SEMOP(2) SEMOP(2)
NAME
semop - semaphore operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semop (semid, sops, nsops)
int semid;
struct sembuf **sops;
unsigned nsops;
DESCRIPTION
semop is used to automatically 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 semaphore-operation structures. Nsops is
the number of such structures in the array. The contents of
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
follows:
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
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SEMOP(2) SEMOP(2)
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
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(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's semadj value for the specified
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SEMOP(2) SEMOP(2)
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 (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 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 semzcnt associated with the
specified semaphore is decremented, and the
calling process resumes execution in the
manner prescribed in signal(2).
semop will fail if one or more of the following are 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
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SEMOP(2) SEMOP(2)
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 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
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.
SEE ALSO
exec(2), exit(2), fork(2), intro(2), semctl(2), semget(2).
DIAGNOSTICS
If semop 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
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SEMOP(2) SEMOP(2)
system, a value of -1 is returned and errno is set to EIDRM.
Upon successful completion, a value of zero is returned.
Otherwise, a value of -1 is returned and errno is set to
indicate the error.
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