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Diffstat (limited to 'lib/libc/sys/semop.2')
| -rw-r--r-- | lib/libc/sys/semop.2 | 289 |
1 files changed, 289 insertions, 0 deletions
diff --git a/lib/libc/sys/semop.2 b/lib/libc/sys/semop.2 new file mode 100644 index 0000000..a08f65e --- /dev/null +++ b/lib/libc/sys/semop.2 @@ -0,0 +1,289 @@ +.\" +.\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com> +.\" +.\" All rights reserved. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR +.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES +.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. +.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT, +.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT +.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, +.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY +.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT +.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF +.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. +.\" +.\" $FreeBSD$ +.\" +.Dd September 22, 1995 +.Dt SEMOP 2 +.Os +.Sh NAME +.Nm semop +.Nd atomic array of operations on a semaphore set +.Sh LIBRARY +.Lb libc +.Sh 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" +.Sh 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: +.\" +.\" From <sys/sem.h> +.\" +.Bd -literal +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 , +.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 ensure 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 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 EACCES +Permission denied due to mismatch between operation and mode of +semaphore set. +.It Bq Er EAGAIN +The semaphore's value would have resulted in the process being put to sleep +and +.Dv IPC_NOWAIT +was specified. +.It Bq Er E2BIG +Too many operations were specified. +.Bq Dv SEMOPM +.It Bq Er EFBIG +.\" +.\" I'd have thought this would be EINVAL, but the source says +.\" EFBIG. +.\" +.Va sem_num +was not in the range of valid semaphores for the set. +.It Bq Er EIDRM +The semaphore set was removed from the system. +.It Bq Er EINTR +The +.Fn semop +system call was interrupted by a signal. +.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 SEE ALSO +.Xr semctl 2 , +.Xr semget 2 , +.Xr sigaction 2 +.Sh BUGS +The +.Fn semop +system call +may block waiting for memory even if +.Dv IPC_NOWAIT +was specified. |
