semop

Performs operations on a set of semaphores.

Syntax

   #include <sys/types.h>

   #include <sys/ipc.h>

   #include <sys/sem.h>



   int semop (int semid, struct sembuf *sops, int nsops);

Parameters

semid: Passes a semaphore identifier returned by a semget() call.
sops: Passes a pointer to an array of semaphore operation structures where each element is of type struct sembuf (defined in the <sys/sem.h> header). Semaphore operation structures define operations to perform on the semaphore set. For details on using semaphore operation structures, refer to the "Description" section below.
nsops: Passes the number of valid semaphore operation structures in the array pointed to by sops.

Return Values

0: Success.
-1: An error occurred, and errno is set to indicate the error condition.

Description

The semop() function performs operations on the set of semaphores associated with the semaphore identifier specified by semid.

The sops argument points to an array where each of nsops elements contains a semaphore operation structure. Each semaphore operation specified by the sem_op field is performed on the semaphore specified by sem_num. The operation is further defined by the sem_flg field. No semaphore operations are performed until blocking conditions on all of the semaphores specified in the array are removed.

If the value of sem_op is less than 0 and the calling process has write permission, one of the following operations occurs depending upon the current semaphore value and the value of sem_flag:

Table 2-1 Operations when sem_op <0

Semaphore Value	`sem_flag` Value	Operation
>= absolute value of `sem_op`	0 or `IPC_NOWAIT`	A new semaphore value is calculated as the result of subtracting the absolute value of `sem_op` from the current semaphore value. The call to `semop()` returns successfully to the calling process.
>= absolute value of `sem_op`	`SEM_UNDO`	A new semaphore value is calculated as the result of subtracting the absolute value of `sem_op` from the current semaphore value. The absolute value of `sem_op` is added to the calling process's semaphore adjust value of the specified semaphore. The call to `semop()` returns successfully to the calling process.
< absolute value of `sem_op`	`IPC_NOWAIT`	`semop()` returns -1, sets `errno` to `EAGAIN`, and returns control to the calling process.
< absolute value of `sem_op`	`IPC_NOWAIT` not specified	The `semncnt` field is incremented, indicating the number of processes waiting for the semaphore value of the specified semaphore to become greater than zero. Process execution is suspended until one of the following conditions occurs: The semaphore value becomes greater than or equal to the absolute value of `sem_op`. When this occurs, the `semncnt` field is decremented by 1 and execution continues as described above when semaphore value >= absolute value of `sem_op`. The semaphore identifier is removed from the system. `semop()` returns with a value of -1 and `errno` is set to `EIRDM`. A signal is caught by the suspended process. When this occurs, the `semncnt` field is decremented by 1 and the calling process resumes execution in the manner defined by the signal facility.

If the value of sem_op is equal to 0 and the calling process has read permission, one of the following operations occurs depending upon the current semaphore value and the value of sem_flag:

Table 2-2 Operations when sem_op=0

Semaphore

Value

sem_flag

Value

Operation

0 Any value semop() executes the next semaphore operation in the array pointed to by semops, or returns successfully to the calling process if there are no more valid semaphore operations.

<>0 IPC_NOWAIT semop() returns -1, sets errno to EAGAIN, and returns control to the calling process.

<>0

0 or SEM_UNDO

The semzcnt field is incremented, indicating the number of processes waiting for the semaphore value of the specified semaphore to become zero. Process execution is suspended until one of the following conditions occurs:

The semaphore value becomes zero. When this occurs, the semzcnt field is decremented by 1 and execution continues as described above when semaphore value = 0.
The specified semaphore identifier is removed from the system. semop() returns with a value of -1 and errno is set to EIRDM.
A signal is caught by the suspended process. When this occurs, the semncnt field is decremented by 1 and the calling process resumes execution in the manner defined by the signal facility.

If the value of sem_op is greater than 0 and the calling process has write permission, one of the following operations occurs depending upon the current semaphore value and the value of sem_flag.

Table 2-3 Operations when sem_op>0

Semaphore Value	`sem_flag` Value	Operation
Any value	`SEM_UNDO` not specified	A new semaphore value is calculated as the result of adding the value of `sem_op` to the current semaphore value of the specified semaphore. The call to `semop()` returns successfully to the calling process.
Any value	`SEM_UNDO`	A new semaphore value is calculated as the result of adding the value of `sem_op` to the current semaphore value of the specified semaphore. The value of `sem_op` is subtracted from the calling process's semaphore adjust value of the specified semaphore. The call to `semop()` returns successfully to the calling process.

If semop() is successful, the value of sempid for each semaphore specified in the array pointed to by sops is set equal to the PID of the calling process. The value of sem_otime in the data structure associated with the semaphore identifier is set to the current time.

Implementation Considerations

If a process suspended during execution of semop() receives a signal, control returns to the user with errno set to EINTR. Disabled signals are ignored.

The maximum number of semaphore UNDO entries per process is 64. Semaphore adjust values can be maintained for up to 64 distinct semaphores (elements of semaphore sets).

An MPE/iX system manager can use the MPE/iX SVIPC utility to interactively configure:

the maximum semaphore value
the maximum semaphore adjust value
the maximum nsops value
The maximum sem_op value

Refer to the section "Managing SVID IPC Services" for more information.

Errors

If an error occurs, errno is set to one of the following values.

`E2BIG`	CAUSE	`nsops` specifies a value greater than the system-defined limit.
	ACTION	Check the `nsops` value and make sure it is within the system-defined range.
`EACCES`	CAUSE	The calling process does not have permission.
	ACTION	Ensure the process has write permission (to modify a semaphore value) or read permission (to test a semaphore for 0).
`EAGAIN`	CAUSE	`sem_flg` specifies `IPC_NOWAIT` and the calling process would suspend on the specified operation.
	ACTION	None. Application dependent.
`EFAULT`	CAUSE	The system detected a NULL or bad address in attempting to use the `sops` argument.
	ACTION	Check to see that the `sops` argument has been properly defined.
`EFBIG`	CAUSE	`sem_num` is either less than zero or greater than or equal to the number of semaphores in the semaphore set associated with `semid`.
	ACTION	Check the `sem_num` value to make sure it specifies a valid semaphore in the semaphore set identified by `semid`.
`EIDRM`	CAUSE	The semaphore set specified by `semid` was removed while `semop()` was suspended on a semaphore operation.
	ACTION	None.
`EINTR`	CAUSE	`semget()` was interrupted by a signal.
	ACTION	None. Application dependent.
`EINVAL`	CAUSE	`semid` is not a valid semaphore identifier, or the calling process requested a `SEM_UNDO` for a number of semaphores that would exceed the system-defined limit.
	ACTION	Check that `semid` specifies a valid semaphore identifier and that it has not been removed from the system.
`ENOSPC`	CAUSE	The number of maximum undo entries (64) for this process would exceed the system-defined limit.
	ACTION	None. There were no undo table entries available to record the `SEM_UNDO` information. Examine the application to see if `SEM_UNDO` is required for that many semaphores.
`ERANGE`	CAUSE	The resulting semaphore value or semaphore adjust value would exceed the system-defined limit.
	ACTION	None. Application dependent.
`ESYSERR`	CAUSE	An operating system error occurred that does not map directly to any of the above errors.
	ACTION	Examine the MPE/iX process error stack for the type of system error.

semop

Technical documentation

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