semop(2) SYSTEM CALLS 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 fol-
lows, 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 immedi-
ately.
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
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semop(2) SYSTEM CALLS semop(2)
decremented, the absolute value of sem_op is sub-
tracted 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.
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&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 decre-
mented, 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:
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semop(2) SYSTEM CALLS semop(2)
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&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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