Linux C语言 信号量 sem_init() sem_wait() sem_timedwait() sem_post() sem_destroy()

sem_init

SEM_INIT(3)                                                          Linux Programmer's Manual                                                         SEM_INIT(3)

NAME
       sem_init - initialize an unnamed semaphore	//初始化一个未命名的信号量

SYNOPSIS
       #include <semaphore.h>

       int sem_init(sem_t *sem, int pshared, unsigned int value);

       Link with -pthread.

DESCRIPTION
       sem_init() initializes the unnamed semaphore at the address pointed to by sem.  The value argument specifies the initial value for the semaphore.
       //在 sem 指向的地址初始化未命名的信号量。 
       //value 参数指定信号量的初始值。

       The pshared argument indicates whether this semaphore is to be shared between the threads of a process, or between processes.
       //pshared 参数指示该信号量是在进程的线程之间还是在进程之间共享。

       If  pshared  has  the  value 0, then the semaphore is shared between the threads of a process, and should be located at some address that is visible to all
       threads (e.g., a global variable, or a variable allocated dynamically on the heap).
       //如果 pshared 的值为 0，则信号量在进程的线程之间共享，并且应该位于对所有线程可见的某个地址
       //（例如，全局变量或在堆上动态分配的变量）。

       If pshared is nonzero, then the semaphore is shared between processes, and should be located in a region of shared memory (see  shm_open(3),  mmap(2),  and
       shmget(2)).   (Since  a child created by fork(2) inherits its parent's memory mappings, it can also access the semaphore.)  Any process that can access the
       shared memory region can operate on the semaphore using sem_post(3), sem_wait(3), and so on.
       //如果 pshared 不为零，则信号量在进程之间共享，并且应该位于共享内存的区域中
       //（请参阅 shm_open(3)、mmap(2) 和 shmget(2)）。 
       //（由于 fork(2) 创建的子代继承了其父代的内存映射，它也可以访问信号量。）
	   //任何可以访问共享内存区域的进程都可以使用 sem_post(3)、sem_wait(3) 等对信号量进行操作。

       Initializing a semaphore that has already been initialized results in undefined behavior.
       //初始化已经初始化的信号量会导致未定义的行为。

RETURN VALUE
       sem_init() returns 0 on success; on error, -1 is returned, and errno is set to indicate the error.
       //sem_init() 成功返回 0； 出错时，返回 -1，并设置 errno 以指示错误。

ERRORS
       EINVAL value exceeds SEM_VALUE_MAX.

       ENOSYS pshared is nonzero, but the system does not support process-shared semaphores (see sem_overview(7)).

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌───────────┬───────────────┬─────────┐
       │Interface  │ Attribute     │ Value   │
       ├───────────┼───────────────┼─────────┤
       │sem_init() │ Thread safety │ MT-Safe │
       └───────────┴───────────────┴─────────┘
CONFORMING TO
       POSIX.1-2001.

NOTES
       Bizarrely, POSIX.1-2001 does not specify the value that should be returned by a successful call to sem_init().  POSIX.1-2008 rectifies this, specifying the
       zero return on success.
       //奇怪的是，POSIX.1-2001 没有指定成功调用 sem_init() 应该返回的值。 
       //POSIX.1-2008 纠正了这一点，指定了成功时返回0。

SEE ALSO
       sem_destroy(3), sem_post(3), sem_wait(3), sem_overview(7)

COLOPHON
       This  page  is part of release 4.04 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the latest version
       of this page, can be found at http://www.kernel.org/doc/man-pages/.

Linux                                                                       2015-03-02                                                                 SEM_INIT(3)
 Manual page sem_init(3) line 16/57 (END) (press h for help or q to quit)

sem_wait、sem_timedwait

SEM_WAIT(3)                                                          Linux Programmer's Manual                                                         SEM_WAIT(3)

NAME
       sem_wait, sem_timedwait, sem_trywait - lock a semaphore

SYNOPSIS
       #include <semaphore.h>

       int sem_wait(sem_t *sem);

       int sem_trywait(sem_t *sem);

       int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout);

       Link with -pthread.

