semctl -%uA0信号灯控制操作

内容简介

#include <sys/types.h> 
#include <sys/ipc.h> 
#include <sys/sem.h>

int semctl(int semid, int semnum, int cmd, ...)

描述

semctl() performs the control operation specified by%uA0cmd%uA0on the semaphore set identified by%uA0semid, or on the%uA0semnum-th semaphore of that set. (The semaphores in a set are numbered starting at 0.)

This function has three or four arguments, depending on%uA0cmd. When there are four, the fourth has the type%uA0union semun. The%uA0calling program%uA0must define this union as follows:

union semun {
    int              val    /* Value for SETVAL */
    struct semid_ds *buf    /* Buffer for IPC_STAT, IPC_SET */
    unsigned short  *array  /* Array for GETALL, SETALL */
    struct seminfo  *__buf  /* Buffer for IPC_INFO
                                (Linux specific) */
}

The%uA0semid_dsꃚta structure is defined in <sys/sem.h> as follows:

struct semid_ds {
    struct ipc_perm sem_perm  /* Ownership and permissions
    time_t          sem_otime /* Last semop time */
    time_t          sem_ctime /* Last change time */
    unsigned short  sem_nsems /* No. of semaphores in set */
}

The%uA0ipc_perm%uA0structure is defined in <sys/ipc.h> as follows (the highlighted fields are settable using%uA0IPC_SET):

struct ipc_perm {
    key_t key            /* Key supplied to semget() */
    uid_t uid            /* Effective UID of owner */
    gid_t gid            /* Effective GID of owner */
    uid_t cuid           /* Effective UID of creator */
    gid_t cgid           /* Effective GID of creator */
    unsigned short mode  /* Permissions */
    unsigned short seq   /* Sequence number */
}

Valid values for%uA0cmd%uA0are:

标签

描述

IPC_STAT

Copy information from the kernel data structure associated withsemid%uA0into the%uA0semid_ds%uA0structure pointed to by%uA0arg.buf. The argument%uA0semnum%uA0is ignored. The calling process must have read permission on the semaphore set.

IPC_SET

Write the values of some members of the%uA0semid_ds%uA0structure pointed to by%uA0arg.buf%uA0to the kernel data structure associated with this semaphore set, updating also its%uA0sem_ctime%uA0member. The following members of the structure are updated:sem_perm.uid,%uA0sem_perm.gid, and (the least significant 9 bits of)sem_perm.mode. The effective UID of the calling process must match the owner (sem_perm.uid) or creator (sem_perm.cuid) of the semaphore set, or the caller must be privileged. The argument%uA0semnum%uA0is ignored.

IPC_RMID

Immediately remove the semaphore set, awakening all processes blocked in%uA0semop() calls on the set (with an error return and%uA0errno%uA0set to%uA0EIDRM). The effective user ID of the calling process must match the creator or owner of the semaphore set, or the caller must be privileged. The argumentsemnum%uA0is ignored.

IPC_INFO%uA0(Linux specific)

%uA0

Returns information about system-wide semaphore limits and parameters in the structure pointed to by%uA0arg.__buf. This structure is of type%uA0seminfo, defined in%uA0<sys/sem.h>%uA0if the _GNU_SOURCE feature test macro is defined:

struct  seminfo {
    int semmap  /* # of entries in semaphore map
                    unused */
    int semmni  /* Max. # of semaphore sets */
    int semmns  /* Max. # of semaphores in all
                    semaphore sets */
    int semmnu  /* System-wide max. # of undo
                    structures unused */
    int semmsl  /* Max. # of semaphores in a set */
    int semopm  /* Max. # of operations for semop() */
    int semume  /* Max. # of undo entries per
                    process unused */
    int semusz  /* size of struct sem_undo */
    int semvmx  /* Maximum semaphore value */
    int semaem  /* Max. value that can be recorded for
                    semaphore adjustment (SEM_UNDO) */
}

The%uA0semmsl,%uA0semmns,%uA0semopm, and%uA0semmni%uA0settings can be changed via%uA0/proc/sys/kernel/sem see%uA0proc(5) for details.

SEM_INFO%uA0(Linux specific)

%uA0

Returns a%uA0seminfo%uA0structure containing the same information as for%uA0IPC_INFO, except that the following fields are returned with information about system resources consumed by semaphores: the%uA0semusz%uA0field returns the number of semaphore sets that currently exist on the system and the%uA0semaem%uA0field returns the total number of semaphores in all semaphore sets on the system.

SEM_STAT%uA0(Linux specific)

%uA0

Returns a%uA0semid_ds%uA0structure as for%uA0IPC_STAT. However, thesemid%uA0argument is not a semaphore identifier, but instead an index into the kernel’s internal array that maintains information about all semaphore sets on the system.

GETALL

Return%uA0semval%uA0(i.e., the current value) for all semaphores of the set into%uA0arg.array.%uA0The argument%uA0semnum%uA0is ignored. The calling process must have read permission on the semaphore set.

GETNCNT

The system call returns the value of%uA0semncnt%uA0(i.e., the number of processes waiting for the value of this semaphore to increase) for the%uA0semnum-th semaphore of the set (i.e. the number of processes waiting for an increase of%uA0semval%uA0for thesemnum-th semaphore of the set). The calling process must have read permission on the semaphore set.

