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semop(2) semop(2)

NAME

semop - semaphore operations

SYNOPSIS

#include <sys/sem.h>

int semop(

int semid,

struct sembuf *sops,

size_t nsops

);

DESCRIPTION

semop() is used to atomically perform an array of semaphore operations on the set of semaphores associ-

ated 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:

ushort 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. Semaphore array operations are atomic in that none of the semaphore operations are

performed until blocking conditions on all of the semaphores in the array have been removed.

sem_op speciﬁes one of three semaphore operations as follows:

If sem_op is a negative integer, one of the following occurs:

If semval (see semaphore identiﬁer in glossary(9)) 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 glossary(9) and 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() returns immediately.

If semval is less than the absolute value of sem_op and (sem_ﬂg & IPC_NOWAIT ) is ‘‘false’’,

semop() increments 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 −1is

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(5).

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 semaphore.

If sem_op is zero, one of the following occurs:

If semval is zero,

semop() proceeds to the next semaphore operation speciﬁed by sops, or

returns immediately if this is the last operation.

If semval is not equal to zero and (sem_ﬂg & IPC_NOWAIT ) is ‘‘true’’, semop() returns

immediately.

If semval is not equal to zero and (sem_ﬂg & IPC_NOWAIT ) is ‘‘false’’,

semop() incre-

ments the semzcnt associated with the speciﬁed semaphore and suspends execution of the

HP-UX Release 11i: December 2000 − 1 − Section 2−−291

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