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# semctl() - Unix, Linux System Call

# NAME

semctl - semaphore control operations

# SYNOPSIS

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

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

# DESCRIPTION

semctl() performs the control operation specified by cmd on the semaphore set identified by semid, or on the semnum-th semaphore of that set. (The semaphores in a set are numbered starting at 0.)

This function has three or four arguments, depending on cmd. When there are four, the fourth has the type union semun. The calling program must 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 semid_ds data 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 ipc_perm structure is defined in <sys/ipc.h> as follows (the highlighted fields are settable using IPC_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 cmd are:

Tag | Description
--- | ---
IPC_STAT | Copy information from the kernel data structure associated with semid into the semid_ds structure pointed to by arg.buf. The argument semnum is ignored. The calling process must have read permission on the semaphore set.
IPC_SET | Write the values of some members of the semid_ds structure pointed to by arg.buf to the kernel data structure associated with this semaphore set, updating also its sem_ctime member. The following members of the structure are updated: sem_perm.uid, sem_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 semnum is ignored.
IPC_RMID | Immediately remove the semaphore set, awakening all processes blocked in semop() calls on the set (with an error return and errno set to EIDRM). 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 argument semnum is ignored.
IPC_INFO (Linux specific)
| Returns information about system-wide semaphore limits and parameters in the structure pointed to by arg.__buf. This structure is of type seminfo, defined in <sys/sem.h> if 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 semmsl, semmns, semopm, and semmni settings can be changed via /proc/sys/kernel/sem; see proc(5) for details.
SEM_INFO (Linux specific)
| Returns a seminfo structure containing the same information as for IPC_INFO, except that the following fields are returned with information about system resources consumed by semaphores: the semusz field returns the number of semaphore sets that currently exist on the system; and the semaem field returns the total number of semaphores in all semaphore sets on the system.
SEM_STAT (Linux specific)
| Returns a semid_ds structure as for IPC_STAT. However, the semid argument 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 semval (i.e., the current value) for all semaphores of the set into arg.array. The argument semnum is ignored. The calling process must have read permission on the semaphore set.
GETNCNT | The system call returns the value of semncnt (i.e., the number of processes waiting for the value of this semaphore to increase) for the semnum-th semaphore of the set (i.e. the number of processes waiting for an increase of semval for the semnum-th semaphore of the set). The calling process must have read permission on the semaphore set.
GETPID | The system call returns the value of sempid for the semnum-th semaphore of the set (i.e. the PID of the process that executed the last semop() call for the semnum-th semaphore of the set). The calling process must have read permission on the semaphore set.
GETVAL | The system call returns the value of semval for the semnum-th semaphore of the set. The calling process must have read permission on the semaphore set.
GETZCNT | The system call returns the value of semzcnt (i.e., the number of processes waiting for the value of this semaphore to become zero) for the semnum-th semaphore of the set (i.e. the number of processes waiting for semval of the semnum-th semaphore of the set to become 0). The calling process must have read permission on the semaphore set.
SETALL | Set semval for all semaphores of the set using arg.array, updating also the sem_ctime member of the semid_ds structure associated with the set. Undo entries (see semop(2)) are cleared for altered semaphores in all processes. If the changes to semaphore values would permit blocked semop() calls in other processes to proceed, then those processes are woken up. The argument semnum is ignored. The calling process must have alter (write) permission on the semaphore set.
SETVAL | Set the value of semval to arg.val for the semnum-th semaphore of the set, updating also the sem_ctime member of the semid_ds structure associated with the set. Undo entries are cleared for altered semaphores in all processes. If the changes to semaphore values would permit blocked semop() calls in other processes to proceed, then those processes are woken up. The calling process must have alter permission on the semaphore set.

# RETURN VALUE

On failure semctl() returns -1 with errno indicating the error.

Otherwise the system call returns a nonnegative value depending on cmd as follows:

Tag | Description
--- | ---
GETNCNT | the value of semncnt.
GETPID | the value of sempid.
GETVAL | the value of semval.
GETZCNT | the value of semzcnt.
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 SEM_STAT operations to obtain information about all semaphore sets on the system.)
SEM_INFO | As for IPC_INFO.
SEM_STAT | the identifier of the semaphore set whose index was given in semid.

All other cmd values return 0 on success.

# ERRORS

On failure, errno will be set to one of the following:

Tag | Description
--- | ---
EACCES | The argument cmd has one of the values GETALL, GETPID, GETVAL, GETNCNT, GETZCNT, IPC_STAT, SEM_STAT, SETALL, or SETVAL and the calling process does not have the required permissions on the semaphore set and does not have the CAP_IPC_OWNER capability.
EFAULT | The address pointed to by arg.buf or arg.array isn’t accessible.
EIDRM | The semaphore set was removed.
EINVAL | Invalid value for cmd or semid. Or: for a SEM_STAT operation, the index value specified in semid referred to an array slot that is currently unused.
EPERM | The argument cmd has the value IPC_SET or IPC_RMID but the effective user ID of the calling process is not the creator (as found in sem_perm.cuid) or the owner (as found in sem_perm.uid) of the semaphore set, and the process does not have the CAP_SYS_ADMIN capability.
ERANGE | The argument cmd has the value SETALL or SETVAL and the value to which semval is to be set (for some semaphore of the set) is less than 0 or greater than the implementation limit SEMVMX.

# NOTES

The IPC_INFO, SEM_STAT and SEM_INFO operations are used by the ipcs(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 struct semid_ds were 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 cmd.)

In some earlier versions of glibc, the semun union 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 not defined, the macro _SEM_SEMUN_UNDEFINED is defined in <sys/sem.h>.

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

Tag | Description
--- | ---
SEMVMX | Maximum value for semval: implementation dependent (32767).

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

Under Linux, semctl() is not a system call, but is implemented via the system call ipc(2).

# CONFORMING TO

SVr4, POSIX.1-2001.

# SEE ALSO

- ipc (2)

- semget (2)

- semop (2)

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