SEMOP(2) DOMAIN/IX 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.
If sem_op is a negative integer, one of the following 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's 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
Printed 12/4/86 SEMOP-1
SEMOP(2) DOMAIN/IX SYS5 SEMOP(2)
(sem_flg & IPC_NOWAIT)
is "false", semop will increment the semncnt associated
with the specified semaphore and suspend execution of the
calling process until one of the following conditions
occurs.
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 sub-
tracted 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 decre-
mented and the calling 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 cal-
ling process's 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 immediately.
⊕ 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
SEMOP-2 Printed 12/4/86
SEMOP(2) DOMAIN/IX SYS5 SEMOP(2)
(sem_flg & IPC_NOWAIT)
is "false", semop will increment the semzcnt associated
with the specified semaphore and suspend 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 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 semzcnt
associated with the specified semaphore is decre-
mented and the calling 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.
A successful call returns the value of semval at the time of
the call for the last operation in the array that sops
points to. A failed call returns -1 and sets errno as indi-
cated below.
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 maximum.
[EACCES] Operation permission is denied to the calling pro-
cess (see intro(2)).
[EAGAIN] The operation would result in suspension of the
calling process but
Printed 12/4/86 SEMOP-3
SEMOP(2) DOMAIN/IX SYS5 SEMOP(2)
(sem_flg & IPC_NOWAIT)
is "true".
[ENOSPC] The limit on the number of individual processes
requesting a 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 over-
flow 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)
SEMOP-4 Printed 12/4/86