SEMCTL(2) | System Calls Manual | SEMCTL(2)

# NAME

`semctl` — semaphore control operations

# SYNOPSIS

`#include <sys/sem.h>`

int
`semctl`(int semid, int semnum, int cmd, union semun arg);

# 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). arg 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 as follows in `<sys/sem.h>`:

    struct semid_ds {
    	struct ipc_perm	 sem_perm;	/* operation permissions */
    	struct sem	*sem_base;	/* semaphore set */
    	u_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 looks like this:

    struct ipc_perm {
    	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;	/* r/w permission (see chmod(2)) */
    	u_short	seq;	/* sequence # */
    			/* (to generate unique msg/sem/shm id) */
    	key_t	key;	/* user specified msg/sem/shm key */
    };

`semctl`() provides the following operations:

`GETVAL`
  Return the value of the semaphore.
`SETVAL`
  Set the value of the semaphore to arg.val.
`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 for all the semaphores associated with semid.
`SETALL`
  Set the values for all the semaphores that are associated with the semaphore identifier semid to the corresponding values in arg.array.
`IPC_STAT`
  Gather statistics about a semaphore and place the information in the
  semid_ds
  structure pointed to by arg.buf (see above).
`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. This operation can only be executed by the superuser, 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 message queue.
`IPC_RMID`
  Remove the semaphores associated with semid from the system and destroy the data structures associated with it. Only the superuser 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 message queue (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:

[`EPERM`]
  cmd is equal to `IPC_SET` or `IPC_RMID` and the caller is not the superuser, nor does the effective UID match either the sem_perm.uid or sem_perm.cuid fields of the data structure associated with the message queue.
[`EACCES`]
  The caller has no operation permission for this semaphore.
[`EINVAL`]
  semid is not a valid message semaphore identifier.

  cmd is not a valid command.

[`EFAULT`]
  arg.buf or arg.array specify an invalid address.
[`ERANGE`]
  cmd is equal to `SETVAL` or `SETALL` and arg.val or the values in arg.array are greater than the system-imposed limit.

# SEE ALSO

ipcrm(1), ipcs(1), semget(2), semop(2)

November 21, 2021 | Debian