HP-UX Reference (11i v2 07/12) - 2 System Calls (vol 5)

s
semop(2) semop(2)
NAME
semop(), semtimedop() - semaphore operations
SYNOPSIS
#include <sys/sem.h>
#include <sys/time.h>
int semop(
int semid,
struct sembuf *sops,
size_t nsops
);
int semtimedop(
int semid,
struct sembuf *sops,
size_t nsops,
const struct timespec *timeout
);
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 glos-
sary(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 -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(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.
356 Hewlett-Packard Company − 1 − HP-UX 11i Version 2: December 2007 Update