Ubuntu Manpage: semop, semtimedop - System V semaphore operations

NAME

       semop, semtimedop - System V semaphore operations

SYNOPSIS

       #include <sys/types.h>
       #include <sys/ipc.h>
       #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 *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; /* ID of process that did last op */

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

RETURN VALUE

       If  successful,  semop()  and  semtimedop()  return 0; otherwise they return -1 with errno indicating the
       error.

ERRORS

       On failure, errno is set to one of the following:

       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.

       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.

VERSIONS

       semtimedop() first appeared in Linux 2.5.52, and was subsequently backported into kernel  2.4.22.   Glibc
       support for semtimedop() first appeared in version 2.3.3.

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, SVr4.

NOTES

The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux or by any version of POSIX.
However, some old implementations required the inclusion of these header files, and the SVID also
documented their inclusion. Applications intended to be portable to such old systems may need to include
these header files.

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(2) 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  kernels  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 kernel 2.6.11.

EXAMPLE

       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);
           }

COLOPHON

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Linux                                              2015-08-08                                           SEMOP(2)