SEMOP
Section: System Calls (2)
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BSD mandoc
NetBSD
NAME
semop
- semaphore operations
SYNOPSIS
Fd #include <sys/types.h>
Fd #include <sys/ipc.h>
Fd #include <sys/sem.h>
Ft int
Fn semop int semid struct sembuf *sops int nsops
DESCRIPTION
Fn semop
provides a number of atomic operations on a set of semaphores. The semaphore
set is specified by
Fa semid .
Fa sops
is an array of semaphore operations,
Fa nsops
is the number of operations in this array. The
sembuf
structures in the array contain the following members:
u_short sem_num; /* semaphore # */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */
Each operation (specified in
sem_op
is applied to semaphore number
sem_num
in the set of semaphores specified by
Fa semid .
The value of
sem_op
determines the action taken in the following way:
-
sem_op
is less than 0. The current process is blocked until the value of the
semaphore is greater than or equal to the absolute value of
sem_op
The absolute value of
sem_op
is then subtracted from the value of the semaphore, and the calling
process continues. Negative values of
sem_op
are thus used to enter critical regions.
-
sem_op
is greater than 0. Its value is added to the value of the specified
semaphore. This is used to leave critical regions.
-
sem_op
is equal to 0. The calling process is blocked until the value of the
specified semaphore reaches 0.
The behaviour of each operation is influenced by the flags set in
sem_flg
in the following way:
- IPC_NOWAIT
-
In the case where the calling process would normally block, waiting
for a semaphore to reach a certain value, IPC_NOWAIT makes the
call return immediately, returning a value of -1 and setting
errno
to EAGAIN.
- SEM_UNDO
-
Keep track of the changes that this call makes to the value of a semaphore,
so that they can be undone when the calling process terminates. This is
useful to prevent other processes waiting on a semaphore to block forever,
should the process that has the semaphore locked terminate in a critical
section.
RETURN VALUES
Upon successful completion, a value of 0 is returned. Otherwise, -1 is
returned and the global variable
errno
is set to indicate the error.
ERRORS
Fn semop
will fail if:
- Bq Er EINVAL
-
There is no semaphore associated with
Fa semid .
The semaphore set was removed while the process was waiting for one of
its semaphores to reach a certain value.
- Bq Er EACCESS
-
The calling process has no permission to access the specified semaphore set.
- Bq Er E2BIG
-
The value of
Fa nsops
is too big. The maximum is specified in MAX_SOPS in <sys/sem.h>
- Bq Er EFBIG
-
sem_num
in one of the sem_buf structures is less than 0, or greater than the actual
number of semaphores in the set specified by
Fa semid .
- Bq Er ENOSPC
-
SEM_UNDO was requested, and there is not enough space left in the kernel to
store the unfo information.
- Bq Er EAGAIN
-
The requested operation can not immediately be performed, and IPC_NOWAIT
was set in
sem_flg
- Bq Er EFAULT
-
Fa sops
points to an illegal address.
SEE ALSO
semctl(2),
semget(2)
BUGS
In case of a removed semaphore identifier,
errno
should be set to EIDRM, but NetBSD does not define this error.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUES
-
- ERRORS
-
- SEE ALSO
-
- BUGS
-
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Time: 16:28:59 GMT, April 18, 2022