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.\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>
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.\" $FreeBSD: src/lib/libc/sys/semop.2,v 1.18 2003/01/25 21:27:37 alfred Exp $
.\"
.Dd September 22, 1995
.Dt SEMOP 2
.Os
.Sh NAME
.Nm semop
.Nd atomic array of operations on a semaphore set
.Sh SYNOPSIS
.In sys/sem.h
.Ft int
.Fo semop
.Fa "int semid"
.Fa "struct sembuf *sops"
.Fa "size_t nsops"
.Fc
.Sh DESCRIPTION
The
.Fn semop
system call
atomically performs the array of operations indicated by
.Fa sops
on the semaphore set indicated by
.Fa semid .
The length of
.Fa sops
is indicated by
.Fa nsops .
Each operation is encoded in a
.Vt "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 sops ,
.Va sem_op
and
.Va sem_flg
determine an operation to be performed on semaphore number
.Va sem_num
in the set.
The values
.Dv SEM_UNDO
and
.Dv IPC_NOWAIT
may be
.Em OR Ns 'ed
into the
.Va sem_flg
member in order to modify the behavior of the given operation.
.Pp
The operation performed depends as follows on the value of
.Va sem_op :
.\"
.\" This section is based on the description of semop() in
.\" Stevens, _Advanced Programming in the UNIX Environment_,
.\" and the semop(2) description in The Open Group Unix2 specification.
.\"
.Bl -bullet
.It
When
.Va sem_op
is positive and the process has alter permission,
the semaphore's value is incremented by
.Va sem_op Ns 's
value.
If
.Dv SEM_UNDO
is specified, the semaphore's adjust on exit value is decremented by
.Va sem_op Ns 's
value.
A positive value for
.Va sem_op
generally corresponds to a process releasing a resource
associated with the semaphore.
.It
The behavior when
.Va sem_op
is negative and the process has alter permission,
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
.Va sem_op ,
then the value is decremented by the absolute value of
.Va sem_op .
If
.Dv SEM_UNDO
is specified, the semaphore's adjust on exit
value is incremented by the absolute value of
.Va sem_op .
.It
If the current value of the semaphore is less than the absolute value of
.Va sem_op ,
one of the following happens:
.\" XXX a *second* sublist?
.Bl -bullet
.It
If
.Dv IPC_NOWAIT
was specified, then
.Fn semop
returns immediately with a return value of
.Er EAGAIN .
.It
Otherwise, the calling process is put to sleep until one of the following
conditions is satisfied:
.\" XXX We already have two sublists, why not a third?
.Bl -bullet
.It
Some other process removes the semaphore with the
.Dv IPC_RMID
option of
.Xr semctl 2 .
In this case,
.Fn semop
returns immediately with a return value of
.Er EIDRM .
.It
The process receives a signal that is to be caught.
In this case, the process will resume execution as defined by
.Xr sigaction 2 .
.It
The semaphore's
value is greater than or equal to the absolute value of
.Va sem_op .
When this condition becomes true, the semaphore's value is decremented
by the absolute value of
.Va sem_op ,
the semaphore's adjust on exit value is incremented by the
absolute value of
.Va sem_op .
.El
.El
.El
.Pp
A negative value for
.Va sem_op
generally means that a process is waiting for a resource to become
available.
.It
When
.Va sem_op
is zero and the process has read permission,
one of the following will occur:
.Bl -bullet
.It
If the current value of the semaphore is equal to zero
then
.Fn semop
can return immediately.
.It
If
.Dv IPC_NOWAIT
was specified, then
.Fn semop
returns immediately with a return value of
.Er EAGAIN .
.It
Otherwise, the calling process is put to sleep until one of the following
conditions is satisfied:
.\" XXX Another nested sublists
.Bl -bullet
.It
Some other process removes the semaphore with the
.Dv IPC_RMID
option of
.Xr semctl 2 .
In this case,
.Fn semop
returns immediately with a return value of
.Er EIDRM .
.It
The process receives a signal that is to be caught.
In this case, the process will resume execution as defined by
.Xr sigaction 2 .
.It
The semaphore's value becomes zero.
.El
.El
.El
.Pp
For each semaphore a process has in use, the kernel maintains an
.Dq "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
.Rv -std semop
.Sh ERRORS
The
.Fn semop
system call will fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er E2BIG
Too many operations are specified.
.Bq Dv SEMOPM
.\" ===========
.It Bq Er EACCES
Permission is denied, due to a mismatch between the operation
and the mode of the semaphore set.
.\" ===========
.It Bq Er EAGAIN
The semaphore's value would result
in the process being put to sleep and
.Dv IPC_NOWAIT
is specified.
.\" ===========
.It Bq Er EFBIG
.\"
.\" I'd have thought this would be EINVAL,
.\" but the source says EFBIG.
.\"
.Va sem_num
is not in the range of valid semaphores for the set.
.\" ===========
.It Bq Er EIDRM
The semaphore set is removed from the system.
.\" ===========
.It Bq Er EINTR
The
.Fn semop
system call is interrupted by a signal.
.\" ===========
.It 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
.Dv SEM_UNDO
structures.
.\" ===========
.It Bq Er ENOSPC
The system
.Dv SEM_UNDO
pool
.Bq Dv SEMMNU
is full.
.\" ===========
.It 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.
.El
.Sh LEGACY SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/ipc.h>
.Fd #include <sys/sem.h>
.Pp
The include files
.In sys/types.h
and
.In sys/ipc.h
are necessary.
.Sh SEE ALSO
.Xr semctl 2 ,
.Xr semget 2 ,
.Xr sigaction 2 ,
.Xr compat 5
.Sh BUGS
The
.Fn semop
system call
may block waiting for memory even if
.Dv IPC_NOWAIT
was specified.