path: root/man2/semop.2

.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
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.\" Modified 1996-10-22, Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2002-01-08, Michael Kerrisk <mtk16@ext.canterbury.ac.nz>
.\" Modified 2003-04-28, Ernie Petrides <petrides@redhat.com>
.\" Modified 2004-05-27, Michael Kerrisk <mtk16@ext.canterbury.ac.nz>
.\"
.TH SEMOP 2 2003-04-28 "Linux 2.4" "Linux Programmer's Manual" 
.SH NAME
semop, semtimedop \- semaphore operations
.SH SYNOPSIS
.nf
.B
#include <sys/types.h>
.B
#include <sys/ipc.h>
.B
#include <sys/sem.h>
.fi
.sp
.BI "int semop(int " semid ,
.BI "struct sembuf *" sops ,
.BI "unsigned " nsops );
.sp
.BI "int semtimedop(int " semid ,
.BI "struct sembuf *" sops ,
.BI "unsigned " nsops ,
.BI "struct timespec *" timeout );
.SH DESCRIPTION
A semaphore is represented by an anonymous structure
including the following members:
.sp
.in +4n
.nf
unsigned short  semval;   /* semaphore value */
unsigned short  semzcnt;  /* # waiting for zero */
unsigned short  semncnt;  /* # waiting for increase */
pid_t           sempid;   /* process that did last op */
.sp
.in -4n
.fi
The function
.B semop
performs operations on selected members of the semaphore set indicated by
.IR semid .
Each of the
.I nsops
elements in the array pointed to by
.I sops
specifies an operation to be performed on a semaphore by a
.B "struct sembuf"
including the following members:
.sp
.in +4n
.nf
unsigned short sem_num;	/* semaphore number */
short sem_op;			/* semaphore operation */
short sem_flg;			/* operation flags */
.sp
.in -4n
.fi
Flags recognized in
.I sem_flg
are
.B IPC_NOWAIT
and
.BR SEM_UNDO .
If an operation asserts
.BR SEM_UNDO ,
it will be undone when the process exits.
.PP
The set of operations contained in
.I sops
is performed
.IR atomically ,
that is, the operations are performed at the same time, and only
if they can all be simultaneously performed.
The behaviour of the system call if not all operations can be
performed immediately depends on the presence of the
.B IPC_NOWAIT
flag in the individual
.I sem_flg
fields, as noted below.

Each operation is performed on the
.IR sem_num \-th
semaphore of the semaphore set, where the first semaphore of the set
is semaphore
.BR 0 .
There are three types of operation, distinguished by the value of
.IR sem_op .
.PP
If
.I sem_op
is a positive integer, the operation adds this value to
the semaphore value
.RI  ( semval ).
Furthermore, if
.B SEM_UNDO
is asserted for this operation, the system updates the process undo count
.RI ( semadj )
for this semaphore.
This operation can always proceed \- it never forces a process to wait.
The calling process must have alter permission on the semaphore set.
.PP
If
.I sem_op
is zero, the process must have read access permission on the semaphore
set.
This is a "wait-for-zero" operation: if
.I semval
is zero, the operation can immediately proceed.
Otherwise, if
.B IPC_NOWAIT
is asserted in
.IR sem_flg ,
the system call fails with
.I errno
set to
.B EAGAIN
(and none of the operations in
.I sops
is performed).
Otherwise
.I semzcnt
(the count of processes waiting until this semaphore's value becomes zero)
is incremented by one and the process sleeps until
one of the following occurs:
.IP \(bu
.I semval
becomes 0, at which time the value of
.I semzcnt
is decremented.
.IP \(bu
The semaphore set
is removed: the system call fails, with
.I errno
set to
.BR EIDRM .
.IP \(bu
The calling process catches a signal:
the value of
.I semzcnt
is decremented and the system call fails, with
.I errno
set to
.BR EINTR .
.IP \(bu
The time limit specified by
.I timeout
in a
.B semtimedop
call expires: the system call fails, with
.I errno
set to
.BR EAGAIN .
.PP
If
.I sem_op
is less than zero, the process must have alter permission on the
semaphore set.
If
.I semval
is greater than or equal to the absolute value of
.IR sem_op ,
the operation can proceed immediately:
the absolute value of
.I sem_op
is subtracted from
.IR semval ,
and, if
.B SEM_UNDO
is asserted for this operation, the system updates the process undo count
.RI ( semadj )
for this semaphore.
If the absolute value of
.I sem_op
is greater than
.IR semval ,
and
.B IPC_NOWAIT
is asserted in
.IR sem_flg ,
the system call fails, with
.I errno
set to
.B EAGAIN
(and none of the operations in
.I sops
is performed).
Otherwise
.I semncnt
(the counter of processes waiting for this semaphore's value to increase)
is incremented by one and the process sleeps until
one of the following occurs:
.IP \(bu
.I semval
becomes greater than or equal to the absolute value of
.IR sem_op ,
at which time the value of
.I semncnt
is decremented, the absolute value of
.I sem_op
is subtracted from
.I semval
and, if
.B SEM_UNDO
is asserted for this operation, the system updates the process undo count
.RI ( semadj )
for this semaphore.
