Manpage of SEMCTL

SEMCTL

Section: Linux Programmer's Manual (2)
Updated: 2016-10-08
Index
 

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 cmdon 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_dsdata 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 long   sem_nsems; /* No. of semaphores in set */
};

The ipc_permstructure 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 */
};

Valid values for cmdare:

IPC_STAT
Copy information from the kernel data structure associated with semidinto the semid_dsstructure pointed to by arg.buf. The argument semnumis ignored. The calling process must have read permission on the semaphore set.
IPC_SET
Write the values of some members of the semid_dsstructure pointed to by arg.bufto the kernel data structure associated with this semaphore set, updating also its sem_ctimemember. 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 semnumis ignored.
IPC_RMID
Immediately remove the semaphore set, awakening all processes blocked in semop(2) calls on the set (with an error return and errnoset 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 semnumis 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_SOURCEfeature 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 semmnisettings can be changed via /proc/sys/kernel/sem; see proc(5) for details.
SEM_INFO (Linux-specific)
Return a seminfostructure containing the same information as for IPC_INFO, except that the following fields are returned with information about system resources consumed by semaphores: the semuszfield returns the number of semaphore sets that currently exist on the system; and the semaemfield returns the total number of semaphores in all semaphore sets on the system.
SEM_STAT (Linux-specific)
Return a semid_dsstructure as for IPC_STAT. However, the semidargument 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 semnumis ignored. The calling process must have read permission on the semaphore set.
GETNCNT
Return the value of semncntfor the semnum-th semaphore of the set (i.e., the number of processes waiting for an increase of semvalfor the semnum-th semaphore of the set). The calling process must have read permission on the semaphore set.
GETPID
Return the value of sempidfor 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 the value of semvalfor the semnum-th semaphore of the set. The calling process must have read permission on the semaphore set.
GETZCNT
Return the value of semzcntfor the semnum-th semaphore of the set (i.e., the number of processes waiting for semvalof the semnum-th semaphore of the set to become 0). The calling process must have read permission on the semaphore set.
SETALL
Set semvalfor all semaphores of the set using arg.array, updating also the sem_ctimemember of the semid_dsstructure 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 semnumis ignored. The calling process must have alter (write) permission on the semaphore set.
SETVAL
Set the value of semvalto arg.valfor the semnum-th semaphore of the set, updating also the sem_ctimemember of the semid_dsstructure 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 errnoindicating the error.

Otherwise, the system call returns a nonnegative value depending on cmdas 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_STAToperations 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 cmdvalues return 0 on success.  

ERRORS

On failure, errnowill be set to one of the following:
EACCES
The argument cmdhas one of the values GETALL, GETPID, GETVAL, GETNCNT, GETZCNT, IPC_STAT, SEM_STAT, SETALL, or SETVALand the calling process does not have the required permissions on the semaphore set and does not have the CAP_IPC_OWNERcapability in the user namespace that governs its IPC namespace.
EFAULT
The address pointed to by arg.bufor arg.arrayisn't accessible.
EIDRM
The semaphore set was removed.
EINVAL
Invalid value for cmdor semid. Or: for a SEM_STAToperation, the index value specified in semidreferred to an array slot that is currently unused.
EPERM
The argument cmdhas the value IPC_SETor IPC_RMIDbut 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_ADMINcapability.
ERANGE
The argument cmdhas the value SETALLor SETVALand the value to which semvalis 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_nsemsfield of the semid_dsstructure 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_STATand SEM_INFOoperations are used by the ipcs(1) program to provide information on allocated resources. In the future these may modified or moved to a /procfilesystem interface.

Various fields in a struct semid_ds were typed as shortunder Linux 2.2 and have become longunder 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_64flag in cmd.)

In some earlier versions of glibc, the semununion 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_UNDEFINEDis 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 sempidas 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 sempidvalue.

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 sempidfor the other operations that update the value of a semaphore: the SETVALand SETALLoperations, as well as the semaphore adjustments performed on process termination as a consequence of the use of the SEM_UNDOflag (see semop(2)).

Linux also updates sempidfor SETVALoperations and semaphore adjustments. However, somewhat inconsistently, up to and including 4.5, Linux did not update sempidfor SETALLoperations. This was rectified in Linux 4.6.  

SEE ALSO

ipc(2), semget(2), semop(2), capabilities(7), sem_overview(7), svipc(7)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
The sempid value
SEE ALSO

This document was created by man2html, using the manual pages.
Time: 16:30:06 GMT, March 14, 2017 Click Here!