semop (2)

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Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>

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(The comments found at the beginning of the groff file "man2/semop.2freebsd".)

NAME

semop - atomic array of operations on a semaphore set

LIBRARY

Lb libc

SYNOPSIS

In sys/types.h In sys/ipc.h In sys/sem.h Ft int Fn semop int semid struct sembuf *array size_t nops

DESCRIPTION

The Fn 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 Fn 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, Fn 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 Fn semop can return immediately.
  • If IPC_NOWAIT was specified, then Fn 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, Fn 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 ensure that a resource is released if a process terminates unexpectedly.

RETURN VALUES

Rv -std semop

ERRORS

The Fn 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 Fn 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.

SEE ALSO

semctl(2), semget(2), sigaction(2)

BUGS

The Fn semop system call may block waiting for memory even if IPC_NOWAIT was specified.