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Pthreads: Difference between revisions
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* No, you shouldn't be using the [http://www.mjmwired.net/kernel/Documentation/volatile-considered-harmful.txt volatile keyword]. | * No, you shouldn't be using the [http://www.mjmwired.net/kernel/Documentation/volatile-considered-harmful.txt volatile keyword]. | ||
==Pthreads signal model== | |||
POSIX threads specify that all threads within a process share a PID, and that each has its own, distinct thread id. PIDs have the integer type pid_t, and only positive PIDs can define actual processes. TIDs have the opaque type pthread_t; they cannot be ordered, but can be compared for equality. | |||
Other processes can direct signals only to a PID, not the thread IDs within another process (TIDs of a given process have no meaning outside that process). Within the process, signals can be directed to particular TIDs. Use of raise(s) | |||
in a threaded program is equivalent to pthread_kill(pthread_self(),s). Use of kill(p,s) in a multithreaded program having PID p is a special case. | |||
Signal dispositions (handlers) are per-process. Signal masks are per-thread, and inherited upon thread creation (sigprocmask() is undefined in threaded programs; pthread_sigmask() must be used). Alternate signal stacks descriptors | |||
are per-thread, and inherited upon thread creation. | |||
Both the process as a whole and threads within that process have distinct sets of pending signals (signals which were blocked at the time of delivery). The former is composed of those process-directed signals which could not be | |||
dispatched to a single thread. The latter are made up of thread-directed signals which were being blocked by the specified thread. | |||
A process-directed signal will be delivered to only one particular thread. For a single-threaded process, the demultiplexing is trivial and immediate; in this case, the process's and (single) thread's pending signals are the same. Otherwise, if threads are blocking on the signal in sigwait(), one of them is arbitrarily chosen. Otherwise, if threads are not blocking the signal, one of them is arbitrarily chosen. The signal otherwise remains pending across the process until it can be directed to a thread. | |||
Synchronous signals, and those directed to a TID using pthread_kill() within the process, can be handled only by the specified thread. If the signal is masked by that thread, it will be marked pending. | |||
POSIX cancellations are dispatched the same way as a thread-directed signal. | |||
==Compiling code with pthreads== | ==Compiling code with pthreads== | ||
First off, POSIX requires _REENTRANT be defined in the scope of all code run in a multithreaded manner. This is most typically achieved via -D_REENTRANT on the gcc command line. Some gcc versions / target architectures provide a -pthread option which sets -D_REENTRANT (as well as setting up pthread linking), as evidenced below:<pre>Date: Sun, 8 Apr 2007 23:33:23 -0400 | First off, POSIX requires _REENTRANT be defined in the scope of all code run in a multithreaded manner. This is most typically achieved via -D_REENTRANT on the gcc command line. Some gcc versions / target architectures provide a -pthread option which sets -D_REENTRANT (as well as setting up pthread linking), as evidenced below:<pre>Date: Sun, 8 Apr 2007 23:33:23 -0400 | ||
