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   semctl    ( 2 )

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Примечание (Note)

The IPC_INFO, SEM_STAT, and SEM_INFO operations are used by the ipcs(1) program to provide information on allocated resources. In the future these may modified or moved to a /proc filesystem interface.

Various fields in a struct semid_ds were typed as short under Linux 2.2 and have become long under Linux 2.4. To take advantage of this, a recompilation under glibc-2.1.91 or later should suffice. (The kernel distinguishes old and new calls by an IPC_64 flag in cmd.)

In some earlier versions of glibc, the semun union was defined in <sys/sem.h>, but POSIX.1 requires that the caller define this union. On versions of glibc where this union is not defined, the macro _SEM_SEMUN_UNDEFINED is defined in <sys/sem.h>.

The following system limit on semaphore sets affects a semctl() call:

SEMVMX Maximum value for semval: implementation dependent (32767).

For greater portability, it is best to always call semctl() with four arguments.

The sempid value POSIX.1 defines sempid as the "process ID of [the] last operation" on a semaphore, and explicitly notes that this value is set by a successful semop(2) call, with the implication that no other interface affects the sempid value.

While some implementations conform to the behavior specified in POSIX.1, others do not. (The fault here probably lies with POSIX.1 inasmuch as it likely failed to capture the full range of existing implementation behaviors.) Various other implementations also update sempid for the other operations that update the value of a semaphore: the SETVAL and SETALL operations, as well as the semaphore adjustments performed on process termination as a consequence of the use of the SEM_UNDO flag (see semop(2)).

Linux also updates sempid for SETVAL operations and semaphore adjustments. However, somewhat inconsistently, up to and including Linux 4.5, the kernel did not update sempid for SETALL operations. This was rectified in Linux 4.6.