#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h>
int semop(int semid, struct sembuf *sops, unsigned nsops);
unsigned short semval; /* semaphore value */ unsigned short semzcnt; /* # waiting for zero */ unsigned short semncnt; /* # waiting for increase */ pid_t sempid; /* process that did last op */The function semop performs operations on selected members of the semaphore set indicated by semid. Each of the nsops elements in the array pointed to by sops specifies an operation to be performed on a semaphore by a struct sembuf including the following members:
unsigned short sem_num; /* semaphore number */ short sem_op; /* semaphore operation */ short sem_flg; /* operation flags */Flags recognized in sem_flg are IPC_NOWAIT and SEM_UNDO. If an operation asserts SEM_UNDO, it will be undone when the process exits.
The set of operations contained in sops is performed atomically, that is, the operations are performed at the same time, and only if they can all be simultaneously performed. The behaviour of the system call if not all operations can be performed immediately depends on the presence of the IPC_NOWAIT flag in the individual sem_flg fields, as noted below.
Each operation is performed on the sem_num-th semaphore of the semaphore set, where the first semaphore of the set is semaphore 0. There are three types of operation, distinguished by the value of sem_op.
If sem_op is a positive integer, the operation adds this value to the semaphore value (semval). Furthermore, if SEM_UNDO is asserted for this operation, the system updates the process undo count (semadj) for this semaphore. This operation can always proceed - it never forces a process to wait. The calling process must have alter permission on the semaphore set.
If sem_op is zero, the process must have read access permission on the semaphore set. This is a "wait-for-zero" operation: if semval is zero, the operation can immediately proceed. Otherwise, if IPC_NOWAIT is asserted in sem_flg, the system call fails with errno set to EAGAIN (and none of the operations in sops is performed). Otherwise semzcnt (the count of processes waiting until this semaphore's value becomes zero) is incremented by one and the process sleeps until one of the following occurs:
If sem_op is less than zero, the process must have alter permission on the semaphore set. If semval is greater than or equal to the absolute value of sem_op, the operation can proceed immediately: the absolute value of sem_op is subtracted from semval, and, if SEM_UNDO is asserted for this operation, the system updates the process undo count (semadj) for this semaphore. If the absolute value of sem_op is greater than semval, and IPC_NOWAIT is asserted in sem_flg, the system call fails, with errno set to EAGAIN (and none of the operations in sops is performed). Otherwise semncnt (the counter of processes waiting for this semaphore's value to increase) is incremented by one and the process sleeps until one of the following occurs:
On successful completion, the sempid value for each semaphore specified in the array pointed to by sops is set to the process ID of the calling process. In addition, the sem_otime is set to the current time.
semop is never automatically restarted after being interrupted by a signal handler, regardless of the setting of the SA_RESTART flags when establishing a signal handler.
semadj is a per-process integer which is simply the (negative) count of all semaphore operations performed specifying the SEM_UNDO flag. When a semaphore's value is directly set using the SETVAL or SETALL request to semctl(2), the corresponding semadj values in all processes are cleared.
The semval, sempid, semzcnt, and semnct values for a semaphore can all be retrieved using appropriate semctl(2) calls.
The followings are limits on semaphore set resources affecting a semop call:
The implementation has no intrinsic limits for the adjust on exit maximum value (SEMAEM), the system wide maximum number of undo structures (SEMMNU) and the per-process maximum number of undo entries system parameters.