NAME

semctl — semaphore control operations

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/sem.h>

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

DESCRIPTION

The semctl() system call provides a number of control operations on the semaphore specified by semnum and semid. The operation to be performed is specified in cmd (see below). The fourth argument is optional and depends upon the operation requested. If required, it is a union of the following fields:

    int     val;            /* value for SETVAL */ 
    struct  semid_ds *buf;  /* buffer for IPC_{STAT,SET} */ 
    u_short *array;         /* array for GETALL & SETALL */

The semid_ds structure used in the IPC_SET and IPC_STAT commands is defined in <sys/sem.h> and contains the following members:

    struct ipc_perm sem_perm; /* operation permissions */ 
    unsigned short sem_nsems; /* number of sems in set */ 
    time_t sem_otime;         /* last operation time */ 
    time_t sem_ctime;         /* last change time */

The ipc_perm structure used inside the semid_ds structure is defined in <sys/ipc.h> and contains the following members:

    uid_t cuid;  /* creator user id */ 
    gid_t cgid;  /* creator group id */ 
    uid_t uid;	 /* user id */ 
    gid_t gid;	 /* group id */ 
    mode_t mode; /* permission (lower 9 bits) */

semctl() provides the following operations:

GETVAL

Return the value of the semaphore.

SETVAL

Set the value of the semaphore to arg.val, where arg is the fourth argument to semctl().

GETPID

Return the pid of the last process that did an operation on this semaphore.

GETNCNT

Return the number of processes waiting to acquire the semaphore.

GETZCNT

Return the number of processes waiting for the value of the semaphore to reach 0.

GETALL

Return the values of all the semaphores associated with semid.

SETALL

Set the values of all the semaphores that are associated with the semaphore identifier semid to the corresponding values in arg.array, where arg is the fourth argument to semctl().

IPC_STAT

Gather information about a semaphore and place the information in the structure pointed to by arg.buf, where arg is the fourth argument to semctl().

IPC_SET

Set the value of the sem_perm.uid, sem_perm.gid and sem_perm.mode fields in the structure associated with the semaphore. The values are taken from the corresponding fields in the structure pointed to by arg.buf, there arg is the fourth argument to semctl(). This operation can only be executed by the super-user, or a process that has an effective user id equal to either sem_perm.cuid or sem_perm.uid in the data structure associated with the semaphore.

IPC_RMID

Remove the semaphores associated with semid from the system and destroy the data structures associated with it. Only the super-user or a process with an effective uid equal to the sem_perm.cuid or sem_perm.uid values in the data structure associated with the semaphore can do this.

The permission to read or change a semaphore (see semop(2)) is determined by the sem_perm.mode field in the same way as is done with files (see chmod(2)), but the effective uid can match either the sem_perm.cuid field or the sem_perm.uid field, and the effective gid can match either sem_perm.cgid or sem_perm.gid.

RETURN VALUES

For the GETVAL, GETPID, GETNCNT, and GETZCNT operations, semctl() returns one of the values described above if successful. All other operations will make semctl() return 0 if no errors occur. Otherwise -1 is returned and errno set to reflect the error.

ERRORS

semctl() will fail if:

[EACCES]

The caller has no operation permission for this semaphore.

[EFAULT]

arg.buf or arg.array specifies an invalid address.

[EINVAL]

semid is not a valid message semaphore identifier.

cmd is not a valid command.

[EPERM]

cmd is equal to IPC_SET or IPC_RMID and the caller is not the super-user, nor does the effective uid match either the sem_perm.uid or sem_perm.cuid fields of the data structure associated with the semaphore.

[ERANGE]

cmd is equal to SETVAL or SETALL and the value to be set is greater than the system semaphore maximum value.

SEE ALSO

semget(2), semop(2)

STANDARDS

The semctl system call conforms to X/Open System Interfaces and Headers Issue 5 (“XSH5”).

HISTORY

Semaphores appeared in the first release of AT&T System V UNIX.