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       sem_timedwait(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600

DESCRIPTION
       sem_wait()  decrements  (locks) the semaphore pointed to by sem.  If the semaphore's value is greater than zero, then the decrement proceeds, and the func‐
       tion returns, immediately.  If the semaphore currently has the value zero, then the call blocks until either it becomes possible to perform  the  decrement
       (i.e., the semaphore value rises above zero), or a signal handler interrupts the call.

       sem_trywait()  is  the  same  as sem_wait(), except that if the decrement cannot be immediately performed, then call returns an error (errno set to EAGAIN)
       instead of blocking.

       sem_timedwait() is the same as sem_wait(), except that abs_timeout specifies a limit on the amount of time that the call should block if the decrement can‐
       not  be  immediately  performed.   The  abs_timeout  argument points to a structure that specifies an absolute timeout in seconds and nanoseconds since the
       Epoch, 1970-01-01 00:00:00 +0000 (UTC).  This structure is defined as follows:

           struct timespec {
               time_t tv_sec;      /* Seconds */
               long   tv_nsec;     /* Nanoseconds [0 .. 999999999] */
           };

       If the timeout has already expired by the time of the call, and the semaphore could not be locked immediately, then sem_timedwait() fails  with  a  timeout
       error (errno set to ETIMEDOUT).

       If the operation can be performed immediately, then sem_timedwait() never fails with a timeout error, regardless of the value of abs_timeout.  Furthermore,
       the validity of abs_timeout is not checked in this case.

RETURN VALUE
       All of these functions return 0 on success; on error, the value of the semaphore is left unchanged, -1 is returned, and errno is set to indicate the error.

ERRORS
       EINTR  The call was interrupted by a signal handler; see signal(7).

       EINVAL sem is not a valid semaphore.

       The following additional error can occur for sem_trywait():

       EAGAIN The operation could not be performed without blocking (i.e., the semaphore currently has the value zero).

       The following additional errors can occur for sem_timedwait():

       EINVAL The value of abs_timeout.tv_nsecs is less than 0, or greater than or equal to 1000 million.

       ETIMEDOUT
              The call timed out before the semaphore could be locked.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌───────────────────────────┬───────────────┬─────────┐
       │Interface                  │ Attribute     │ Value   │
       ├───────────────────────────┼───────────────┼─────────┤
       │sem_wait(), sem_trywait(), │ Thread safety │ MT-Safe │
       │sem_timedwait()            │               │         │
       └───────────────────────────┴───────────────┴─────────┘
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008.

NOTES
       A signal handler always interrupts a blocked call to one of these functions, regardless of the use of the sigaction(2) SA_RESTART flag.

EXAMPLE
       The (somewhat trivial) program shown below operates on an unnamed semaphore.  The program expects two command-line arguments.  The first argument specifies
       a seconds value that is used to set an alarm timer to generate a SIGALRM signal.  This handler performs a sem_post(3) to increment the  semaphore  that  is
       being  waited  on  in main() using sem_timedwait().  The second command-line argument specifies the length of the timeout, in seconds, for sem_timedwait().
       The following shows what happens on two different runs of the program:

           $ ./a.out 2 3
           About to call sem_timedwait()
           sem_post() from handler
           sem_timedwait() succeeded
           $ ./a.out 2 1
           About to call sem_timedwait()
           sem_timedwait() timed out

   Program source

       #include <unistd.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <semaphore.h>
       #include <time.h>
       #include <assert.h>
       #include <errno.h>
       #include <signal.h>

       sem_t sem;

       #define handle_error(msg) \
           do { perror(msg); exit(EXIT_FAILURE); } while (0)

       static void
       handler(int sig)
       {
           write(STDOUT_FILENO, "sem_post() from handler\n", 24);
           if (sem_post(&sem) == -1) {
               write(STDERR_FILENO, "sem_post() failed\n", 18);
               _exit(EXIT_FAILURE);
           }
       }

       int
       main(int argc, char *argv[])
       {
           struct sigaction sa;
           struct timespec ts;
           int s;

           if (argc != 3) {
               fprintf(stderr, "Usage: %s <alarm-secs> <wait-secs>\n",
                       argv[0]);
               exit(EXIT_FAILURE);
           }

           if (sem_init(&sem, 0, 0) == -1)
               handle_error("sem_init");

           /* Establish SIGALRM handler; set alarm timer using argv[1] */

           sa.sa_handler = handler;
           sigemptyset(&sa.sa_mask);
           sa.sa_flags = 0;
           if (sigaction(SIGALRM, &sa, NULL) == -1)
               handle_error("sigaction");

           alarm(atoi(argv[1]));

