SEMCTL

Section: Linux Programmer's Manual (2)
Updated: 2020-12-21
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NAME

semctl - System V 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 System V 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; /* Creation time/time of last
                                  modification via semctl() */
    unsigned long   sem_nsems; /* No. of semaphores in set */ };

The fields of the semid_ds structure are as follows:

sem_perm: This is an ipc_perm structure (see below) that specifies the access permissions on the semaphore set.
sem_otime: Time of last semop(2) system call.
sem_ctime: Time of creation of semaphore set or time of last semctl() IPCSET, SETVAL, or SETALL operation.
sem_nsems: Number of semaphores in the set. Each semaphore of the set is referenced by a nonnegative integer ranging from 0 to sem_nsems-1.

The ipc_perm structure is defined as follows (the highlighted fields are settable using IPC_SET):

struct ipc_perm {
    key_t          __key; /* Key supplied to semget(2) */
    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 */ };

The least significant 9 bits of the mode field of the ipc_perm structure define the access permissions for the shared memory segment. The permission bits are as follows:

0400	Read by user
0200	Write by user
0040	Read by group
0020	Write by group
0004	Read by others
0002	Write by others

In effect, "write" means "alter" for a semaphore set. Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.

Valid values for cmd are:

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(2) 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): Return 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;  /* Number of entries in semaphore
                    map; unused within kernel */
    int semmni;  /* Maximum number of semaphore sets */
    int semmns;  /* Maximum number of semaphores in all
                    semaphore sets */
    int semmnu;  /* System-wide maximum number of undo
                    structures; unused within kernel */
    int semmsl;  /* Maximum number of semaphores in a
                    set */
    int semopm;  /* Maximum number of operations for
                    semop(2) */
    int semume;  /* Maximum number of undo entries per
                    process; unused within kernel */
    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): Return 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): Return 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.
SEM_STAT_ANY (Linux-specific, since Linux 4.17): Return a seminfo structure containing the same information as for SEM_STAT. However, sem_perm.mode is not checked for read access for semid meaning that any user can employ this operation (just as any user may read /proc/sysvipc/sem to obtain the same information).
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: Return the semncnt value for the semnum-th semaphore of the set (i.e., the number of processes waiting for the semaphore's value to increase). The calling process must have read permission on the semaphore set.
GETPID: Return the sempid value for the semnum-th semaphore of the set. This is the PID of the process that last performed an operation on that semaphore (but see NOTES). The calling process must have read permission on the semaphore set.
GETVAL: Return semval (i.e., the semaphore value) for the semnum-th semaphore of the set. The calling process must have read permission on the semaphore set.
GETZCNT: Return the semzcnt value for the semnum-th semaphore of the set (i.e., the number of processes waiting for the semaphore value to become 0). The calling process must have read permission on the semaphore set.
SETALL: Set the semval values 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(2) 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 semaphore value (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(2) 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:

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 or SEM_STAT_ANY 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.
SEM_STAT_ANY: as for SEM_STAT.

All other cmd values return 0 on success.

ERRORS

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

EACCES: The argument cmd has one of the values GETALL, GETPID, GETVAL, GETNCNT, GETZCNT, IPC_STAT, SEM_STAT, SEM_STAT_ANY, 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 in the user namespace that governs its IPC namespace.
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.

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4.

POSIX.1 specifies the sem_nsems field of the semid_ds structure as having the type unsigned short, and the field is so defined on most other systems. It was also so defined on Linux 2.2 and earlier, but, since Linux 2.4, the field has the type unsigned long.

NOTES

The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux or by any version of POSIX. However, some old implementations required the inclusion of these header files, and the SVID also documented their inclusion. Applications intended to be portable to such old systems may need to include these header files.

The IPC_INFO, SEM_STAT, and SEM_INFO operations are used by the ipcs(1) program to provide information on allocated resources. In the future these may modified or moved to a /proc filesystem interface.

Various fields in a struct semid_ds were typed as short under Linux 2.2 and have become long 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 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:

SEMVMX: Maximum value for semval: implementation dependent (32767).

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

The sempid value

POSIX.1 defines sempid as the "process ID of [the] last operation" on a semaphore, and explicitly notes that this value is set by a successful semop(2) call, with the implication that no other interface affects the sempid value.

While some implementations conform to the behavior specified in POSIX.1, others do not. (The fault here probably lies with POSIX.1 inasmuch as it likely failed to capture the full range of existing implementation behaviors.) Various other implementations also update sempid for the other operations that update the value of a semaphore: the SETVAL and SETALL operations, as well as the semaphore adjustments performed on process termination as a consequence of the use of the SEM_UNDO flag (see semop(2)).

Linux also updates sempid for SETVAL operations and semaphore adjustments. However, somewhat inconsistently, up to and including Linux 4.5, the kernel did not update sempid for SETALL operations. This was rectified in Linux 4.6.

EXAMPLES

See shmop(2).

COLOPHON

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