Ubuntu Manpage: semop, semtimedop - System V semaphore operations

NAME

       semop, semtimedop - System V semaphore operations

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/sem.h>

       int semop(int semid, struct sembuf *sops, size_t nsops);
       int semtimedop(int semid, struct sembuf *sops, size_t nsops,
                      const struct timespec *_Nullable timeout);

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

       semtimedop():
           _GNU_SOURCE

DESCRIPTION

Each semaphore in a System V semaphore set has the following associated values:

unsigned short semval; /* semaphore value */
unsigned short semzcnt; /* # waiting for zero */
unsigned short semncnt; /* # waiting for increase */
pid_t sempid; /* PID of process that last
modified the semaphore value */

semop() performs operations on selected semaphores in the set indicated by semid. Each of
the nsops elements in the array pointed to by sops is a structure that specifies an
operation to be performed on a single semaphore. The elements of this structure are of
type struct sembuf, containing the following members:

unsigned short sem_num; /* semaphore number */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */

Flags recognized in sem_flg are IPC_NOWAIT and SEM_UNDO. If an operation specifies
SEM_UNDO, it will be automatically undone when the process terminates.

The set of operations contained in sops is performed in array order, and atomically, that
is, the operations are performed either as a complete unit, or not at all. The behavior
of the system call if not all operations can be performed immediately depends on the
presence of the IPC_NOWAIT flag in the individual sem_flg fields, as noted below.

Each operation is performed on the sem_num-th semaphore of the semaphore set, where the
first semaphore of the set is numbered 0. There are three types of operation,
distinguished by the value of sem_op.

If sem_op is a positive integer, the operation adds this value to the semaphore value
(semval). Furthermore, if SEM_UNDO is specified for this operation, the system subtracts
the value sem_op from the semaphore adjustment (semadj) value for this semaphore. This
operation can always proceed—it never forces a thread to wait. The calling process must
have alter permission on the semaphore set.

If sem_op is zero, the process must have read permission on the semaphore set. This is a
"wait-for-zero" operation: if semval is zero, the operation can immediately proceed.
Otherwise, if IPC_NOWAIT is specified in sem_flg, semop() fails with errno set to EAGAIN
(and none of the operations in sops is performed). Otherwise, semzcnt (the count of
threads waiting until this semaphore's value becomes zero) is incremented by one and the
thread sleeps until one of the following occurs:

• semval becomes 0, at which time the value of semzcnt is decremented.

• The semaphore set is removed: semop() fails, with errno set to EIDRM.

• The calling thread catches a signal: the value of semzcnt is decremented and semop()
fails, with errno set to EINTR.

If sem_op is less than zero, the process must have alter permission on the semaphore set.
If semval is greater than or equal to the absolute value of sem_op, the operation can
proceed immediately: the absolute value of sem_op is subtracted from semval, and, if
SEM_UNDO is specified for this operation, the system adds the absolute value of sem_op to
the semaphore adjustment (semadj) value for this semaphore. If the absolute value of
sem_op is greater than semval, and IPC_NOWAIT is specified in sem_flg, semop() fails, with
errno set to EAGAIN (and none of the operations in sops is performed). Otherwise, semncnt
(the counter of threads waiting for this semaphore's value to increase) is incremented by
one and the thread sleeps until one of the following occurs:

• semval becomes greater than or equal to the absolute value of sem_op: the operation now
proceeds, as described above.

• The semaphore set is removed from the system: semop() fails, with errno set to EIDRM.

• The calling thread catches a signal: the value of semncnt is decremented and semop()
fails, with errno set to EINTR.

On successful completion, the sempid value for each semaphore specified in the array
pointed to by sops is set to the caller's process ID. In addition, the sem_otime is set
to the current time.

semtimedop()
semtimedop() behaves identically to semop() except that in those cases where the calling
thread would sleep, the duration of that sleep is limited by the amount of elapsed time
specified by the timespec structure whose address is passed in the timeout argument.
(This sleep interval will be rounded up to the system clock granularity, and kernel
scheduling delays mean that the interval may overrun by a small amount.) If the specified
time limit has been reached, semtimedop() fails with errno set to EAGAIN (and none of the
operations in sops is performed). If the timeout argument is NULL, then semtimedop()
behaves exactly like semop().

Note that if semtimedop() is interrupted by a signal, causing the call to fail with the
error EINTR, the contents of timeout are left unchanged.

RETURN VALUE

       On success, semop() and semtimedop() return 0.  On failure, they return -1, and set  errno
       to indicate the error.

ERRORS

       E2BIG  The argument nsops is greater than SEMOPM, the maximum number of operations allowed
              per system call.

       EACCES The calling process does not have the permissions required to perform the specified
              semaphore  operations,  and  does not have the CAP_IPC_OWNER capability in the user
              namespace that governs its IPC namespace.