           /* Calculate relative interval as current time plus
              number of seconds given argv[2] */

           if (clock_gettime(CLOCK_REALTIME, &ts) == -1)
               handle_error("clock_gettime");

           ts.tv_sec += atoi(argv[2]);

           printf("main() about to call sem_timedwait()\n");
           while ((s = sem_timedwait(&sem, &ts)) == -1 && errno == EINTR)
               continue;       /* Restart if interrupted by handler */

           /* Check what happened */

           if (s == -1) {
               if (errno == ETIMEDOUT)
                   printf("sem_timedwait() timed out\n");
               else
                   perror("sem_timedwait");
           } else
               printf("sem_timedwait() succeeded\n");

           exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
       }

SEE ALSO
       clock_gettime(2), sem_getvalue(3), sem_post(3), sem_overview(7), time(7)

COLOPHON
       This page is part of release 4.04 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the  latest  version
       of this page, can be found at http://www.kernel.org/doc/man-pages/.

Linux                                                                       2015-08-08                                                                 SEM_WAIT(3)
 Manual page sem_wait(3) line 135/176 (END) (press h for help or q to quit)

sem_post

SEM_POST(3)                                                          Linux Programmer's Manual                                                         SEM_POST(3)

NAME
       sem_post - unlock a semaphore

SYNOPSIS
       #include <semaphore.h>

       int sem_post(sem_t *sem);

       Link with -pthread.

DESCRIPTION
       sem_post()  increments (unlocks) the semaphore pointed to by sem.  If the semaphore's value consequently becomes greater than zero, then another process or
       thread blocked in a sem_wait(3) call will be woken up and proceed to lock the semaphore.

RETURN VALUE
       sem_post() returns 0 on success; on error, the value of the semaphore is left unchanged, -1 is returned, and errno is set to indicate the error.

ERRORS
       EINVAL sem is not a valid semaphore.

       EOVERFLOW
              The maximum allowable value for a semaphore would be exceeded.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌───────────┬───────────────┬─────────┐
       │Interface  │ Attribute     │ Value   │
       ├───────────┼───────────────┼─────────┤
       │sem_post() │ Thread safety │ MT-Safe │
       └───────────┴───────────────┴─────────┘
CONFORMING TO
       POSIX.1-2001.

NOTES
       sem_post() is async-signal-safe: it may be safely called within a signal handler.

EXAMPLE
       See sem_wait(3).

SEE ALSO
       sem_getvalue(3), sem_wait(3), sem_overview(7)

COLOPHON
       This page is part of release 4.04 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the  latest  version
       of this page, can be found at http://www.kernel.org/doc/man-pages/.

Linux                                                                       2015-03-02                                                                 SEM_POST(3)
 Manual page sem_post(3) line 9/50 (END) (press h for help or q to quit)

sem_destroy

SEM_DESTROY(3)                                                       Linux Programmer's Manual                                                      SEM_DESTROY(3)

NAME
       sem_destroy - destroy an unnamed semaphore

SYNOPSIS
       #include <semaphore.h>

       int sem_destroy(sem_t *sem);

       Link with -pthread.

DESCRIPTION
       sem_destroy() destroys the unnamed semaphore at the address pointed to by sem.

       Only a semaphore that has been initialized by sem_init(3) should be destroyed using sem_destroy().

       Destroying a semaphore that other processes or threads are currently blocked on (in sem_wait(3)) produces undefined behavior.

       Using a semaphore that has been destroyed produces undefined results, until the semaphore has been reinitialized using sem_init(3).

RETURN VALUE
       sem_destroy() returns 0 on success; on error, -1 is returned, and errno is set to indicate the error.

ERRORS
       EINVAL sem is not a valid semaphore.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────┬───────────────┬─────────┐
       │Interface     │ Attribute     │ Value   │
       ├──────────────┼───────────────┼─────────┤
       │sem_destroy() │ Thread safety │ MT-Safe │
       └──────────────┴───────────────┴─────────┘
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008.

NOTES
       An  unnamed  semaphore  should  be  destroyed with sem_destroy() before the memory in which it is located is deallocated.  Failure to do this can result in
       resource leaks on some implementations.

SEE ALSO
       sem_init(3), sem_post(3), sem_wait(3), sem_overview(7)

COLOPHON
       This page is part of release 4.04 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the  latest  version
       of this page, can be found at http://www.kernel.org/doc/man-pages/.

Linux                                                                       2015-08-08                                                              SEM_DESTROY(3)
 Manual page sem_destroy(3) line 9/50 (END) (press h for help or q to quit)