The semop function performs operations on semaphores in the
semaphore set specified by semid. These operations are supplied
in a user-defined array of semaphore operation sembuf structures
specified by sops. Each sembuf structure includes the following
member variables:
struct sembuf { /* semaphore operation structure */
unsigned short sem_num; /* semaphore number */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags SEM_UNDO and IPC_NOWAIT */
Each semaphore operation specified by the sem_op variable is
performed on the corresponding semaphore specified by the semid
function argument and the sem_num variable.
The sem_op variable specifies one of three semaphore operations:
1. If sem_op is a negative integer and the calling process has
change permission, one of the following occurs:
o If semval (see <sem.h>) 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 non-zero, the absolute value of sem_op is added to the
calling process' semadj value for the specified semaphore.
o If semval is less than the absolute value of sem_op
and (sem_flg &IPC_NOWAIT) is nonzero, semop returns
immediately.
o If semval is less than the absolute value of sem_op and
(sem_flg &IPC_NOWAIT) is 0, semop increments the semncnt
associated with the specified semaphore and suspends
execution of the calling thread until one of the following
conditions occurs:
- The value of 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 nonzero,
the absolute value of sem_op is added to the calling
process' semadj value for the specified semaphore.
- The semid for which the calling thread is awaiting
action is removed from the system. When this occurs,
errno is set equal to EIDRM and -1 is returned.
- The calling thread receives a signal that is to be
intercepted. When this occurs, the value of semncnt
associated with the specified semaphore is decremented,
and the calling thread is resumes execution in the
manner prescribed in sigaction.
2. If sem_op is a positive integer and the calling process has
change permission, the value of sem_op is added to semval
and, if (sem_flg &SEM_UNDO) is nonzero, the value of sem_op
is subtracted from the calling process' semadj value for the
specified semaphore.
3. If sem_op is 0 and the calling process has read permission,
one of the following occurs:
o If semval is 0, semop returns immediately.
o If semval is nonzero and (sem_flg &IPC_NOWAIT) is nonzero,
semop returns immediately.
o If semval is nonzero and (sem_flg &IPC_NOWAIT) is 0, semop
increments the semzcnt associated with the specified
semaphore and suspends execution of the calling thread
until one of the following occurs:
- The value of semval becomes 0, at which time the value
of semzcnt associated with the specified semaphore is
decremented.
- The semid for which the calling thread is awaiting
action is removed from the system. When this occurs,
errno is set equal to EIDRM and -1 is returned.
- The calling thread receives a signal that is to be
intercepted. When this occurs, the value of semzcnt
associated with the specified semaphore is decremented,
and the calling thread resumes execution in the manner
prescribed in sigaction.
On 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.