semop(2) semop(2)
NAME [Toc] [Back]
semop, semtimedop - semaphore operations
SYNOPSIS [Toc] [Back]
#include <sys/sem.h>
#include <sys/time.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
);
DESCRIPTION [Toc] [Back]
semop() is used to atomically perform an array of semaphore operations
on the set of semaphores associated with the semaphore identifier
specified by semid. sops is a pointer to the array of semaphoreoperation
structures. nsops is the number of such structures in the
array. The contents of each structure includes the following members:
ushort sem_num; /* semaphore number */
short sem_op /* semaphore operation */
short sem_flg; /* operation flags */
Each semaphore operation specified by sem_op is performed on the
corresponding semaphore specified by semid and sem_num. Semaphore
array operations are atomic in that none of the semaphore operations
are performed until blocking conditions on all of the semaphores in
the array have been removed.
sem_op specifies one of three semaphore operations as follows:
+ If sem_op is a negative integer, one of the following occurs:
If semval (see semaphore identifier in glossary(9)) is greater
than or equal to the absolute value of sem_op, the absolute
value of sem_op is subtracted from semval. Also, if (sem_flg
& SEM_UNDO) is ``true'', the absolute value of sem_op is added
to the calling process's semadj value (see glossary(9) and
exit(2)) for the specified semaphore.
If semval is less than the absolute value of sem_op and
(sem_flg & IPC_NOWAIT) is ``true'', semop() returns
immediately.
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semop(2) semop(2)
If semval is less than the absolute value of sem_op and
(sem_flg & IPC_NOWAIT) is ``false'', semop() increments the
semncnt associated with the specified semaphore and suspend
execution of the calling process until one of the following
conditions occur:
semval becomes greater than or equal to the absolute
value of sem_op. When this occurs, the value of semncnt
associated with the specified semaphore is decremented,
the absolute value of sem_op is subtracted from semval
and, if (sem_flg & SEM_UNDO) is ``true'', the absolute
value of sem_op is added to the calling process's semadj
value for the specified semaphore.
The semid for which the calling process is awaiting
action is removed from the system (see semctl(2)). When
this occurs, errno is set equal to [EIDRM], and a value
of -1 is returned.
The calling process receives a signal that is to be
caught. When this occurs, the value of semncnt
associated with the specified semaphore is decremented,
and the calling process resumes execution in the manner
prescribed in signal(5).
+ If sem_op is a positive integer, the value of sem_op is added
to semval and, if (sem_flg & SEM_UNDO) is ``true'', the value
of sem_op is subtracted from the calling process's semadj
value for the specified semaphore.
+ If sem_op is zero, one of the following occurs:
If semval is zero, semop() proceeds to the next semaphore
operation specified by sops , or returns immediately if this
is the last operation.
If semval is not equal to zero and (sem_flg & IPC_NOWAIT) is
``true'', semop() returns immediately.
If semval is not equal to zero and (sem_flg & IPC_NOWAIT) is
``false'', semop() increments the semzcnt associated with the
specified semaphore and suspends execution of the calling
process until one of the following occurs:
semval becomes zero, at which time the value of semzcnt
associated with the specified semaphore is decremented.
The semid for which the calling process is awaiting
action is removed from the system. When this occurs,
errno is set equal to [EIDRM], and a value of -1 is
returned.
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semop(2) semop(2)
The calling process receives a signal that is to be
caught. When this occurs, the value of semzcnt
associated with the specified semaphore is decremented,
and the calling process resumes execution in the manner
prescribed in signal(5).
semtimedop() is identical to semop() except that an additional
constraint is provided by the timeout parameter. If semtimedop() must
suspend the calling process longer than the time indicated by timeout,
or if the timeout expires while the calling process is suspended in
semtimedop(), the call returns an error.
If the structure pointed to by the timeout parameter is zero-valued,
semtimedop() returns with an error immediately if it would otherwise
have to suspend the calling program.
If the timeout parameter is the NULL pointer, semtimedop() behaves
exactly like semop().
EXAMPLES [Toc] [Back]
The following call to semop() atomically performs a "P" or "get"
operation on the second semaphore in the semaphore set and a "V" or
"release" operation on the third semaphore in the set. This example
assumes the process has a valid semid which represents a set of 4
semaphores as shown on the semget(2) manual page. It also assumes
that the semvals of the semaphores in the set have been initialized as
shown in the semctl(2) manual entry.
struct sembuf sops[4];
sops[0].sem_num = 1;
sops[0].sem_op = -1; /* P (get) */
sops[0].sem_flg = 0;
sops[1].sem_num = 2;
sops[1].sem_op = 1; /* V (release) */
sops[1].sem_flg = 0;
semop (mysemid, sops, 2);
RETURN VALUE [Toc] [Back]
If semop() or semtimedop() returns due to the receipt of a signal, a
value of -1 is returned to the calling process and errno is set to
[EINTR]. If it returns due to the removal of a semid from the system,
a value of -1 is returned and errno is set to [EIDRM].
Upon successful completion, a non-negative value is returned.
Otherwise, a value of -1 is returned and errno is set to indicate the
error.
ERRORS [Toc] [Back]
semop() fails if one or more of the following is true for any of the
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semop(2) semop(2)
semaphore operations specified by sops:
[EINVAL] semid is not a valid semaphore identifier.
[EFBIG] sem_num is less than zero or greater than or equal
to the number of semaphores in the set associated
with semid.
[E2BIG] nsops is greater than the system-imposed maximum.
[EACCES] Operation permission is denied to the calling
process (see glossary(9)).
[EAGAIN] The operation would result in suspension of the
calling process but (sem_flg & IPC_NOWAIT) is
``true''.
[ENOSPC] The limit on the number of individual processes
requesting an SEM_UNDO would be exceeded.
[EINVAL] The number of individual semaphores for which the
calling process requests a SEM_UNDO would exceed
the limit.
[ERANGE] An operation would cause a semval to overflow the
system-imposed limit.
[ERANGE] An operation would cause a semadj value to
overflow the system-imposed limit.
[EFAULT] sops points to an illegal address. The reliable
detection of this error will be implementation
dependent.
semtimedop() fails if one or more of the following is true:
[EFAULT] The timeout argument points to an illegal address.
[EINVAL] The timeout argument has one or more invalid
fields. For example, tv_sec or tv_nsec is less
than 0 or tv_nsec is greater than or equal to one
billion.
[EAGAIN] The timeout expired before the semaphore
operations could be completed.
Upon successful completion, the value of sempid for each semaphore
specified in the array pointed to by sops is set equal to the process
ID of the calling process. The value of sem_otime in the data
structure associated with the semaphore identifier will be set to the
current time.
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semop(2) semop(2)
WARNINGS [Toc] [Back]
Check all references to signal(5) for appropriateness on systems that
support sigvector(2). sigvector(2) can affect the behavior described
on this page.
SEE ALSO [Toc] [Back]
ipcs(1), exec(2), exit(2), fork(2), semctl(2), semget(2), stdipc(3C),
signal(5).
STANDARDS CONFORMANCE [Toc] [Back]
semop(): SVID2, SVID3, XPG2, XPG3, XPG4
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