semop(2) UNIX System V semop(2)
NAME
semop - semaphore operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semop(int semid, struct sembuf *sops, sizet nsops);
DESCRIPTION
semop is used to perform atomically 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 semnum; /* semaphore number */
short semop; /* semaphore operation */
short semflg; /* 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, depending
on whether its value is negative, positive, or zero:
If sem_op is a negative integer, one of the following occurs: {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&SEMUNDO) 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&IPCNOWAIT) is true, semop returns immediately.
If semval is less than the absolute value of sem_op and
(sem_flg&IPCNOWAIT) is false, semop increments the semncnt
associated with the specified semaphore and suspends 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&SEMUNDO) is
true, the absolute value of sem_op is added to the calling
process's semadj value for the specified semaphore.
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semop(2) UNIX System V semop(2)
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&SEMUNDO) is true, the value of sem_op is subtracted
from the calling process's semadj value for the specified semaphore.
{ALTER}
If sem_op is zero, one of the following occurs: {READ}
If semval is zero, semop returns immediately.
If semval is not equal to zero and (sem_flg&IPCNOWAIT) is true,
semop returns immediately.
If semval is not equal to zero and (sem_flg&IPCNOWAIT) 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.
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 fails 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 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)].
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semop(2) UNIX System V semop(2)
EAGAIN The operation would result in suspension of the calling
process but (sem_flg&IPCNOWAIT) is true.
ENOSPC The limit on the number of individual processes requesting
an SEMUNDO would be exceeded.
EINVAL The number of individual semaphores for which the calling
process requests a SEMUNDO 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
intro(2), exec(2), exit(2), fork(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 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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