SEMOP(2) DOMAIN/IX Reference Manual (SYS5) SEMOP(2)
NAME
semop - semaphore operations
USAGE
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semop (semid, sops, nsops)
int semid;
struct sembuf **sops;
int nsops;
DESCRIPTION
Semop performs a variety of semaphore operations on the set
of semaphores associated with the semaphore identifier
semid. Sops is a pointer to the array of semaphore-
operation structures. Nsops is the number of such struc-
tures in the array. Each structure includes the following
members:
short 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.
Sem_op specifies one of three semaphore operations as fol-
lows:
If sem_op is a negative integer, one of the follow-
ing will occur: {ALTER}
If semval (see intro(2)) 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' semadj value (see
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 will return immediately.
If semval is less than the absolute value of
sem_op and (sem_flg & IPC_NOWAIT) is ``false'',
semop will increment the semncnt associated
with the specified semaphore and suspend execu-
tion of the calling process until one of the
following conditions occurs.
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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' 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 speci-
fied semaphore is decremented, and the cal-
ling process resumes execution in the manner
prescribed in signal(2).
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' semadj value for the specified
semaphore. {ALTER}
If sem_op is zero, one of the following will occur:
{READ}
If semval is zero, semop will return immedi-
ately.
If semval is not equal to zero and (sem_flg &
IPC_NOWAIT) is ``true'', semop will return
immediately.
If semval is not equal to zero and (sem_flg &
IPC_NOWAIT) is ``false'', semop will increment
the semzcnt associated with the specified sema-
phore and suspend execution of the calling pro-
cess 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 to EIDRM, and
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a value of -1 is returned.
The calling process receives a signal that
is to be caught. When this occurs, the
value of semzcnt associated with the speci-
fied semaphore is decremented, and the cal-
ling process resumes execution in the manner
prescribed in signal(2).
RETURN VALUE
If semop returns because it received a signal, it returns a
value of -1 to the calling process and sets errno to EINTR.
If it returns because a semid was removed from the system,
it returns -1 and sets errno to EIDRM.
Upon successful completion, the value of semval at the time
of the call for the last operation in the array that sops
points to is returned. Otherwise, a value of -1 is returned
and errno is set to indicate the error.
ERRORS
Semop will fail if one or more of the following is true for
any of the 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 max-
imum.
[EACCES] Operation permission is denied to the calling
process (see intro(2)).
[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
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overflow the system-imposed limit.
[EFAULT] Sops points to an illegal address.
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.
RELATED INFORMATION
exec(2), exit(2), fork(2), semctl(2), semget(2)
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