       EAGAIN An operation could not proceed immediately and either IPC_NOWAIT was  specified  in
              sem_flg or the time limit specified in timeout expired.

       EFAULT An address specified in either the sops or the timeout argument isn't accessible.

       EFBIG  For  some operation the value of sem_num is less than 0 or greater than or equal to
              the number of semaphores in the set.

       EIDRM  The semaphore set was removed.

       EINTR  While blocked in this system call, the thread caught a signal; see signal(7).

       EINVAL The semaphore set doesn't exist, or semid  is  less  than  zero,  or  nsops  has  a
              nonpositive value.

       ENOMEM The  sem_flg  of  some  operation  specified  SEM_UNDO and the system does not have
              enough memory to allocate the undo structure.

       ERANGE For some  operation  sem_op+semval  is  greater  than  SEMVMX,  the  implementation
              dependent maximum value for semval.

STANDARDS

       POSIX.1-2008.

VERSIONS

       Linux 2.5.52 (backported into Linux 2.4.22), glibc 2.3.3.  POSIX.1-2001, SVr4.

NOTES

The sem_undo structures of a process aren't inherited by the child produced by fork(2),
but they are inherited across an execve(2) system call.

semop() is never automatically restarted after being interrupted by a signal handler,
regardless of the setting of the SA_RESTART flag when establishing a signal handler.

A semaphore adjustment (semadj) value is a per-process, per-semaphore integer that is the
negated sum of all operations performed on a semaphore specifying the SEM_UNDO flag. Each
process has a list of semadj values—one value for each semaphore on which it has operated
using SEM_UNDO. When a process terminates, each of its per-semaphore semadj values is
added to the corresponding semaphore, thus undoing the effect of that process's operations
on the semaphore (but see BUGS below). When a semaphore's value is directly set using the
SETVAL or SETALL request to semctl(2), the corresponding semadj values in all processes
are cleared. The clone(2) CLONE_SYSVSEM flag allows more than one process to share a
semadj list; see clone(2) for details.

The semval, sempid, semzcnt, and semnct values for a semaphore can all be retrieved using
appropriate semctl(2) calls.

Semaphore limits
The following limits on semaphore set resources affect the semop() call:

SEMOPM Maximum number of operations allowed for one semop() call. Before Linux 3.19, the
default value for this limit was 32. Since Linux 3.19, the default value is 500.
On Linux, this limit can be read and modified via the third field of
/proc/sys/kernel/sem. Note: this limit should not be raised above 1000, because of
the risk of that semop() fails due to kernel memory fragmentation when allocating
memory to copy the sops array.

SEMVMX Maximum allowable value for semval: implementation dependent (32767).

The implementation has no intrinsic limits for the adjust on exit maximum value (SEMAEM),
the system wide maximum number of undo structures (SEMMNU) and the per-process maximum
number of undo entries system parameters.

BUGS

       When  a  process  terminates,  its set of associated semadj structures is used to undo the
       effect of all of the semaphore operations it  performed  with  the  SEM_UNDO  flag.   This
       raises  a  difficulty:  if one (or more) of these semaphore adjustments would result in an
       attempt to decrease a semaphore's value below zero, what should an implementation do?  One
       possible  approach  would  be  to  block  until  all  the  semaphore  adjustments could be
       performed.  This is however undesirable since it could force process termination to  block
       for  arbitrarily  long  periods.   Another  possibility is that such semaphore adjustments
       could be ignored altogether (somewhat analogously to failing when IPC_NOWAIT is  specified
       for a semaphore operation).  Linux adopts a third approach: decreasing the semaphore value
       as  far  as  possible  (i.e.,  to  zero)  and  allowing  process  termination  to  proceed
       immediately.

       In  Linux 2.6.x, x <= 10, there is a bug that in some circumstances prevents a thread that
       is waiting for a semaphore value to become zero from being woken up when  the  value  does
       actually become zero.  This bug is fixed in Linux 2.6.11.

EXAMPLES

       The following code segment uses semop() to atomically wait for the value of semaphore 0 to
       become zero, and then increment the semaphore value by one.

           struct sembuf sops[2];
           int semid;

           /* Code to set semid omitted */

           sops[0].sem_num = 0;        /* Operate on semaphore 0 */
           sops[0].sem_op = 0;         /* Wait for value to equal 0 */
           sops[0].sem_flg = 0;

           sops[1].sem_num = 0;        /* Operate on semaphore 0 */
           sops[1].sem_op = 1;         /* Increment value by one */
           sops[1].sem_flg = 0;

           if (semop(semid, sops, 2) == -1) {
               perror("semop");
               exit(EXIT_FAILURE);
           }

       A further example of the use of semop() can be found in shmop(2).