.IP \(bu
The semaphore set is removed from the system: the system call fails with
.I errno
set to
.BR EIDRM .
.IP \(bu
The calling process catches a signal:
the value of
.I semncnt
is decremented and the system call fails with
.I errno
set to
.BR EINTR .
.IP \(bu
The time limit specified by
.I timeout
in a
.B semtimedop
call expires: the system call fails, with
.I errno
set to
.BR EAGAIN .
.PP
On successful completion, the
.I sempid
value for each semaphore specified in the array pointed to by
.I sops
is set to the process ID of the calling process.
In addition, the
.I sem_otime
.\" and
.\" .I sem_ctime
is set to the current time.
.PP
The function
.B semtimedop
behaves identically to the function
.B semop
except that in those cases were the calling process would sleep,
the duration of that sleep is limited by the amount of elapsed
time specified by the
.B timespec
structure whose address is passed in the
.I timeout
parameter.  If the specified time limit has been reached,
the system call fails with
.I errno
set to
.B EAGAIN
(and none of the operations in
.I sops
is performed).
If the
.I timeout
parameter is
.BR NULL ,
then
.B semtimedop
behaves exactly like
.BR semop .
.SH "RETURN VALUE"
If successful the system call returns
.BR 0 ,
otherwise it returns
.B \-1
with
.I errno
indicating the error.
.SH ERRORS
On failure,
.I errno
is set to one of the following:
.TP
.B E2BIG
The argument
.I nsops
is greater than
.BR SEMOPM ,
the maximum number of operations allowed per system
call.
.TP
.B EACCES
The calling process does not have the permissions required
to perform the specified semaphore operations,
and does not have the
.B CAP_IPC_OWNER
capability.
.TP
.B EAGAIN
An operation could not proceed immediately and either
.BR IPC_NOWAIT
was asserted in its
.I sem_flg
or the time limit specified in
.I timeout
expired.
.TP
.B EFAULT
An address specified in either the
.I sops
or
.I timeout
parameters isn't accessible.
.TP
.B EFBIG
For some operation the value of
.I sem_num
is less than 0 or greater than or equal to the number
of semaphores in the set.
.TP
.B EIDRM
The semaphore set was removed.
.TP
.B EINTR
While blocked in this system call, the process caught a signal.
.TP
.B EINVAL
The semaphore set doesn't exist, or
.I semid
is less than zero, or
.I nsops
has a non-positive value.
.TP
.B ENOMEM
The
.I sem_flg
of some operation asserted
.B SEM_UNDO
and the system does not have enough memory to allocate the undo
structure.
.TP
.B ERANGE
For some operation
.I sem_op+semval
is greater than
.BR SEMVMX ,
the implementation dependent maximum value for
.IR semval .
.SH NOTES
The
.I sem_undo
structures of a process aren't inherited across a
.BR fork (2)
system call, but they are inherited across a
.BR execve (2)
system call.
.PP
.B semop
is never automatically restarted after being interrupted by a signal handler,
regardless of the setting of the
.B SA_RESTART
flags when establishing a signal handler.
.PP
.I semadj
is a per\-process integer which is simply the (negative) count
of all semaphore operations performed specifying the
.B SEM_UNDO
flag.
When a semaphore's value is directly set using the
.B SETVAL
or
.B SETALL
request to
.BR semctl (2),
the corresponding
.I semadj
values in all processes are cleared.
.PP
The \fIsemval\fP, \fIsempid\fP, \fIsemzcnt\fP, and \fIsemnct\fP values
for a semaphore can all be retrieved using appropriate
.BR semctl (2)
calls.
.PP
The followings are limits on semaphore set resources affecting a
.B semop
call:
.TP
.B SEMOPM
Maximum number of operations allowed for one
.B semop
call (32).
.TP
.B SEMVMX
Maximum allowable value for
.IR semval :
implementation dependent (32767).
.PP
The implementation has no intrinsic limits for
the adjust on exit maximum value
.RB ( SEMAEM ),
the system wide maximum number of undo structures
.RB ( SEMMNU )
and the per\-process maximum number of undo entries system parameters.
.SH BUGS
When a process terminates, its set of associated
.I semadj
structures is used to undo the effect of all of the
semaphore operations it performed with the
.B SEM_UNDO
flag.
This raises a difficulty: if one (or more) of these semaphore adjustments
would result in an attempt to decrease a semaphore's value below zero,
what should an implementation do?
One possible approach would be to block until all the semaphore
adjustments could be performed.
This is however undesirable since it could force process termination to
block for arbitrarily long periods.
Another possibility is that such semaphore adjustments could be ignored
altogether (somewhat analogously to failing when
.B IPC_NOWAIT
is specified for a semaphore operation).
Linux adopts a third approach: decreasing the semaphore value
as far as possible (i.e., to zero) and allowing process
termination to proceed immediately.
.SH "CONFORMING TO"
SVr4, SVID.  SVr4 documents additional error conditions EINVAL, EFBIG,
ENOSPC.
.SH "SEE ALSO"
.BR semctl (2),
.BR semget (2),
.BR sigaction (2),
.BR ipc (5),
.BR capabilities (7)