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semop (2)

semop (2) ( Solaris man: Системные вызовы )

>> semop (2) ( FreeBSD man: Системные вызовы )

semop (2) ( Русские man: Системные вызовы )

semop (2) ( Linux man: Системные вызовы )

semop (3) ( POSIX man: Библиотечные вызовы )

BSD mandoc

NAME



semop

 - atomic array of operations on a semaphore set

LIBRARY

Lb libc

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 indicated by Fa array on the semaphore set indicated by Fa semid . The length of Fa array is indicated by Fa nops . Each operation is encoded in a Vt 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 Fa 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

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.
The behavior when sem_op is negative and the process has alter permission, depends on the current value of the semaphore:
- 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 semaphore's adjust on exit value is incremented by the absolute value of sem_op
- If the current value of the semaphore is less than the absolute value of sem_op one of the following happens:
  - If IPC_NOWAIT was specified, then semop ();
    returns immediately with a return value of Er EAGAIN .
  - Otherwise, the calling process is put to sleep until one of the following conditions is satisfied:
    - Some other process removes the semaphore with the IPC_RMID option of semctl(2). In this case, semop ();
      returns immediately with a return value of Er EIDRM .
    - The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2).
    - The semaphore's value is greater than or equal to the absolute value of sem_op When this condition becomes true, the semaphore's value is decremented by the absolute 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.
When sem_op is zero and the process has read permission, one of the following will occur:
- If the current value of the semaphore is equal to zero then semop ();
  can return immediately.
- If IPC_NOWAIT was specified, then semop ();
  returns immediately with a return value of Er EAGAIN .
- Otherwise, the calling process is put to sleep until one of the following conditions is satisfied:
  - Some other process removes the semaphore with the IPC_RMID option of semctl(2). In this case, semop ();
    returns immediately with a return value of Er EIDRM .
  - The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2).
  - 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 voluntarily or involuntarily, the adjust on exit value for each semaphore is added to the semaphore's value. This can be used to insure that a resource is released if a process terminates unexpectedly.

RETURN VALUES

Rv -std semop

ERRORS

The semop ();
system call will fail if:

Bq Er EINVAL: No semaphore set corresponds to Fa semid , or the process would exceed the system-defined limit for the number of per-process SEM_UNDO structures.
Bq Er EACCES: Permission denied due to mismatch between operation and mode of semaphore set.
Bq Er EAGAIN: The semaphore's value would have resulted in the process being put to sleep and IPC_NOWAIT was specified.
Bq Er E2BIG: Too many operations were specified. Bq Dv SEMOPM
Bq Er EFBIG: sem_num was not in the range of valid semaphores for the set.
Bq Er EIDRM: The semaphore set was removed from the system.
Bq Er EINTR: The semop ();
system call was interrupted by a signal.
Bq Er ENOSPC: The system SEM_UNDO pool Bq Dv SEMMNU is full.
Bq Er ERANGE: The requested operation would cause either the semaphore's current value Bq Dv SEMVMX or its adjust on exit value Bq Dv SEMAEM to exceed the system-imposed limits.