GETPID

The system call returns the value of%uA0sempid%uA0for the%uA0semnum-th semaphore of the set (i.e. the PID of the process that executed the last%uA0semop() call for the%uA0semnum-th semaphore of the set). The calling process must have read permission on the semaphore set.

GETVAL

The system call returns the value of%uA0semval%uA0for the%uA0semnum-th semaphore of the set. The calling process must have read permission on the semaphore set.

GETZCNT

The system call returns the value of%uA0semzcnt%uA0(i.e., the number of processes waiting for the value of this semaphore to become zero) for the%uA0semnum-th semaphore of the set (i.e. the number of processes waiting for%uA0semval%uA0of the%uA0semnum-th semaphore of the set to become 0). The calling process must have read permission on the semaphore set.

SETALL

Set%uA0semval%uA0for all semaphores of the set using%uA0arg.array,updating also the%uA0sem_ctime%uA0member of the%uA0semid_ds%uA0structure associated with the set. Undo entries (see%uA0semop(2)) are cleared for altered semaphores in all processes. If the changes to semaphore values would permit blocked%uA0semop() calls in other processes to proceed, then those processes are woken up. The argument%uA0semnum%uA0is ignored. The calling process must have alter (write) permission on the semaphore set.

SETVAL

Set the value of%uA0semval%uA0to%uA0arg.val%uA0for the%uA0semnum-th semaphore of the set, updating also the%uA0sem_ctime%uA0member of the%uA0semid_ds%uA0structure associated with the set. Undo entries are cleared for altered semaphores in all processes. If the changes to semaphore values would permit blocked%uA0semop() calls in other processes to proceed, then those processes are woken up. The calling process must have alter permission on the semaphore set.

返回值

On failure%uA0semctl() returns -1 with%uA0errno%uA0indicating the error.

Otherwise the system call returns a nonnegative value depending on%uA0cmd%uA0as follows:

标签	描述
GETNCNT	the value of%uA0semncnt.
GETPID	the value of%uA0sempid.
GETVAL	the value of%uA0semval.
GETZCNT	the value of%uA0semzcnt.
IPC_INFO	the index of the highest used entry in the kernel’s internal array recording information about all semaphore sets. (This information can be used with repeated%uA0SEM_STAT%uA0operations to obtain information about all semaphore sets on the system.)
SEM_INFO	As for%uA0IPC_INFO.
SEM_STAT	the identifier of the semaphore set whose index was given insemid.

All other%uA0cmd%uA0values return 0 on success.

错误

On failure,%uA0errno%uA0will be set to one of the following:

标签	描述
EACCES	The argument%uA0cmd%uA0has one of the values%uA0GETALL,%uA0GETPID,GETVAL,%uA0GETNCNT,%uA0GETZCNT,%uA0IPC_STAT,%uA0SEM_STAT,SETALL, or%uA0SETVAL%uA0and the calling process does not have the required permissions on the semaphore set and does not have the%uA0CAP_IPC_OWNERꃊpability.
EFAULT	The address pointed to by%uA0arg.buf%uA0or%uA0arg.array%uA0isn’t accessible.
EIDRM	The semaphore set was removed.
EINVAL	Invalid value for%uA0cmd%uA0or%uA0semid. Or: for a%uA0SEM_STAT%uA0operation, the index value specified in%uA0semid%uA0referred to an array slot that is currently unused.
EPERM	The argument%uA0cmd%uA0has the value%uA0IPC_SET%uA0or%uA0IPC_RMID%uA0but the effective user ID of the calling process is not the creator (as found in%uA0sem_perm.cuid) or the owner (as found insem_perm.uid) of the semaphore set, and the process does not have the%uA0CAP_SYS_ADMINꃊpability.
ERANGE	The argument%uA0cmd%uA0has the value%uA0SETALL%uA0or%uA0SETVAL%uA0and the value to which%uA0semval%uA0is to be set (for some semaphore of the set) is less than 0 or greater than the implementation limitSEMVMX.

注意

The%uA0IPC_INFO,%uA0SEM_STAT%uA0and%uA0SEM_INFO%uA0operations are used by the%uA0ipcs(8) program to provide information on allocated resources. In the future these may modified or moved to a /proc file system interface.

Various fields in a%uA0struct semid_ds%uA0were shorts under Linux 2.2 and have become longs 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%uA0cmd.)

In some earlier versions of glibc, the%uA0semun%uA0union was defined in <sys/sem.h>, but POSIX.1-2001 requires that the caller define this union. On versions of glibc where this union is%uA0notꃞfined, the macro%uA0_SEM_SEMUN_UNDEFINED%uA0is defined in <sys/sem.h>.

The following system limit on semaphore sets affects a%uA0semctl() call:

标签	描述
SEMVMX	Maximum value for%uA0semval: implementation dependent (32767).

For greater portability it is best to always call%uA0semctl() with four arguments.

Under Linux,%uA0semctl() is not a system call, but is implemented via the system call%uA0ipc(2).

遵循于

SVr4, POSIX.1-2001.

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内容简介

描述

返回值

错误

注意

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