Книга: Programming with POSIX® Threads

9.3.1 Error detection and reporting

9.3.1 Error detection and reporting

The POSIX standard distinguishes carefully between two categories of error:

1. Mandatory ("if occurs") errors involve circumstances beyond the control of the programmer. These errors must always be detected and reported by the system using a particular error code. If you cannot create a new thread because your process lacks sufficient virtual memory, then the implementation must always tell you. You can't possibly be expected to check whether there's enough memory before creating the thread—for one thing, you have no way to know how much memory would be required.

2. Optional ("if detected") errors are problems that are usually your mistake. You might try to lock a mutex that hadn't been initialized, for example, or try to unlock a mutex that's locked by another thread. Some systems may

not detect these errors, but they're still errors in your code, and you ought to be able to avoid them without help from the system.

While it would be "nice" for the system to detect optional errors and return the appropriate error number, sometimes it takes a lot of time to check or is difficult to check reliably. It may be expensive, for example, for the system to determine the identity of the current thread. Systems may therefore not remember which thread locked a mutex, and would be unable to detect that the unlock was erroneous. It may not make sense to slow down the basic synchronization operations for correct programs just to make it a little easier to debug incorrect programs.

Systems may provide debugging modes where some or all of the optional errors are detected. Digital UNIX, for example, provides "error check" mutexes and a "metered" execution mode, where the ownership of mutexes is always tracked and optional errors in locking and unlocking mutexes are reported. The UNLX98 specification includes "error check" mutexes (Section 10.1.2), so they will soon be available on most UNIX systems.

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