program must make use of such system calls, the calling thread should specify a nonblocking or polling option to the system call. If the call is not successful, then the calling thread should retry; however, to prevent the retry code from becoming a hot loop, a yield or delay function call should be inserted into the path. This gives other threads in the program a chance to run between poll attempts.

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