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semop(2)
NAME
semop - Performs 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 X/Open
standards, 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(): 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 flags 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 flag 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.
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
has caused the calling process to be suspended.
[EFBIG] The sem_num parameter is less than 0 (zero) or greater than or
equal to the number of semaphores in 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] The semid parameter is not a valid semaphore ID, or the number of
semaphores for which SEM_UNDO is requested exceeds the system-
defined limit.
[ENOSPC] The system-defined limit on the number of processes using
SEM_UNDO was exceeded.
[ERANGE] An operation caused a semval 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.
RELATED INFORMATION
Functions: exec(2), exit(2), fork(2), semctl(2), semget(2)
Data Structures: semid_ds(4)
Standards: standards(5)
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