 |
Index for
Section 2 |
|
 |
Alphabetical
listing for S |
|
 |
Bottom of
page |
|
semop(2)
NAME
semop - Perform semaphore operations
SYNOPSIS
#include <sys/sem.h>
int semop(
int semid,
struct sembuf *sops,
size_t nsops );
The following declaration of the nsops parameter does not conform to
current standards and is supported only for backward compatibility:
u_int nsops
Application developers may want to specify #include statements for
<sys/types.h> and <sys/ipc.h> before the one for <sys/sem.h> if programs
are being developed for multiple platforms. The additional #include
statements are not required on Tru64 UNIX systems or by ISO or XSH
specifications, but may be required on other vendors' systems that conform
to these standards.
STANDARDS
Interfaces documented on this reference page conform to industry standards
as follows:
semop(): XSH4.0, XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
PARAMETERS
semid
Specifies the ID of the semaphore set.
sops
Points to the user-defined array of sembuf structures that contain the
semaphore operations.
nsops
The number of sembuf structures in the array.
DESCRIPTION
The semop() function performs operations on the semaphores in the specified
semaphore set. The semaphore operations are defined in the sops array. The
sops array contains nsops elements, each of which is represented by a
sembuf structure.
The sembuf structure (from <sys/sem.h>) is shown here:
struct sembuf {
u_short sem_num;
short sem_op;
short sem_flg;
};
The fields in the sembuf structure are defined as follows:
sem_num
Specifies an individual semaphore within the semaphore set.
sem_op
Specifies the operation to perform on the semaphore.
sem_flg
Specifies various options for the operations. The possible values are:
SEM_UNDO
Instructs the kernel to adjust the process's adjust-on-exit
value for a modified semaphore. When the process exits, the
kernel uses this value to restore the semaphore to the value it
had before any modifications by the process. This option is
used to prevent semaphore locking by a process that no longer
exists.
IPC_NOWAIT
Instructs the kernel to return an error condition if a
requested operation would cause the process to sleep. If the
kernel returns an error condition, none of the requested
semaphore operations are performed.
The sem_op operation is specified as a negative integer, a positive
integer, or 0 (zero). The effects of these three values are described
below.
If sem_op is a negative integer and the calling process has modify
permission, the semop() function does one of the following:
· If the semaphore's current value (in semval) is equal to or greater
than the absolute value of sem_op, the absolute value of sem_op is
subtracted from semval. If SEM_UNDO is set, the absolute value of
sem_op is added to the calling process' adjust-on-exit value for the
semaphore.
· If semval is less than the absolute value of sem_op and IPC_NOWAIT is
set, semop() returns immediately.
· If semval is less than the absolute value of sem_op and IPC_NOWAIT is
not set, semop() increments the semaphore's semncnt value and suspends
the calling process.
If the process is suspended, it sleeps until one of the following occurs:
· The semval value becomes equal to or greater than the absolute value
of sem_op. In this case, the semaphore's semncnt value is decremented;
the absolute value of sem_op is subtracted from semval; and, if
SEM_UNDO is set, the absolute value of sem_op is added to the calling
process's adjust-on-exit value for the semaphore.
· The semaphore set (specified by semid) is removed from the system. In
this case, errno is set equal to [EIDRM] and a value of -1 is returned
to the calling process.
· The calling process catches a signal. In this case, the semaphore's
semncnt value is decremented, and the calling process resumes
execution as directed by the sigaction() function.
If sem_op is a positive integer and the calling process has modify
permission, semop() adds the sem_op value to the semaphore's current semval
value. If SEM_UNDO is set, the sem_op value is subtracted from the calling
process's adjust-on-exit value for the semaphore.
If sem_op is 0 (zero) and the calling process has read permission, semop()
does one of the following:
· If semval is 0, semop() returns immediately.
· If semval is not equal to 0 and IPC_NOWAIT is set, semop() returns
immediately.
· If semval is not equal to 0 and IPC_NOWAIT is not set, semop()
increments the semaphore's semzcnt value and suspends the calling
process.
If the process is suspended, it sleeps until one of the following occurs:
· The semval value becomes 0 (zero). In this case, the semaphore's
semncnt value is decremented.
· The semaphore set (specified by semid) is removed from the system. In
this case, errno is set equal to [EIDRM] and a value of -1 is returned
to the calling process.
· The calling process catches a signal. In this case, the semaphore's
semncnt value is decremented, and the calling process resumes
execution as directed by the sigaction() function.
The calling process must have read (sense) or write (alter) permission to
the semaphore set for all access control policies for each specified
operation. If any operation accesses the semaphore set in a way that is
not allowed according to one of the access control policies, access is
denied.
NOTES
[Tru64 UNIX] Semaphore operations are performed atomically; that is,
either all of the requested operations are performed, or none are. If the
kernel goes to sleep while doing the operations, it restores all of the
semaphores in the set to their previous values, at the start of the semop()
function.
The librt library contains alternative interfaces for interprocess
communication. The names of these routines adhere to the format sem_* and
their reference pages are listed in SEE ALSO.
RETURN VALUES
Upon successful completion, the semop() function returns a value of 0
(zero) and the sempid value for each semaphore that is operated upon is set
to the process ID of the calling process.
If the semop() function fails, a value of -1 is returned and errno is set
to indicate the error.
ERRORS
The semop() function sets errno to the specified values for the following
conditions:
[E2BIG]
The nsops parameter is greater than the system-defined maximum.
[EACCES]
The calling process does not have the required permission.
[EACCES]
The process does not have read or write access permission to the
semaphore set with respect to all access control policies.
[EAGAIN]
Both sem_flg and IPC_NOWAIT are true, but the requested operation would
cause the calling process to be suspended.
[EFAULT]
[Tru64 UNIX] Indicates that the requested address is in some way
invalid, for example, out of bounds.
[EFBIG]
The sem_num value is greater than or equal to the number of semaphores
in the set associated with semid.
[EIDRM]
The semaphore ID specified by the semid parameter has been removed from
the system.
[EINTR]
The semop() function was interrupted by a signal.
[EINVAL]
One of the following conditions applies:
· The semid value is not a valid semaphore ID.
· The number of semaphores for which SEM_UNDO is requested exceeds
the system-defined limit.
· [Tru64 UNIX] The sem_num value is less than zero.
[ENOSPC]
The system-defined limit on the number of processes using SEM_UNDO was
exceeded.
[ERANGE]
An operation caused a semval value to overflow the system-defined
limit, or an operation caused an adjust-on-exit value to exceed the
system-defined limit.
[ENOSYS]
[Tru64 UNIX] The requested operation is not supported by this
implementation.
SEE ALSO
Functions: exec(2), exit(2), fork(2), semctl(2), semget(2), sem_close(3),
sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_post(3),
sem_unlink(3), sem_wait(3)
Data Structures: semid_ds(4)
Standards: standards(5)
|
Index for
Section 2 |
|
|
Alphabetical
listing for S |
|
 |
Top of
page |
|