freebsd/lib/libc/sys/semop.2

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.\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>
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.Dd September 22, 1995
.Dt SEMOP 2
.Os FreeBSD
.Sh NAME
.Nm semop
.Nd atomic array of operations on a semaphore set
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/ipc.h>
.Fd #include <sys/sem.h>
.Ft int
.Fn semop "int semid" "struct sembuf array[]" "unsigned nops"
.Sh DESCRIPTION
.Fn Semop
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
.Fa "struct sembuf" ,
which is defined as follows:
.Bd -literal
.\"
.\" From <sys/sem.h>
.\"
struct sembuf {
        u_short sem_num;        /* semaphore # */
        short   sem_op;         /* semaphore operation */
        short   sem_flg;        /* operation flags */
};
.Ed
.Pp
For each element in
.Fa array ,
.Fa sem_op
and
.Fa sem_flg
determine an operation to be performed on semaphore number
.Fa sem_num
in the set.  The values SEM_UNDO and IPC_NOWAIT may be
.Em OR Ns 'ed
into the
.Fa sem_flg
member in order to modify the behavior of the given operation.
.Pp
The operation performed depends as follows on the value of
.Fa sem_op :
.\"
.\" This section is based on the description of semop() in
.\" Stevens, _Advanced Programming in the UNIX Environment_.
.\"
.Bl -bullet
.It
When
.Fa sem_op
is positive, the semaphore's value is incremented by
.Fa sem_op Ns 's
value.  If SEM_UNDO is specified, the semaphore's adjust on exit
value is decremented by
.Fa sem_op Ns 's
value.  A positive value for
.Fa sem_op
generally corresponds to a process releasing a resource
associated with the semaphore.
.It
The behavior when
.Fa sem_op
is negative depends on the current value of the semaphore:
.Bl -bullet
.It
If the current value of the semaphore is greater than or equal to
the absolute value of
.Fa sem_op ,
then the value is decremented by the absolute value of
.Fa sem_op .
If SEM_UNDO is specified, the semaphore's adjust on exit
value is incremented by the absolute value of
.Fa sem_op .
.It
If the current value of the semaphore is less than
.Fa sem_op Ns 's
value, one of the following happens:
.\" XXX a *second* sublist?
.Bl -bullet
.It
If IPC_NOWAIT was specified, then
.Fn semop
returns immediately with a return value of EAGAIN.
.It
If some other process has removed the semaphore with the IPC_RMID
option of
.Fn semctl ,
then
.Fn semop
returns immediately with a return value of EINVAL.
.It
Otherwise, the calling process is put to sleep until the semaphore's
value is greater than or equal to the absolute value of
.Fa sem_op .
When this condition becomes true, the semaphore's value is decremented
by the absolute value of
.Fa sem_op ,
and the semaphore's adjust on exit value is incremented by the
absolute value of
.Fa sem_op .
.El
.Pp
A negative value for
.Fa sem_op
generally means that a process is waiting for a resource to become
available.
.El
.Pp
.It
When
.Fa sem_op
is zero, the process waits for the semaphore's value to become zero.
If it is already zero, the call to
.Fn semop
can return immediately.  Otherwise, the calling process is put to 
sleep until the semaphore's value becomes zero.
.El
.Pp
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.
.Sh RETURN VALUES
On success,
.Fn semop
returns 0; otherwise, -1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn Semop
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
No semaphore set corresponds to
.Fa semid .
.It Bq Er EACCES
Permission denied due to mismatch between operation and mode of
semaphore set.
.It Bq Er EAGAIN
The semaphore's value was less than
.Fa sem_op ,
and IPC_NOWAIT was specified.
.It Bq Er E2BIG
Too many operations were specified.
.It Bq Er EFBIG
.\"
.\" I'd have thought this would be EINVAL, but the source says
.\" EFBIG.
.\"
.Fa sem_num
was not in the range of valid semaphores for the set.
.Sh SEE ALSO
.Xr semctl 2 ,
.Xr semget 2