NAME

semctl − semaphore control operations

SYNOPSIS

#include <sys/sem.h>

int semctl(int semid,
int semnum,
int cmd, ...
/* arg */
);

DESCRIPTION

semctl() provides a variety of semaphore control operations as specified by cmd.

The following cmds are executed with respect to the semaphore specified by semid and semnum:

GETVAL Return the value of semval (see semaphore identifier in glossary(9)). Requires Read permission.

SETVAL Set the value of semval to arg, where arg is the fourth argument of semctl() taken as an int.  When this cmd is successfully executed, the semadj value corresponding to the specified semaphore in all processes is cleared.  Requires Alter permission. 

GETPID Return the value of sempid.  Requires Read permission. 

GETNCNT Return the value of semncnt.  Requires Read permission. 

GETZCNT Return the value of semzcnt.  Requires Read permission. 

The following cmds return and set, respectively, every semval in the set of semaphores.

GETALL Place semvals into array pointed to by arg, where arg is the fourth argument of semctl() taken as a pointer to unsigned short int.  Requires Read permission. 

SETALL Set semvals according to the array pointed to by arg, where arg is the fourth argument of semctl() taken as a pointer to unsigned short int.  When this cmd is successfully executed, the semadj values corresponding to each specified semaphore in all processes are cleared.  Requires Alter permission. 

The following cmds are also available:

IPC_STAT Place the current value of each member of the data structure associated with semid into the structure pointed to by arg, where arg is the fourth argument of semctl() taken as a pointer to struct semid_ds.  The contents of this structure are defined in glossary(9). Requires Read permission.

IPC_SET Set the value of the following members of the data structure associated with semid to the corresponding value found in the structure pointed to by arg, where arg is the fourth argument of semctl() taken as a pointer to struct semid_ds:

sem_perm.uid
sem_perm.gid
sem_perm.mode/* only low 9 bits */

This cmd can only be executed by a process that has an effective user ID equal to either that of super-user or to the value of either sem_perm.uid or sem_perm.cuid in the data structure associated with semid.

IPC_RMID Remove the semaphore identifier specified by semid from the system and destroy the set of semaphores and data structure associated with it.  This cmd can only be executed by a process that has an effective user ID equal to either that of super-user or to the value of either sem_perm.uid or sem_perm.cuid in the data structure associated with semid.

EXAMPLES

The following call to semctl() initializes the set of 4 semaphores to the values 0, 1, 0, and 1 respectively.  This example assumes the process has a valid semid representing a set of 4 semaphores as shown in the semget(2) manual entry. For an example of performing "P" and "V" operations on the semaphores below, refer to semop(2).

ushort semarray[4];

semarray[0] = 0;
semarray[1] = 1;
semarray[2] = 0;
semarray[3] = 1;

semctl (mysemid, 0, SETALL, semarray);

RETURN VALUE

Upon successful completion, the value returned depends on cmd as follows:

GETVAL The value of semval. 

GETNCNT The value of semncnt. 

GETZCNT The value of semzcnt. 

GETPID The value of sempid. 

All others return a value of 0. 

Otherwise, a value of −1 is returned and errno is set to indicate the error. 

ERRORS

semctl() fails if any of the following conditions are encountered:

[EACCES] Operation permission is denied to the calling process (see semaphore operation permissions in glossary(9).

[EFAULT] cmd is equal to GETVAL, SETVAL, GETALL, SETALL, IPC_STAT, or IPC_SET, and arg.

[EINVAL] semid is not a valid semaphore identifier. 

[EINVAL] semnum is less than zero or greater than or equal sem_nsems. 

[EINVAL] cmd is not a valid command. 

[EPERM] cmd is equal to IPC_RMID or IPC_SET and the effective user ID of the calling process is not equal to that of super-user and it is not equal to the value of either sem_perm.uid or sem_perm.cuid in the data structure associated with semid.

[ERANGE] cmd is SETVAL or SETALL and the value to which semval is to be set is greater than the system imposed maximum. 

SEE ALSO

ipcrm(1), ipcs(1), semget(2), semop(2), stdipc(3C). 

STANDARDS CONFORMANCE

semctl(): SVID2, XPG2, XPG3, XPG4

Hewlett-Packard Company  —  HP-UX Release 9.0: August 1992