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SEMOP(2) BSD System Calls Manual SEMOP(2) NAME semop -- atomic array of operations on a semaphore set LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> int semop(int semid, struct sembuf *array, size_t nops); DESCRIPTION The semop() system call atomically performs the array of operations indi- cated by array on the semaphore set indicated by semid. The length of array is indicated by nops. Each operation is encoded in a struct sembuf, which is defined as follows: struct sembuf { u_short sem_num; /* semaphore # */ short sem_op; /* semaphore operation */ short sem_flg; /* operation flags */ }; For each element in array, sem_op and sem_flg determine an operation to be performed on semaphore number sem_num in the set. The values SEM_UNDO and IPC_NOWAIT may be OR'ed into the sem_flg member in order to modify the behavior of the given operation. The operation performed depends as follows on the value of sem_op: o When sem_op is positive and the process has alter permission, the semaphore's value is incremented by sem_op's value. If SEM_UNDO is specified, the semaphore's adjust on exit value is decremented by sem_op's value. A positive value for sem_op generally corresponds to a process releasing a resource associated with the semaphore. o The behavior when sem_op is negative and the process has alter per- mission, depends on the current value of the semaphore: o If the current value of the semaphore is greater than or equal to the absolute value of sem_op, then the value is decremented by the absolute value of sem_op. If SEM_UNDO is specified, the sem- aphore's adjust on exit value is incremented by the absolute value of sem_op. o If the current value of the semaphore is less than the absolute value of sem_op, one of the following happens: o If IPC_NOWAIT was specified, then semop() returns immediately with a return value of EAGAIN. o Otherwise, the calling process is put to sleep until one of the following conditions is satisfied: o Some other process removes the semaphore with the IPC_RMID option of semctl(2). In this case, semop() re- turns immediately with a return value of EIDRM. o The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2). o The semaphore's value is greater than or equal to the ab- solute value of sem_op. When this condition becomes true, the semaphore's value is decremented by the abso- lute value of sem_op, the semaphore's adjust on exit value is incremented by the absolute value of sem_op. A negative value for sem_op generally means that a process is waiting for a resource to become available. o When sem_op is zero and the process has read permission, one of the following will occur: o If the current value of the semaphore is equal to zero then semop() can return immediately. o If IPC_NOWAIT was specified, then semop() returns immediately with a return value of EAGAIN. o Otherwise, the calling process is put to sleep until one of the following conditions is satisfied: o Some other process removes the semaphore with the IPC_RMID option of semctl(2). In this case, semop() returns immedi- ately with a return value of EIDRM. o The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2). o The semaphore's value becomes zero. For each semaphore a process has in use, the kernel maintains an "adjust on exit" value, as alluded to earlier. When a process exits, either vol- untarily or involuntarily, the adjust on exit value for each semaphore is added to the semaphore's value. This can be used to ensure that a re- source is released if a process terminates unexpectedly. RETURN VALUES The semop() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. ERRORS The semop() system call will fail if: [EINVAL] No semaphore set corresponds to semid, or the process would exceed the system-defined limit for the number of per-process SEM_UNDO structures. [EACCES] Permission denied due to mismatch between operation and mode of semaphore set. [EAGAIN] The semaphore's value would have resulted in the process being put to sleep and IPC_NOWAIT was speci- fied. [E2BIG] Too many operations were specified. [SEMOPM] [EFBIG] sem_num was not in the range of valid semaphores for the set. [EIDRM] The semaphore set was removed from the system. [EINTR] The semop() system call was interrupted by a signal. [ENOSPC] The system SEM_UNDO pool [SEMMNU] is full. [ERANGE] The requested operation would cause either the sema- phore's current value [SEMVMX] or its adjust on exit value [SEMAEM] to exceed the system-imposed limits. SEE ALSO semctl(2), semget(2), sigaction(2) BUGS The semop() system call may block waiting for memory even if IPC_NOWAIT was specified. BSD September 22, 1995 BSD
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | BUGS
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